SSSAJ: Volume 75: Number 5 • September–October 2011

1789

Soil Sci. Soc. Am. J. 75:1789–1798Posted online 11 Aug. 2011doi:10.2136/sssaj2010.0202Received 14 May 2010.*Corresponding author (upendra.sainju@ars.usda.gov).© Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USAAll rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Sheep Grazing in a Wheat–Fallow System Affects Dryland Soil Properties and Grain Yield

Soil Fertility & Plant Nutrition

Integrated crop–livestock systems were commonly used to sustain crop and live-stock production throughout the world before synthetic fertilizers were intro-

duced in the 20th century (Franzluebbers, 2007). Th e system is still used in many developing countries, especially in Africa and Southeast Asia, where fertilizers are scarce and expensive (Herrero et al., 2010). Extensive application of fertilizers in the 20th century increased crop yields but reduced environmental quality by increasing (i) N leaching from the soil profi le to the groundwater, (ii) surface runoff of N and P from agricultural lands to streams and lakes, causing eutrophication, and (iii) emis-sions of greenhouse gases such as N2O, (Franzluebbers, 2007; Herrero et al., 2010). Increased soil acidity following the application of commercial fertilizers, especially N fertilizers, also led to the development of infertile soils that did not respond well to increased fertilizer application for sustaining crop yields (Herrero et al., 2010). In such cases, integrated crop–livestock systems can be used as an option to improve soil quality and sustain crop yields (Franzluebbers, 2007; Maughan et al., 2009). Th e major benefi ts of these systems are: (i) the production of crops, meat, and milk, (ii) the production of crop residue for animal feed, (iii) the production of manure to apply as fertilizer, (iv) the use of animals as draft power for tillage, and (v) control of weeds and pests (Franzluebbers, 2007; Hatfi eld et al., 2007a,c; Herrero et al., 2010).

Upendra M. Sainju*Andrew W. Lenssen

USDA-ARSNorthern Plains Agricultural Research Lab.1500 North Central Ave.Sidney, MT 59270

Hayes B. GooseyErin SnyderPatrick G. Hatfi eld

Dep. of Animal and Range Sciences230 Linfi eld HallMontana State Univ.Bozeman, MT 59717

Sheep (Ovis aries L.) grazing, an eff ective method of controlling weeds and pests in a wheat (Triticum aestivum L.)–fallow system, may aff ect dryland soil properties and wheat yield. We evaluated the eff ects of fallow manage-ment for weed control and soil water conservation (sheep grazing, herbicide application [chemical], and tillage [mechanical]) and cropping sequence (continuous spring wheat [CSW], spring wheat–fallow [SW-F], and winter wheat–fallow [WW-F]) on soil nutrients and chemical properties in the 0- to 60-cm depth and wheat yield. Th e experiment was conducted in a Blackmore silt loam from 2004 to 2008 in southwestern Montana. Soil P and K concentrations at 0 to 30 cm were lower in the grazing than in the chemical or mechanical treatments. In contrast, soil Na, Ca, and Mg concentrations were greater in the grazing and mechanical than the chemical treatment. Soil Mg concentration at 30 to 60 cm was greater under CSW than WW-F. Soil SO4–S concentration varied with fallow management and cropping sequence. Soil pH, cation exchange capacity (CEC), and electrical conductivity (EC) at 0 to 15 cm were greater in the mechanical than in the chemical or grazing treatments. Annualized wheat yield was greater under CSW than SW-F or WW-F but was not aff ected by fallow management. Sheep grazing aff ected soil nutrients probably by consuming wheat residues but returning them at various levels through feces and urine. In contrast, tillage increased pH, CEC, and EC, probably by incorporating crop residue, feces, and urine into the soil. By applying enough P and K fertilizers to wheat and using less intensive grazing, sheep grazing can be used to sustain wheat yields without seriously aff ecting soil nutrients and chemical properties.

Abbreviations: CEC, cation exchange capacity; CSW, continuous spring wheat; EC, electrical conductivity; SW-F, spring wheat–fallow; WW-F, winter wheat–fallow.

1790 SSSAJ: Volume 75: Number 5 • September–October 2011

In the northern Great Plains, wheat–fallow systems have been used as the conventional dryland farming practice since the last century (Halvorson et al., 2000, 2002). In these systems, land is typically fallowed from 14 to 20 mo. Fallowing is used to conserve soil water, release plant nutrients, control weeds, increase succeed-ing crop yields, and reduce the risk of crop failure (Aase and Pikul, 1995; Jones and Popham, 1997). Using tillage and herbicides to control weeds on fallowed land is eff ective but expensive, result-ing in some of the highest variable costs for small grain production in Montana ( Johnson et al., 1997). Other disadvantages of using these practices are the exposure of soil to erosion due to tillage and an increased risk of contamination of herbicides in soil and water and risks to human and animal health (Fenster, 1997).

Sheep grazing during fallow periods in wheat–fallow systems can be used to eff ectively control weeds (Hatfi eld et al., 2007c) and insects, such as wheat stem sawfl y (Cephus cinctus Norton [Hymenoptera: Cephidae]) (Goosey et al., 2005; Hatfi eld et al., 2007a). During fallow, sheep usually graze on crop residues and weeds. Although grazing can reduce the quantity of crop residue returned to the soil, the number of sheep grazed per unit area can be adjusted in such a way that crop residue cover in the grazing treatment will be similar to that in a conservation tillage system where soil erosion is minimal (Hatfi eld et al., 2007c). Animal feces and urine returned to the soil during grazing can enrich soil nutri-

ents, improve soil quality, and increase crop yields (Franzluebbers and Stuedemann, 2008; Tracy and Zhang, 2008; Maughan et al., 2009). Th e distribution of feces and urine by animals during graz-ing at the soil surface can be uneven; however, distribution can be more uniform with sheep than with cattle (Bos taurus L.) grazing (Abaye et al., 1997). Hatfi eld et al. (2007b) reported that sheep grazing during fallow to control weeds did not infl uence soil or-ganic matter and nutrient levels compared with an ungrazed treat-ment in north-central Montana. Abaye et al. (1997) found that sheep grazing increased the soil bulk density and extractable P and grass yields compared with cattle grazing.

Levels of nutrients such as P, K, Ca, Mg, and SO4–S in the soil can infl uence crop yields and quality. Similarly, soil properties such as pH, CEC, and EC can aff ect nutrient availability and plant growth due to chemical reactions and nutrient toxicity levels. Although sheep grazing can return part of the nutrients consumed in wheat residue through feces and urine to the soil, little is known about the eff ect of sheep grazing on dryland soil nutrients and chemical properties and crop yields compared with tillage or herbicide application to con-trol weeds during fallow in wheat–fallow systems. We hypothesized that sheep grazing would result in similar or increased dryland soil nutrients and chemical properties and wheat yields compared with tillage or herbicide application and that the eff ect would be more pro-nounced in CSW than in SW-F and WW-F. Our objectives were to: (i) evaluate the eff ects of fallow management for weed control (graz-ing, chemical, and mechanical) and cropping sequence (CSW, SW-F, and WW-F) on dryland soil P, K, Ca, Mg, Na, SO4–S, CEC, EC, and pH levels at the 0- to 60-cm depth and wheat grain and biomass yields from 2004 to 2008; and (ii) relate soil nutrients and chemical properties with wheat yields in southwestern Montana.

MATERIALS AND METHODSSite Description and Treatments

Th e experiment was conducted from 2004 to 2008 at the Fort Ellis Research and Extension Center, Montana State University, Bozeman (45°40´ N, 111°2´ W; altitude 1468 m). Th e site has a mean monthly air temperature ranging from −5.7°C in January to 18.9°C in July. Th e mean annual precipitation (113-yr average) is 465 mm, 60% of which occurs during the crop growing season (April–September) (Table 1). Th e soil is a Blackmore silt loam (a fi ne-silty, mixed, superactive, frigid Typic Argiustoll) with 0 to 4% slopes and contains 250 g kg−1 sand, 500 g kg−1 silt, and 250 g kg−1 clay. Soil nutrient and chemical properties of samples from the 0- to 15- and 15- to 30-cm depths taken from two com-posite cores per plot in the spring of 2004 before the initiation of the experiment were not signifi cantly diff erent among treatments. As a result, average values across treatments are shown in Table 2. Previous cropping history for the last 10 yr was perennial grass pasture containing a mixture of smooth bromegrass (Bromus iner-mis Leyss.), intermediate wheatgrass [Th inopyrum intermedium (Host) Barkworth & D. R. Dewey], and Canada bluegrass (Poa compressa L.), followed by 1 yr of fallow.

Treatments consisted of three fallow management practices for weed control and soil water conservation (grazing, chemical,

Table 1. Monthly total precipitation from 2004 to 2008 near the experimental site.

Month 2004 2005 2006 2007 2008 113-yr avg.

——————————— mm ————————————

Jan. 7 7 11 4 8 22

Feb. 14 7 2 24 7 19

Mar. 6 22 19 10 13 34

Apr. 33 42 66 48 54 46

May 73 37 34 70 71 73

June 66 42 68 59 73 74

July 45 17 17 3 18 35

Aug. 40 31 18 24 14 32

Sept. 36 23 26 30 9 44

Oct. 31 47 73 52 8 38

Nov. 5 30 5 21 24 28

Dec. 6 17 7 6 15 22

Total 363 321 345 350 315 465

Table 2. Soil chemical properties at the 0- to 15 and 15- to 30-cm depths at the initiation of the experiment in April 2004.

Property 0–15 cm 15–30 cm

Organic C, g kg−1 33.0 17.2

P, mg kg−1 61.2 41.6

K, mg kg−1 424 296

Ca, mg kg−1 3640 3670

Mg, mg kg−1 496 579

Na, mg kg−1 22.3 26.0

SO4–S, kg ha−1 16.2 12.4

Cation exchange capacity, cmolc kg−1 23.5 24.0

Electrical conductivity, S m−1 0.041 0.042

pH 6.68 6.46

SSSAJ: Volume 75: Number 5 • September–October 2011 1791

and mechanical) as the main plot and three cropping sequences (CSW, SW-F, and WW-F) as the split-plot arrangement in a randomized complete block with three replications. Th e grazing treatment consisted of grazing with a group of western white-faced sheep at a stocking rate of 29 to 153 sheep d ha−1 during fallow periods in fenced plots. Sheep grazed on weeds and wheat residue aft er grain harvest during 3 out of 8 mo of the fallow pe-riod in CSW to 10 out of 20 mo in SW-F. Grazing ended when about 47 kg ha−1 or less of wheat residue and weeds remained in the plot. Th e chemical treatment consisted of applying herbicides, such as a mixture of glyphosate [N-(phosphonomethyl)glycine] at 1.17 L ha−1 and the dimethylamine salt of dicamba (3,6-di-chloro-2-methoxybenzoic acid) at 1.75 L ha−1, to control weeds at planting and during fallow periods. Th e mechanical treatment consisted of tilling the plots from two times in CSW (preplanting and post-harvest) to four times (preplanting, post-harvest, and fallow periods) in SW-F and WW-F to control weeds with a John Deere 100 Flexicoil harrow (Deere & Co., Moline, IL) to a depth of 15 cm. Th e split-plot size was 91.4 by 15.2 m.

Crop ManagementTh e rates of N fertilizer applied to spring and winter wheat

ranged from 200 kg N ha−1 in CSW and WW-F to 250 kg N ha−1 in SW-F. Th e rates depended on yield goals, which ranged from 3.9 Mg ha−1 in CSW to 4.8 Mg ha−1 in SW-F and WW-F. Soil NO3–N content to a depth of 60 cm measured aft er grain har-vest in the fall every year was used to adjust the N rate before N fertilizer was applied to spring and winter wheat. Nitrogen fer-tilizer as urea (45% N) was broadcast in April to May to both spring and winter wheat. For spring wheat, N fertilizer was incor-porated to a depth of 15 cm using the Flexicoil harrow. For win-ter wheat, N fertilizer was applied at the surface. Because the soil contained higher levels of extractable P and K (Table 2) (Agvise Laboratories, 2009), no P and K fertilizers were applied.

From 2004 to 2008, spring wheat (cv. McNeal, foundation seed, Montana State Univ., Bozeman) was planted at 90 kg ha−1 in late April to early May and winter wheat (cv. Promontory, foundation seed, Montana State Univ., Bozeman) was planted at 73 kg ha−1 in late September to early October using a double disk opener with a row spacing of 30 cm. Growing-season broadleaf weeds were controlled with selective post-emergence herbicides. In late August to early September, 2 d before grain harvest, total wheat yield containing stems, leaves, and grains were harvested from two 0.5-m2 quadrats. Th ese were oven dried at 60°C for 2 to 3 d, and the dry matter yield was determined aft er weighing. Grain yields for spring and winter wheat (at 12–13% moisture content) were determined from an area of 1389 m2 using a com-bine harvester each year. Th e biomass (stems + leaves) yield was determined aft er deducting the grain yield from the total yield. Aft er grain harvest, wheat residue containing stems and leaves was returned to the soil, except in 2004 when straw from ungrazed plots was removed. Because of the lack of access to a no-till drill and the presence of a large amount of crop residue, all cropped plots in the chemical and mechanical treatments were tilled with

a tandem disk in the late fall following grain harvest to reduce the eff ect of the residues on planting with a conventional-tillage planter. In the grazing treatment, cropped plots were tilled aft er sheep grazing in the late fall. As a result, plots with CSW, SW-F, and WW-F treatments were tilled one time aft er wheat harvest in the chemical and grazing treatments and from two (in CSW) to four times (in SW-F and WW-F) during the preplanting, post-harvest, and fallow periods in the mechanical treatment.

Soil Sampling and AnalysisIn September to October, 2004 to 2007, soil samples were

collected from the 0- to 60-cm depth with a hydraulic probe (5-cm i.d.) attached to a truck from fi ve places within the plot. Th ese were divided into 0 to 15, 15 to 30, and 30 to 60 cm and composited by depth. In 2008, samples were collected from 0 to 30 cm at fi ve places within the plot, separated into 0 to 5, 5 to 10, and 10 to 30 cm, and composited by depth. Th e samples were air dried, ground, and sieved to 2 mm for determining soil nutrients and chemical properties. In 2008, an additional undisturbed soil core (5-cm i.d.) was also collected from 0 to 5, 5 to 10, and 10 to 30 cm to determine the bulk density by dividing the mass of the soil oven dried at 105°C by the volume of the core. Because of the nonsignifi cant eff ects of treatments and interactions, bulk density values of 1.20, 1.34, and 1.61 Mg m−3 at 0 to 5, 5 to 10, and 10 to 30 cm, respectively, were used to convert concentra-tions (g kg−1) of nutrients to contents (kg ha−1).

Soil samples were analyzed for Olsen P, extractable K, Ca, Mg, Na, and SO4–S, CEC, EC, and pH at the Montana State University soil testing laboratory, Bozeman, and Agvise Laboratories, Northwood, ND. Olsen P was determined by ex-tracting the soil with a buff ered alkaline solution (NaHCO3–NaOH) and determining the P concentration in the solution with Mb blue color by using a colorimeter, as described in Kuo (1996). Extractable K, Ca, Mg, and Na were determined by atomic absorption and fl ame emission spectrometry aft er ex-tracting the soil with 1 mol L−1 NH4OAc solution (pH 7.0) (Wright and Stuczynski, 1996). Sulfate-S was determined by the methylene blue method (Tabatabai, 1996). Soil pH was de-termined with a pH meter in 1:2 soil/water solution. Th e CEC was determined by the method described by Sumner and Miller (1996) for arid region soils. Th e EC was determined with a con-ductance meter in a 1:1 soil/water paste (Rhoades, 1996).

Because of staffi ng and budget constraints, only soil samples to a depth of 15 cm from 2004 to 2006, to 60 cm in 2007, and to 30 cm in 2008 were analyzed for P, K, and pH. Similarly, samples to a depth of 15 cm in 2006, to 60 cm in 2007, and to 30 cm in 2008 were analyzed for Ca, Mg, Na, CEC, and EC. For SO4–S, samples to a depth of 30 cm in 2006 and 2008 and to 60 cm in 2007 were used for analysis.

Data AnalysisData on soil nutrients and chemical properties at each

depth and wheat grain and biomass yields were analyzed us-ing the MIXED procedure of SAS (Littell et al., 1996). Fallow

1792 SSSAJ: Volume 75: Number 5 • September–October 2011

management was considered as the main-plot variable and a fi xed eff ect, cropping sequence as the split-plot variable and another fi xed eff ect, and year as the repeated-measure variable. Random variables were replication and replication × fallow management. Values were averaged across cropping sequence phases and used for a cropping sequence in the analysis. For wheat grain and bio-mass yields, data were annualized by dividing the values by 1 in CSW and 2 in SW-F and WW-F because wheat was absent dur-ing the fallow phase in SW-F and WW-F. Means were separated by using the least square means test when treatments and interac-tions were signifi cant (Littell et al., 1996). Statistical signifi cance was evaluated at P ≤ 0.05, unless otherwise stated.

RESULTS AND DISCUSSIONAnnualized Wheat Grain and Biomass Yields

Annualized wheat grain and biomass yields varied signifi -cantly among cropping sequences and years, and biomass yield varied among fallow management practices (data not shown). Interactions were signifi cant for cropping sequence × year for grain and biomass yields, and for fallow management × year for biomass yield. Averaged across fallow management practices, both annualized grain and biomass yields were greater under CSW than SW-F and WW-F in all years except in 2007, when yields were greater under CSW and WW-F than SW-F (Table 3). Averaged across cropping sequences, grain yield did not diff er among fallow management practices in all years. In contrast, biomass yield was greater in the grazing than the mechanical treatment in 2004 and greater in the grazing and mechanical than the chemical treatment in 2008. In 2006, biomass yield was greater in the mechanical than the chemical or grazing treatments. Both grain and biomass yields were greater in 2004 than in other years, regardless of cropping se-

quence and fallow management practice. Averaged across fallow management practices and years, grain and biomass yields were in the order: CSW > WW-F > SW-F. Averaged across cropping sequences and years, biomass yield was in the order: mechanical treatment > grazing treatment = chemical treatment.

Th e greater annualized wheat grain and biomass yields under CSW than SW-F and WW-F in all years except in 2007 was due to continuous cropping. Th e absence of crops during fallow reduced the annualized yields under SW-F and WW-F. Th is is similar to the results of various researchers in dryland cropping systems in the northern Great Plains (Aase and Pikul, 1995; Lenssen et al., 2007; Sainju et al., 2009). In 2007, lower yields under CSW and SW-F than WW-F were probably due to the distribution of precipitation during the crop growing season. Although growing season precipi-tation for spring wheat (April–September) under CSW and SW-F in 2007 was comparable with other years, monthly precipitation in July was 3 mm in 2007 compared with 17 to 45 mm in other years (Table 1). Lower precipitation in July, an active wheat growth pe-riod, probably reduced spring wheat yields under CSW and SW-F in 2007. Growing season precipitation for winter wheat under WW-F (October of the current year to September of the follow-ing year) was much higher than for spring wheat under CSW and WW-F because of the longer growing period. In dryland crop-ping systems, growing season precipitation amount and distribu-tion can infl uence crop yields (Halvorson et al., 2000; Sainju et al., 2009). Greater wheat grain and biomass yields in 2004 than in other years could be a result of increased precipitation (Table 1) and higher soil P and K concentrations, as shown below.

Th e nonsignifi cant eff ect of fallow management on wheat grain yield suggests that sheep grazing did not alter grain yields compared with tillage and herbicide application methods of weed

Table 3. Effects of cropping sequence and fallow management practice on annualized wheat grain and biomass (stems + leaves) yields from 2004 to 2008.

YearCropping sequence† Fallow management‡

MeanCSW SW-F WW-F Chemical Mechanical Grazing

—————————————Mg ha−1————————————————

Annualized grain yield

2004 5.55 aA§ 2.90 aC 3.53 aB 3.92 aA 4.01 aA 4.05 aA 3.99 a

2005 2.68 bA 1.83 bB 1.15 eC 1.84 cA 1.92 bA 1.90 bA 1.89 b

2006 2.57 bA 1.45 cB 1.70 dB 1.89 cA 1.90 bA 1.92 bA 1.90 b

2007 1.86 cB 1.18 cC 2.95 bA 1.89 cA 2.03 bA 2.00 bA 2.00 b

2008 2.61 bA 1.56 bcC 2.22 cB 2.09 bA 2.17 bA 2.14 bA 2.13 b

Mean 3.05 A 1.78 C 2.31 B 2.32 A 2.42 A 2.40 A

Annualized biomass yield

2004 6.60 aA 3.10 aC 3.57 aB 3.61 aAB 3.41 aB 3.89 aA 4.42 a

2005 3.28 bA 1.65 bB 1.94 bcB 2.52 bA 2.17 bcA 2.19 bA 2.29 b

2006 2.96 cA 1.57 bcB 1.64 cB 1.79 bB 2.51 bA 1.87 bcB 2.06 bc

2007 2.18 dA 1.55 bcB 2.25 bA 1.78 bA 2.21 bcA 2.00 bA 2.00 c

2008 1.92 dA 1.17 cB 1.49 cAB 1.08 cB 1.91 cA 1.58 cA 1.53 d

Mean 2.58 A 1.49 C 1.83 B 1.79 B 2.20 A 1.91 B† CSW, continuous spring wheat; SW-F, spring wheat–fallow; and WW-F, winter wheat–fallow.‡ Fallow management practices: chemical, where weeds were controlled with herbicides; grazing, where weeds were controlled with sheep grazing; and mechanical, where weeds were controlled with tillage.§ Numbers followed by different lowercase letters within a column in a set are signifi cantly different at P = 0.05 by the least square means test. Numbers followed by different uppercase letters within a row in a set are signifi cantly different at P = 0.05 by the least square means test.

SSSAJ: Volume 75: Number 5 • September–October 2011 1793

control. Several researchers (Redmon et al., 1995; Landau et al., 2007; Snyder et al., 2007) have also reported that wheat grain yields were similar to or greater with animal grazing than without. In contrast, greater biomass yield with the mechanical than with the chemical and grazing treatments was probably a result of increased root growth or greater availability of soil P and K due to the higher frequency of tillage, as shown below. Th e result suggests that wheat grain and biomass may not always grow in the same proportion; rather, their growth may be altered by management practices. Variations in the amounts of wheat biomass residue returned to the soil among treatments and years and the removal of residue due to consumption by sheep during grazing were expected to infl uence soil nutrients and chemical properties, as discussed below.

Soil Phosphorus, Potassium, and pHSoil P and K concentrations and contents varied among

treatments and years, with signifi cant cropping sequence × fallow management interaction at 0 to 15 cm from 2004 to 2007 and fal-low management at 0 to 30 cm in 2008 (Tables 4, 5, and 6). Both concentrations and contents of P and K were normally lower in the grazing than in the chemical and mechanical treatments for all cropping sequences and depths, with signifi cantly lower values at 0 to 15 cm under SW-F from 2004 to 2005 and at 0 to 30 cm in 2008 (Tables 5 and 6). Th e cropping sequence did not alter P and K levels at any depth, although the levels declined from 2004 to 2007.

Th e lower P and K levels in the grazing than in the chemical and mechanical treatments could be a result of wheat residue re-

moval by sheep due to consumption during grazing. Th is was es-pecially noted under SW-F compared with the other cropping se-quences. Although parts of the P and K from the wheat residue were returned to the soil through sheep feces and urine in the grazing treatment (Follett and Wilkinson, 1995), the amounts were prob-ably not enough to balance nutrient levels in the soil. Most of the P and K in the wheat residue was probably used to increase sheep live weight. Ogejo et al. (2010) reported that, in a day, an average sheep of 29 kg body weight consumed 28 g P and 160 g K through feeds and returned 0.02 g P and 9.0 g K through urine and 1.9 g P and 4.1 g K through feces to the soil. Th is means that only 7 to 8% of P and K consumed by the sheep were returned to the soil through feces and urine. Several researchers (Lorimor et al., 2000; Smith and Frost, 2000; Barker et al., 2001) also found that sheep weighing 27 to 65 kg returned 0.9 to 4.9 g P d−1 and 2.6 to 15.1 g K d−1 through feces due to variations in diet, age, weight, and gender.

Th e fact that grazing especially reduced the P and K levels un-der SW-F compared with the other cropping sequences was prob-ably related to the amount of wheat residue returned to the soil. Th e wheat biomass yield was lower under SW-F than CSW or WW-F (Table 3). It could be possible that sheep grazing further reduced the amount of residue returned to the soil, thereby reducing the P and K levels under SW-F. In the chemical and mechanicals treatments, wheat residue aft er grain harvest was returned to the soil, except in 2004, which could have helped to maintain P and K levels through nutrient cycling in these treatments. Th ese fi ndings are in contrast to those reported by Hatfi eld et al. (2007b), who found that soil P

Table 4. Analysis of variance for soil nutrients and chemical properties at the 0- to 60-cm depth from 2004 to 2007.

Source P K Ca Mg Na SO4–S CEC† EC‡ pH

0–15 cm

Cropping sequence (CS) NS NS NS NS NS NS NS * NS

Fallow management (FM) NS NS NS NS * * NS * NS

CS × FM ** * NS NS NS NS NS NS *

Year (Y) *** *** *** *** *** ** *** *** ***

CS × Y NS NS NS NS NS NS NS NS NS

FM × Y NS NS NS NS NS NS NS NS NS

CS × FM × Y NS NS NS NS NS NS NS NS NS

15–30 cm

CS NS NS NS NS NS * NS NS *

FM NS NS NS NS * NS NS NS NS

CS × FM NS NS NS NS NS NS NS NS NS

Y ND§ ND ND ND ND ** ND ND ND

CS × Y ND ND ND ND ND NS ND ND ND

FM × Y ND ND ND ND ND NS ND ND ND

CS × FM × Y ND ND ND ND ND NS ND ND ND

30–60 cm

CS NS NS NS ** NS NS NS NS NS

FM NS NS * NS * NS * * NS

CS × FM NS NS NS NS NS NS NS NS NS* Signifi cant at P = 0.05; NS, not signifi cant.** Signifi cant at P = 0.01.*** Signifi cant at P = 0.001.† Cation exchange capacity.‡ Electrical conductivity.§ ND, not determined.

1794 SSSAJ: Volume 75: Number 5 • September–October 2011

and K concentrations were not signifi cantly diff erent among sheep-grazed, ungrazed, and tilled treatments under dryland cropping systems in western Montana. Th eir studies, however, were limited to 2 yr in contrast to the 5 yr of study in this experiment. Similarly, Li et al. (2008) reported that soil P and K concentrations were not signifi cantly diff erent between sheep-grazed and ungrazed regions in the desert steppe in Inner Mongolia. Continuous removal of crop residue by sheep grazing during a longer period in this experi-ment probably reduced P and K levels. Th e diff erences in soil and climatic conditions among locations infl uencing the amount of crop residue returned to the soil and sheep grazing intensity can also af-fect soil P and K levels. Quiroga et al. (2009) reported that 10 yr of cattle grazing did not alter the soil P concentration in grazed and ungrazed treatments in Argentina. In contrast, Niu et al. (2009) in Australia observed that soil P and K concentrations were greater in sheep-camping than in noncamping sites due to increased animal excreta. Cattle and sheep grazing in pasture can result in similar or increased soil P and K concentrations compared with ungrazed land (Mathews et al., 1994; Abaye et al., 1997).

Th e reductions in P and K concentrations from 2004 to 2007, regardless of treatment, (Table 5) suggest that these nutri-ents are being constantly removed from the soil. Grain harvest can remove substantial amounts of P and K from the soil (Schomberg et al., 2009). Because P and K fertilizers were not applied to the soil due to their high initial concentrations (Table 2), reduced P and K levels from 2004 to 2007 were probably related to increased P and

K removal in grain and the lack of fertilizer application to replace these nutrients in the soil. Although wheat grain yields remained similar among years, except in 2004 (Table 3), P and K fertilizers might need to be applied to crops to sustain wheat yields if soil P concentrations fall below the threshold value of 12 mg P kg−1 and K concentrations below 120 mg K kg−1 (Agvise Laboratories, 2009). Because wheat grain yield was not aff ected by fallow man-agement practices, less intensive sheep grazing that returns more crop residue to the soil may be used to maintain soil P and K levels and to reduce the rates of P and K fertilization.

Soil pH also varied with treatment and year, with signifi -cant cropping sequence × fallow management interaction at 0 to 15 cm and cropping sequence at 15 to 30 cm (Table 4). Th e soil pH at 0 to 15 cm was greater in the mechanical than in the chem-ical or grazing treatments under CSW and WW-F and at 15 to 30 cm was greater under CSW and WW-F than SW-F (Tables 5 and 6). Averaged across treatments, the pH at 0 to 15 cm varied among years (Table 5).

Th e greater soil pH at 0 to 15 cm in the mechanical than in the chemical or grazing treatments could be a result of mixing of the soil and wheat residue due to tillage. Th is was especially true under CSW and WW-F, where wheat grain and biomass yields were greater than under SW-F (Table 3). Tillage may have brought up subsurface soils containing higher concentrations of basic cations, such as Ca, Mg, and Na (Table 2), thereby resulting in greater pH at the surface in the mechanical treatment. Th is is

Table 5. Effects of cropping sequence and fallow management practice on soil P and K concentrations and pH at the 0- to 15-, 15- to 30-, and 30- to 60-cm depths from 2004 to 2007.

Cropping sequence†

Fallow management‡ Year

P concentration K concentration pH

0–15 cm 15–30 cm 30–60 cm 0–15 cm 15–30 cm 30–60 cm 0–15 cm 15–30 cm 30–60 cm

————————–mg kg−1—————————–

CSW chemical 64.8 67.7 44.7 371 263 250 6.55 6.33 6.80

mechanical 82.8 68.0 28.3 473 296 326 7.13 6.77 7.57

grazing 62.3 31.3 14.0 348 226 223 6.90 6.47 7.17

SW-F chemical 86.6 56.0 36.0 516 352 305 6.70 6.28 6.93

mechanical 76.3 47.3 24.7 443 259 233 6.77 6.25 7.03

grazing 60.1 32.2 14.2 350 219 214 6.74 6.22 7.03

WW-F chemical 80.0 64.3 36.2 487 299 328 6.48 6.33 7.05

mechanical 76.6 51.3 25.8 451 277 269 7.09 6.78 7.42

grazing 59.4 32.6 16.8 345 214 217 6.85 6.38 7.12

LSD (0.05) 24.3 NS§ NS 160 NS NS 0.22 NS NS

Means

CSW 70.0 a¶ 55.7 a 29.0 a 397 a 262 a 266 a 6.86 a 6.52 a 7.18 a

SW-F 72.3 a 45.2 a 24.9 a 436 a 277 a 251 a 6.74 a 6.25 b 7.00 a

WW-F 70.2 a 49.4 a 26.3 a 427 a 263 a 271 a 6.81 a 6.50 a 7.19 a

2004 87.2 a ND†† ND 529 a ND ND 6.80 bc ND ND

2005 75.5 b ND ND 502 a ND ND 6.96 a ND ND

2006 56.2 c ND ND 322 b ND ND 6.92 ab ND ND

2007 69.5 b 50.1 26.7 329 b 267 263 6.53 c 6.42 7.12† CSW, continuous spring wheat; SW-F, spring wheat–fallow; and WW-F, winter wheat–fallow.‡ Fallow management practices are chemical, where weeds were controlled with herbicides; grazing, where weeds were controlled by sheep grazing; and mechanical, where weeds were controlled with tillage.§ NS, not signifi cant.¶ Numbers followed by different letters within a column in a set are signifi cantly different at P = 0.05 by the least square means test.†† ND, not determined.

SSSAJ: Volume 75: Number 5 • September–October 2011 1795

again in contrast to that reported by several researchers (Cayley et al., 2002; Hatfi eld et al., 2007b; Li et al., 2008), who found that soil pH was not diff erent among sheep-grazed, ungrazed, and tilled treatments in Australia, Mongolia, and the United States. Sheep urine and feces have pH values ranging from 8.0 to 8.3 (Ogejo et al., 2010). Because of the small amount of urine and feces returned to the soil compared with the amount of feed consumed by the sheep (Ogejo et al., 2010), sheep grazing had little impact in raising the soil pH compared with the other fal-low management practices. A greater amount of basic cations through increased residue returned to the soil may have increased the pH more under CSW and WW-F than SW-F.

Soil Calcium, Magnesium, Sodium, Sulfur, Cation Exchange Capacity, and Electrical Conductivity

Soil Ca, Mg, Na, SO4–S, CEC, and EC varied signifi cantly among treatments and years at multiple soil depths (Tables 4 and 6). Soil Ca and CEC at 30 to 60 cm were greater in the mechanical than in the chemical treatment from 2006 to 2007 and at 0 to 5 cm were greater in the mechanical than in the chemical or grazing treat-ments under CSW and WW-F in 2008 (Tables 7 and 8). Averaged across fallow management practices, Ca and CEC at 10 to 30 cm were greater under CSW and WW-F than SW-F, but CEC at 5

to 10 cm was greater under WW-F than CSW in 2008 (Table 8). Averaged across cropping sequences, Mg at 5 to 10 cm and Na at 15 to 60 cm were greater in the grazing than the chemical or mechani-cal treatments, but SO4–S at 0 to 30 cm was greater in the mechani-cal than the grazing treatment in 2008 (Table 6). Magnesium at 30 to 60 cm and SO4–S at 15 to 30 cm were greater under CSW than WW-F from 2006 to 2007 (Table 7), but SO4–S at 0 to 30 cm was greater under SW-F or WW-F than CSW in 2008 (Table 6). Th e EC at 0 to 15 and 30 to 60 cm was greater under SW-F and WW-F than CSW and greater in the mechanical than the chemical or graz-ing treatments (Tables 6 and 7).

Th e greater Ca and Mg levels under CSW than SW-F or WW-F was probably due to the greater amount of crop residue re-turned to the soil (Table 3). Th e return of nutrient inputs through crop residue can increase soil nutrient levels (Schomberg et al., 2009). Th is resulted in increased CEC but decreased EC under CSW than SW-F or WW-F because CEC measures total cations (Ca, Mg, Na, and K) in the soil (Sumner and Miller, 1996) but EC measures soluble salts containing both cations and anions (Rhoades, 1996). Th e SO4–S content was variable among cropping sequences. While greater SO4–S content under CSW than WW-F from 2006 to 2007 could be a result of increased crop residue returned to the soil, greater SO4–S content under SW-F and WW-F than CSW in

Table 6. Effects of cropping sequence (CS) and fallow management (FM) practice on soil P, K, Mg, Na, and SO4–S contents, pH, and electrical conductivity (EC) at the 0- to 30-cm depth in 2008.

Chemical property

Soil depth

Cropping sequence† Fallow management‡ Analysis of variance

CSW SW-F WW-F Chemical Mechanical Grazing CS FM CS × FM

cm

P content, kg ha−1

0–5 32.4 a§ 36.2 a 32.3 a 34.5 a 35.7 a 30.8 a NS NS NS

5–10 23.8 a 25.9 a 27.9 a 30.4 a 29.3 a 17.8 b NS * NS

10–30 55.1 a 68.9 a 78.9 a 81.2 a 80.7 a 40.1 b NS * NS

K content, kg ha−1

0–5 242 a 261 a 253 a 263 a 271 a 222 b NS * NS

5–10 159 a 164 a 184 a 176 a 191 a 139 b NS * NS

10–30 724 a 697 a 807 a 792 a 859 a 577 b NS * NS

pH 0–5 6.68 a 6.65 a 6.78 a 6.45 a 6.94 a 6.72 a NS NS NS

5–10 6.56 a 6.43 a 6.48 a 6.31 a 6.64 a 6.51 a NS NS NS

10–30 7.27 a 7.16 a 7.27 a 7.06 a 7.34 a 7.31 a NS NS NS

EC, S m−1 0–5 0.033 a 0.038 a 0.037 a 0.035 a 0.037 a 0.035 a NS NS NS

5–10 0.021 b 0.024 ab 0.026 a 0.024 a 0.024 a 0.024 a * NS NS

10–30 0.027 a 0.026 a 0.026 a 0.025 a 0.026 a 0.27 a NS NS NS

Mg content, kg ha−1

0–5 291 a 294 a 285 a 278 a 288 a 304 a NS NS NS

5–10 378 a 387 a 397 a 362 b 382 ab 417 a NS * NS

10–30 2640 a 2561 a 2651 a 2619 a 2593 a 2640 a NS NS NS

Na content, kg ha−1

0–5 13.1 a 12.2 a 11.6 a 11.7 a 12.5 a 12.8 a NS NS NS

5–10 15.6 a 16.4 a 16.6 a 15.2 b 15.2 b 18.4 a NS * NS

10–30 85.1 a 87.5 a 83.7 a 84.8 ab 76.6 b 95.0 a NS * NS

SO4–S content, kg ha−1

0–5 7.5 b 9.5 a 8.9 ab 8.5 ab 10.0 a 7.4 b * * NS

5–10 7.7 b 8.9 ab 10.1 a 9.0 ab 10.6 a 7.1 b * * NS

10–30 34.0 ab 28.8 b 40.8 a 34.0 ab 40.8 a 28.8 b * * NS* Signifi cant at P = 0.05; NS, not signifi cant.† CSW, continuous spring wheat; SW–F, spring wheat-fallow; and WW–F, winter wheat-fallow.‡ Fallow management practices: chemical, where weeds were controlled with herbicides; grazing, where weeds were controlled with sheep grazing; and mechanical, where weeds were controlled with tillage.§ Numbers followed by different letters within a row in a set are signifi cantly different at P = 0.05 by the least square means test.

1796 SSSAJ: Volume 75: Number 5 • September–October 2011

Table 7. Effects of cropping sequence and fallow management practice on soil Ca, Mg, and Na concentrations, SO4–S content, cation exchange capacity (CEC), and electrical conductivity (EC) at the 0- to 60-cm depth from 2006 to 2007.

Chemicalproperty

Soil depth

Cropping sequence† Fallow management‡ Year

CSW SW-F WW-F Chemical Mechanical Grazing 2006 2007

cm

Ca content, mg kg−1 0–15 3632 a§ 3541 a 3531 a 3591 a 3701 a 3412 a 3353 b 3783 a

15–30 3759 a 3494 a 3763 a 3607 a 3852 a 3558 a ND¶ 3672

30–60 5006 a 4871 a 4972 a 4265 b 5646 a 4937 ab ND 4950

Mg content, mg kg−1 0–15 527 a 535 a 525 a 519 a 538 a 531 a 491 b 567 a

15–30 739 a 714 a 699 a 691 a 703 a 756 a ND 717

30–60 1105 a 888 b 949 b 951 a 1023 a 967 a ND 981

Na content, mg kg−1 0–15 18.4 a 18.0 a 17.2 a 18.3 a 16.8 a 18.5 a 16.1 b 19.7 a

15–30 24.0 a 24.1 a 22.2 a 21.3 b 21.8 b 27.1 a ND 23.4

30–60 32.1 a 28.6 a 27.6 a 27.4 b 28.2 ab 32.6 a ND 29.4

SO4–S content, kg ha−1

0–15 24.3 a 20.1 a 21.4 a 23.9 a 23.4 a 18.5 b 19.3 b 24.6 a

15–30 16.9 a 14.8 ab 14.1 b 16.2 a 16.0 a 13.6 a 12.7 b 17.8 a

30–60 28.9 a 31.6 a 31.4 a 30.1 a 32.6 a 29.1 a ND 30.6

CEC, cmolc kg−1 0–15 23.4 a 23.1 a 22.9 a 23.2 a 23.9 a 22.3 a 21.8 b 24.6 a

15–30 25.7 a 24.3 a 25.4 a 24.7 a 25.9 a 24.8 a ND 25.1

30–60 35.0 a 32.6 a 33.6 a 30.1 b 37.6 a 33.5 ab ND 33.7

EC, S m−1 0–15 0.027 b 0.031 a 0.031 a 0.030 ab 0.033 a 0.026 b 0.037 a 0.033 b

15–30 0.029 a 0.029 a 0.032 a 0.030 a 0.032 a 0.028 a ND 0.030

30–60 0.038 a 0.037 a 0.040 a 0.033 b 0.045 a 0.038 ab ND 0.038† CSW, continuous spring wheat; SW-F, spring wheat–fallow; and WW-F, winter wheat–fallow.‡ Fallow management practices: chemical, where weeds were controlled with herbicides; grazing, where weeds were controlled with sheep grazing; and mechanical, where weeds were controlled with tillage.§ Numbers followed by different letters within a row in a set are signifi cantly different at P = 0.05 by the least square means test.¶ ND, not determined.

Table 8. Effects of cropping sequence and fallow management practice on soil Ca content and cation exchange capacity (CEC) at the 0- to 5-, 5- to 10-, and 10- to 30-cm depths in 2008.

Cropping sequence†

Fallow management‡

Ca content CEC

0–5 cm 5–10 cm 10–30 cm 0–5 cm 5–10 cm 10–30 cm

———— Mg ha−1 ———– ———— cmolc kg−1 ———–

CSW chemical 2.05 2.09 12.3 22.0 20.6 26.5

mechanical 2.25 2.34 13.3 24.2 23.1 28.2

grazing 2.23 2.12 13.4 23.8 21.4 28.4

SW-F chemical 2.09 2.14 10.7 22.7 21.2 24.1

mechanical 2.06 2.31 11.7 22.9 22.7 25.8

grazing 2.08 2.25 12.9 22.2 22.7 26.8

WW-F chemical 1.96 2.22 12.0 21.5 22.1 26.3

mechanical 2.33 2.49 14.3 24.7 24.1 29.5

grazing 2.02 2.42 12.7 21.9 24.2 27.6

LSD (0.05) 0.27 NS§ NS 2.0 NS NS

Means

CSW 2.18 a¶ 2.18 a 13.0 a 23.3 a 21.7 b 27.7 a

SW-F 2.08 a 2.23 a 11.8 b 22.6 a 22.2 ab 25.6 b

WW-F 2.10 a 2.37 a 13.0 a 22.7 a 23.5 a 27.8 a† CSW, continuous spring wheat; SW–F, spring wheat-fallow; and WW–F, winter wheat-fallow.‡ Fallow management practices: chemical, where weeds were controlled with herbicides; grazing, where weeds were controlled with sheep grazing; and mechanical, where weeds were controlled with tillage.§ NS, not signifi cant.¶ Numbers followed by different letters within a column are signifi cantly different at P = 0.05 by the least square means test.

SSSAJ: Volume 75: Number 5 • September–October 2011 1797

2008 may be a result of increased mineralization of organic S dur-ing fallow in 2008 with adequate precipitation (Table 1). Sulfur can be mineralized from soil organic matter and leached into the lower profi le (Williams and Haynes, 1992), especially during fallow when soil temperature and water content increase, resulting in increased microbial activity (Halvorson et al., 2002).

Th e greater Ca, SO4–S, CEC, and EC levels in the mechanical than the chemical or grazing treatments were probably due to great-er wheat biomass residue returned to the soil, followed by its incor-poration into the soil due to tillage. Biomass residue returned to the soil aft er grain harvest was greater in the mechanical than the chem-ical or grazing treatments (Table 3). It could be possible that greater amount of nutrients returned to the soil through increased residue, followed by a greater turnover rate of plant nutrients to soil nutri-ents through higher tillage frequency, could have increased these levels in the mechanical treatment. Greater nutrient inputs from crop residues, followed by rapid mineralization of the residue due to tillage, can increase soil nutrient levels (Schomberg et al., 2009). Th is, however, did not occur with Mg and Na. Higher Mg and Na levels in the grazing than in the chemical or mechanical treatments might be a result of greater inputs in sheep feces and urine. It may be possible that sheep feces and urine contained comparatively higher concentrations of Mg and Na than other nutrients. Magnesium and Na concentrations were 0.76 and 80 mg kg−1, respectively, in sheep urine and 1.08 and 345 mg kg−1, respectively, in feces compared with Ca and S concentrations of 0.09 and 1.30 g kg−1, respectively, in urine and 2.58 and 0.91 g kg−1, respectively, in feces (Ogejo et al., 2010). Variations in the amounts of nutrients returned to the soil in sheep urine and feces and crop residues with sheep grazing com-pared with nutrients in ungrazed residues in the other fallow man-agement practices show that grazing redistributed nutrient levels and aff ected soil properties. Th ese results were in contrast to those obtained by Li et al. (2008), who reported that sheep grazing did not alter Ca, Mg, Na, or SO4–S concentrations or EC in a desert steppe in Inner Mongolia. Th e reasons for increased Ca, Mg, Na, SO4–S, and CEC levels from 2006 to 2007 were not known be-cause wheat grain and biomass yields and precipitation were similar in 2006 and 2007 (Tables 1 and 3).

CONCLUSIONSSheep grazing to control weeds during fallow periods in

wheat–fallow systems had a negative (−209 to −4%) eff ect on extractable soil P, K, and SO4–S levels, a moderate (−27 to 15%) eff ect on Ca, pH, CEC, and EC, and a positive (2–21%) eff ect on Mg and Na compared with herbicide application and tillage treatments. Such eff ects were probably a result of reduced wheat residue return to the soil due to consumption by sheep during grazing (for P, K, and SO4–S), mixing of residue due to tillage (for Ca, pH, CEC, and EC), and return of nutrients to the soil through sheep feces and urine (for Mg and Na). Reduced soil P and K levels with grazing were especially noted under the SW-F system where wheat biomass residues returned to the soil were less than under the other cropping sequences. Such changes in soil properties with grazing, however, did not aff ect wheat grain yield,

probably because the soil contained high levels of these nutrients. Annualized grain and biomass yields were greater, however, un-der CSW than SW-F and WW-F due to continuous cropping. Th e cropping sequence had a mixed eff ect on soil nutrients and chemical properties. Provided that adequate amounts of P and K are applied through fertilization if their levels in the soil fall below the threshold values of 12 and 120 mg kg−1, respectively (Agvise Laboratories, 2009), sheep grazing will have a minimal eff ect on wheat yields. Th e other option for maintaining soil nutrient lev-els and chemical properties and sustaining wheat yields would be less intensive sheep grazing that increases the amount of wheat residue and nutrients returned to the soil, which could reduce or eliminate the need for P and K fertilization.

REFERENCESAase, J.K., and J.L. Pikul, Jr. 1995. Crop and soil response to long-term tillage

practices in the northern Great Plains. Agron. J. 87:652–656. doi:10.2134/agronj1995.00021962008700040008x

Abaye, A.O., V.G. Allen, and J.P. Fontenot. 1997. Grazing sheep and cattle together or separately: Eff ects on soils and plants. Agron. J. 89:380–386. doi:10.2134/agronj1997.00021962008900030003x

Agvise Laboratories. 2009. Interpreting a soil test report. Agvise Laboratories, Northwood, ND.

Barker, J.C., J.P. Jublena, and F.R. Walls. 2001. Animal and poultry manure production and characterization. North Carolina State Univ. Coop. Ext., Raleigh.

Cayley, J.W.D., M.R. McCaskill, and G.A. Kearney. 2002. Changes in pH and organic carbon were minimal in a long-term fi eld study in the western district of Victoria. Aust. J. Soil Res. 53:115–126.

Fenster, C.R. 1997. Conservation tillage in the northern Great Plains. J. Soil Water Conserv. 32:37–42.

Follett, R.F., and S.R. Wilkinson. 1995. Nutrient management in forages. p. 55–82. In R.F Barnes et al. (ed.) Forages. Vol. 2. Iowa State Univ. Press, Ames.

Franzluebbers, A.J. 2007. Integrated crop–livestock systems in the southeastern USA. Agron. J. 99:361–372. doi:10.2134/agronj2006.0076

Franzluebbers, A.J., and J.A. Stuedemann. 2008. Early response of soil organic fractions to tillage and integrated crop–livestock production. Soil Sci. Soc. Am. J. 72:613–625. doi:10.2136/sssaj2007.0121

Goosey, H.B., P.G. Hatfi eld, A.W. Lenssen, S.L. Blodgett, and R.W. Kott. 2005. Th e potential role of sheep in dryland grain production systems. Agric. Ecosyst. Environ. 111:349–353. doi:10.1016/j.agee.2005.06.003

Halvorson, A.D., A.L. Black, J.M. Krupinsky, S.D. Merrill, B.J. Wienhold, and D.L. Tanaka. 2000. Spring wheat response to tillage and nitrogen fertilization in rotation with sunfl ower and winter wheat. Agron. J. 92:136–144.

Halvorson, A.D., B.J. Wienhold, and A.L. Black. 2002. Tillage, nitrogen, and cropping system eff ects on soil carbon sequestration. Soil Sci. Soc. Am. J. 66:906–912. doi:10.2136/sssaj2002.0906

Hatfi eld, P.G., S.L. Blodgett, T.M. Spezzano, H.B. Goosey, A.W. Lenssen, R.W. Kott, and C.B. Marlow. 2007a. Incorporating sheep into dryland grain production systems: I. Impact on overwintering larval populations of wheat stem sawfl y, Cephus cinctus Norton (Hymenoptera: Cephidae). Small Rumin. Res. 67:209–215. doi:10.1016/j.smallrumres.2005.10.002

Hatfi eld, P.G., H.B. Goosey, T.M. Spezzano, S.L. Blodgett, A.W. Lenssen, and R.W. Kott. 2007b. Incorporating sheep into dryland grain production systems: III. Impact on changes in soil bulk density and soil nutrient profi les. Small Rumin. Res. 67:222–232. doi:10.1016/j.smallrumres.2005.10.003

Hatfi eld, P.G., A.W. Lenssen, T.M. Spezzano, S.L. Blodgett, H.B. Goosey, R.W. Kott, and C.B. Marlow. 2007c. Incorporating sheep into dryland grain production systems: II. Impact on changes in biomass and weed density. Small Rumin. Res. 67:216–221. doi:10.1016/j.smallrumres.2005.10.004

Herrero, M., P.K. Th orton, A.M. Notenbaert, S. Wood, S. Msangi, H.A. Freeman, et al. 2010. Smart investments in sustainable food productions: Revisiting mixed crop–livestock systems. Science 327:822–825. doi:10.1126/science.1183725

Johnson, J.B., W.E. Zidack, S.M. Capalbo, J.M. Antle, and D.F. Webb. 1997. Pests, pesticide use, and pesticide costs on larger central and eastern Montana farms with annually-planted dryland crops. Spec. Rep. 23. Dep. of Agric. Econ., Montana State Univ., Bozeman.

1798 SSSAJ: Volume 75: Number 5 • September–October 2011

Jones, O.R., and T.W. Popham. 1997. Cropping and tillage systems for dryland grain production in the southern High Plains. Agron. J. 89:222–232. doi:10.2134/agronj1997.00021962008900020012x

Kuo, S. 1996. Phosphorus. p. 869–919. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical analysis. SSSA Book Ser. 5. SSSA, Madison, WI.

Landau, S., I. Schoembaum, D. Barkar, E.D. Unger, A. Genizi, and J. Kigel. 2007. Grazing, mulching, and removal of wheat straw in a no-till system in a semi-arid environment. Aust. J. Agric. Res. 58:907–912. doi:10.1071/AR06422

Lenssen, A.W., G.D. Johnson, and G.R. Carlson. 2007. Cropping sequence and tillage system infl uence annual crop production and water use in semiarid Montana. Field Crops Res. 100:32–43. doi:10.1016/j.fcr.2006.05.004

Li, C., X. Hao, M. Zhao, G. Han, and W.D. Willms. 2008. Infl uence of historic sheep grazing on vegetation and soil properties of a desert steppe in Inner Mongolia. Agric. Ecosyst. Environ. 128:109–116. doi:10.1016/j.agee.2008.05.008

Littell, R.C., G.A. Milliken, W.W. Stroup, and R.D. Wolfi nger. 1996. SAS system for mixed models. SAS Inst., Cary, NC.

Lorimor, J., W. Powers, and A. Sutton. 2000. Manure characteristics. Manure management system series. MWPS-18, Section 1. Midwest Plant Serv., Ames, IA.

Mathews, B.W., L.E. Sollenberger, V.D. Nair, and C.R. Staples. 1994. Impact of grazing management on soil nitrogen, phosphorus, potassium, and sulfur distribution. J. Environ. Qual. 23:1006–1013. doi:10.2134/jeq1994.00472425002300050022x

Maughan, M.W., J.P.C. Flores, I. Anghinoni, G. Bollero, F.G. Fernandez, and B.G. Tracy. 2009. Soil quality and corn yield under crop–livestock integration in Illinois. Agron. J. 101:1503–1510. doi:10.2134/agronj2009.0068

Niu, Y., G. Li, L. Li, K.Y. Chan, and A. Oates. 2009. Sheep camping infl uences soil properties and pasture production in an acidic soil of New South Wales, Australia. Can. J. Soil Sci. 89:235–244. doi:10.4141/CJSS08004

Ogejo, J.A., S. Wildeus, P. Knight, and R.B. Wilke. 2010. Estimating goat and sheep manure production and their nutrient contribution in the Chesapeake Bay watershed. Appl. Eng. Agric. 26:1061–1065.

Quiroga, A., R. Fernandez, and E. Noellemeyer. 2009. Grazing eff ect on soil properties in conventional and no-till systems. Soil Tillage Res. 105:164–170. doi:10.1016/j.still.2009.07.003

Redmon, L.A., G.W. Horn, E.G. Krenzer, Jr., and D.J. Bernardo. 1995. A review of livestock grazing and wheat grain yield: Boom or bust. Agron. J. 87:137–147. doi:10.2134/agronj1995.00021962008700020001x

Rhoades, J.D. 1996. Salinity: Electrical conductivity and total dissolved solids. p. 437–474. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical analysis. SSSA Book Ser. 5. SSSA, Madison, WI.

Sainju, U.M., A.W. Lenssen, T. Caesar-TonTh at, and R.G. Evans. 2009. Dryland crop yields and soil organic matter as infl uenced by long-term tillage and cropping sequence. Agron. J. 101:243–251. doi:10.2134/agronj2008.0080x

Schomberg, H.H., D.M. Endale, M.B. Jenkins, R.R. Sharpe, D.S. Fisher, M.L. Cabrera, and V. McCracken. 2009. Soil test nutrient changes induced by poultry litter under conventional tillage and no-tillage. Soil Sci. Soc. Am. J. 73:154–163. doi:10.2136/sssaj2007.0431

Smith, K.A., and J.P. Frost. 2000. Nitrogen excretion by farm livestock with respect to land spreading requirements and controlling nitrogen losses to ground and surface waters: 1. Cattle and sheep. Bioresour. Technol. 71:173–181. doi:10.1016/S0960-8524(99)00061-9

Snyder, E.E., H.B. Goosey, P.G. Hatfi eld, and A.W. Lenssen. 2007. Sheep grazing on wheat–summer fallow and the impact on soil nitrogen, moisture, and crop yield. Proc. Am. Soc. Anim. Sci. West. Sect. 58:221–224.

Sumner, M.E., and W.P. Miller. 1996. Cation exchange capacity and exchange coeffi cients. p. 1201–1229. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical analysis. SSSA Book Ser. 5. SSSA, Madison, WI.

Tabatabai, M.A. 1996. Sulfur. p. 921–960. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical analysis. SSSA Book Ser. 5. SSSA, Madison, WI.

Tracy, B.J., and Y. Zhang. 2008. Soil compaction, corn yield response, and soil nutrient pool dynamics within an integrated crop–livestock system in Illinois. Crop Sci. 48:1211–1218. doi:10.2135/cropsci2007.07.0390

Williams, P.H., and R.J. Haynes. 1992. Balance sheet of phosphorus, sulfur, and potassium in a long-term grazed pasture supplied with superphosphate. Fert. Res. 31:51–60. doi:10.1007/BF01064227

Wright, R.J., and T.M. Stuczynski. 1996. Atomic absorption and fl ame emission spectrometry. p. 65–90. In D.L. Sparks (ed.) Methods of soil analysis. Part 3. Chemical analysis. SSSA Book Ser. 5. SSSA, Madison, WI.