The nutritional value of pressed potato vine silage

1982) PARFITT, et al: POTATO VINE SILAGE 415

THE NUTRI TI ONAL VALUE OF PRESSED POTATO VINE SILAGE 1

Dan E. Parfitt, S.J. Peloquin and N.A. Jorgensen 2

Abstract

Potato (Solanum tuberosum L.) vines were studied to determine if they could be successfully ensiled and fed to ruminant animals. Russet Burbank potato vine and alfalfa (Medicago sativa L.) silages were fed to goats (Capra hircus) to measure in vivo digestibility and to determine if potato glycoalkaloids were aversive or toxic. No differences in the feeding patterns between control and experimental animals were observed. Alfalfa silage was more digestible than the Russet Burbank potato vine silage due to a higher content of digestible cell wall materials. Digestibility differences between potato clones and harvest dates were found when in vivo digestibility tests were performed. W729R was the most digestible clone studied, especially for the early harvest dates. Most of the clones had vines that were more digestible, in vitro, than alfalfa. Vine glycoalkaloid contents of the experimental clones were similar to the values reported in the literature. There was no apparent correlation between vine glycoalkaloid content and percent digestible dry matter. A positive correlation of 0.38 between digestible dry matter and dry vine yield was found. Potato vine silage, produced after pressing the vines to remove water, was an acceptable feed for ruminant animals. Additional in vivo testing is necessary to determine if toxic effects due to glycoalkaloids or pesticides from long term feeding will be a problem.

Resumen

Se estudi6 el rastrojo de la papa (Solanum tuberosum L.) para determinar si es-factible ensilarlo con 6xito y utilizarlo para alimentar rumiantes. A grupos de cabras (Capra hircus) se les di6 ensilaje de alfalfa (Medicago sativa L.) y de rastrojo de papa Russet Burbank para medir digestibilidad en vivo y determinar si los glicoalcaloides de la papa eran t6xicos o causaban aversi6n. No se encontraron diferencias en los h~bitos de alimentaci6n entre los testigos y los tratamientos. El ensilaje de alfalfa fue mhs digestible que el de rastrojo de papa, debido a un contenido mayor de pared celular digestible.

~Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, grants from the National Science Foundation (PCM-77-24330) and the International Potato Center, and a gift from Frito-Lay, Inc. ZFormer graduate student and Professor, Departments of Horticulture and Genetics, and Pro- fessor, Department of Dairy Science, University of Wisconsin, Madison, WI 93706. Present address of D.E.P.: Pomology Department, University of California, Davis, CA 95616. Received for publication March 4, 1982. KEY WORDS: Feeding value, haulm, potato, nutrition, ruminant animals, silage, Solanum tuberosum L., vine.

416 AMERICAN POTATO JOURNAL (Vol. 59

Cuando se hicieron pruebas de digestibilidad en vivo, se encontraron diferencias en digestibilidad entre clones de papa y edades de corte. E1 W729R fue el mhs digestible entre los clones estudiados, especialmente cuando el corte se hizo a edad temprana. La mayor ia de los clones tenia rastrojo que era m~ts digestible, in vitro, que la alfalfa. Los contenidos de glicoalcaloides del rastrojo fueron similares a aqu611os referidos en la literatura. No hubo correlaci6n clara entre los glicoalcaioides del rastrojo y el porcentaje de materia seca digestible. Se encontr6 una correlaci6n positiva de 0.38 entre materia seca digestible y producci6n de rastrojo seco. El ensilaje de rastrojo de papa, producido despu6s de comprimir el rastrojo para extraerle el agua, fue un alimento aceptable para rumiantes. Se reguieren pruebas adicionales en vivo para determinar si se presentaran problemas debido al afecto de los glicoalcaloides o los pesticidas en periodos largos de alimentaci6n con dicho ensilaje.

Introduction

Potatoes (Solanum tuberosum L.) produce large quantities of vines that are not presently utilized (7). One possible use for these vines is to ensile them and feed the silage to livestock. Unfortunately, pota to vines are usually too high in moisture to make good silage. With the development of alfalfa pressing machinery, the possibility now exists for removing excess water f rom the vines and successfully ensiling them (8). The nutritional value of forages is a function of three parameters: digestibility, voluntary intake, and palatability (10). A forage should not contain toxic materials and should have the necessary constituents for animal growth.

Several methods have been developed to measure the amount of digestible material in forages. A common in vivo measure of digestibility is the total digestible nutrients (TDN) procedure. TDN is determined by compar- ing the amounts of plant material in the feed and feces. Several in vitro procedures have been developed that are more convenient than in vivo methods. In vitro procedures permit rapid measurement of a large number of samples. The Tilley-Terry in vitro digestibility procedure is frequently used. Results from in vivo and in vitro procedures are highly correlated, with r = 0.88 to 0.98 (1).

Several chemical assays are available for determining the nutritional value of forages. Three assays that are often used are the cell wall constituents (CWC), acid detergent fiber (ADF), and acid detergent lignin (ADL) procedures. CWC is a measure of total fiber (does not contain soluble cel- lular materials); ADF is a measure of lignin, cellulose, and ash fractions; and ADL is a measure of lignin and ash (11). Estimates for cellulose and hemicellulose content are derived f rom these assays by subtraction. Soluble nutrients are digestible; the cell wall components are partially digested by ruminants. Celluloses are digested more slowly than hemicelluloses or soluble sugars. Lignin is indigestible, and may be related to in vivo digestibility (1).


Voluntary intake is an important forage parameter that can affect the weight gain of ruminant animals. Expressed as a percentage of digestible intake for alfalfa, voluntary intake was responsible for 47°7o and digestibility accounted for 26°7o (11). Voluntary intake is positively correlated with digestibility. The rate at which a forage is digested and passes through a ruminant animal 's digestive system may affect the voluntary intake of the forage. Voluntary intake levels of a given forage can vary among animal species and between animals within a species (3, 5). A positive correlation between palatability and voluntary intake was found in a number of studies (5). Palatabil- ity may not be an important concept if only one type of feed is available to the animal. That would usually be the situation when animals are fed silage in feedlots.

Materials and Methods

Four male castrated goats (Capra hircus) were used in a total collection digestion trial with a 14 day adjustment period followed by a seven day collection period to evaluate pressed and ensiled Russet Burbank potato vines, pressed alfalfa (Medicago sativa L.) silage, and conventional alfalfa silage control. Two of the goats were fed ensiled Russet Burbank potato vines (harvested 112 days after planting in 1976), for three weeks. The potato vines were pressed with a screw press to reduce the moisture content of the vines from 90°70 to 50°7o before ensiling. This experiment permitted evaluation of digestibility, cell wall constituents, voluntary intake, and acute toxicity of the vines.

Labora tory tests for the nutritional value of potato vine cultivars at three harvest dates in 1976 (105, 120, and 135 days f rom planting) were car- ried out in 1977 and 1978. CWC, ADF, and ADL procedures were performed to determine the amounts of soluble nutrients, hemicellulose, cellulose, and lignin. Vines f rom the 1976 yield trial were assayed for total protein content, in vitro digestible dry matter, and glycoalkaloid concentration. Two hundred grams to 500 g vine samples f rom each clone and harvest date were dried at 80°C for 48 hours, and were ground to pass through a 0.425 m m (40 mesh) screen. Total protein content was determined by a semi-micro Kjeldahl procedure.

In vitro digestible dry matter assays were performed via the Tilley-Terry in vitro rumen fermentation procedure. One forth gram samples of dried vine were placed in 50 ml centrifuge tubes with 20 ml of a buffer solution and five mililiters of rumen fluid f rom a cow. Stoppers were placed in the tubes after the tubes were flushed with CO2 (the reaction is anaerobic). The tubes were incubated for 48 hours at 39°C and the mixtures were filtered. The solid residues were dried and weighed. The amounts of digestible dry matter (DDM) were calculated by subtracting the residue weights from the initial sample weights. Controls and dehydrated alfalfa standards were used


to adjust the results to a common base. Four harvest dates (90, 105, 120, and 135 days from planting) were analyzed.

Glycoalkaloid assays were performed on vine samples from the 135 day harvest of 1976. The extraction procedure was a modification of a bisolvent extraction method (12). One half gram vine samples were extracted with 15 ml of hot 2:1 methanol-chloroform solution for three hours. Ten milliliters of 0.006 N HC1 was added to each tube. The samples were heated to 70°C to flocculate the glycoalkaloids. The flocculated materials were washed twice with 25 ml of dilute ammonium hydroxide and dried. The precipitates were dissolved in 15 ml of 100% methanol. Ten milliliter aliquots of the methanol solutions were assayed for total glycoalkaloid concentration. A nonaqueous acid-base titration procedure (2) was modified slightly and used to determine TGA concentrations in the aliquots.

Factorial variance analyses were used to analyze the protein and digestibility data from the 1976 potato vine samples. A randomized complete block variance analysis was performed on the glycoalkaloid data. Covariance analyses were performed to permit the calculation of treatment correlations for dry weight vine yield vs. digestibility, and glycoalkaloid concentration vs. digestibility.

Results

The results of the feeding and digestion study are presented in Tables 1 and 2. Results are presented for normally ensiled alfalfa and pressed alfalfa silage, as well as pressed potato vine silage. The goats consumed quantities

TABLE 1. - - In vivo digestibility o f ensiled alfalfa and Russet Burbank potato vines.

We of dry wt. Pressed potato vine Pressed alfalfa Regular alfalfa

Digestible dry matter 53.1 60.4 60.9 Digestible protein 60.3 64.3 69.6 Digestible cell wall 53.4 61.6 56.8

TABLE 2. - - Chemical analysis o f alfalfa and Russet Burbank potato vine silages.

% of dry wt. Pressed potato vine Pressed alfalfa Regular alfalfa

Crude protein 11.4 15.2 18.5 CWC 60.6 62.7 52.5 ADF 54.9 49.0 40.7 HemiceUulose 5.7 13.6 11.8 Cellulose 37.3 40.0 33.1 Lignin + ash (ADL) 10.6 9.0 7.6

1982) PARFITT, et a# POTATO VINE SILAGE 419

o f p o t a t o vine silage tha t were c o m p a r a b l e tO the a l f a l f a silage. The goats exh ib i t ed no toxic reac t ions f rom consuming the p o t a t o vines.

The p o t a t o vine silage was s o m e w h a t lower in d iges t ib le d ry mat te r (in vivo), p ro te in , and cell wal ls t han the a l fa l fa silage (Table 1). The p o t a t o vine si lage con ta ined less p ro t e in and hemicel lu lose than the pressed a l fa l fa si lage, whi le the pressed a l f a l f a silage had less p ro t e in and m o r e cell wal l c o m p o n e n t s than the n o r m a l a l f a l f a silage (Table 2). The quant i t ies o f l ignin were s o m e w h a t higher in the pressed p o t a t o vines t han in the pressed a l fa l fa . Lignin content o f n o r m a l l y ensi led a l f a l f a was lower than in the silages tha t were pressed. L imi t ed obse rva t ion numbers p rec luded stat is t ical ana lys is o f the d a t a in Tab le I a n d 2.

The re were s igni f icant in v i t ro digest ible d ry m a t t e r d i f fe rences for vines be tween clones , da tes , and specif ic c lone × da te c o m b i n a t i o n s (Tables 3 and 4). In v i t ro digest ibi l i ty values were genera l ly very high for the p o t a t o vines.

TABLE 3. - - F values for protein content, % digestible dry matter, and vine glycoalkaloid content o f potato vines harvested in 1976.

Source Protein Digestible Glycoalkaloid content dry matter content

Block 2.22 0.41 0.82 Clone 1.27 4.92"* 3.12" Date 32.49** 11.98** Clone x date 0.60 1.94*

*, ** -Significant at the 5% and 1% level, respectively.

TABLE 4. - - M e a n in vitro % digestible dry matter for potato vines harvested in 1976.

Days after planting Means for

Clone 90 105 120 135 clones Kennebec 73.8 76.2 71.8 71.4 73.3 Russet Burbank 73.7 72.1 73.8 67.1 71.7 Superior 74.4 74.5 71.3 71.4 72.9 W729R 80.6 75.7 73.4 73.2 75.7 PSDT 5 77.1 73.0 71.6 73.1 73.7 PSDT 17 77.2 72.4 69.3 67.6 71.6 PSDT 24 72.3 73.6 70.2 74.8 72.7 PSDT 41 71.0 69.8 72.4 65.2 69.6

Means for dates 75.0 73.4 71.7 70.5 Alfalfa standard = 69.2

5% LSD for clones = 2.3 5% LSD for dates = 1.6 5% LSD for clonex date:4.6


Only three clone × date combinations had less digestible dry matter than the alfalfa standard. The potato vines contained the most digestible dry matter early in the season. W729R, PSDT 5, and Kennebec were the most digestible clones, while PSDT 41, Russet Burbank, and PSDT 17 were the least digestible clones.

Differences between the protein content of potato vines harvested on different dates were observed (Table 3). The vine protein content of all clones declined later in the growing season. Mean vine protein contents of 17.0°70, 15.0070, and 11.4070 were found at 105,120, and 135 days from planting, respectively. The 5070 LSD for these values was 1.4%.

Pressed potato vines were tested for protein content to determine whether pressing the juice f rom the vines lowered the protein content o f the ensilable forage. Chopped vines (112 day harvest) had a protein content o f 18.0070 _+ 0.1%0 and pressed vine material had a protein content of 11.0070 + 0 . 1 % . These results are in agreement with the analysis f rom the feeding study. Protein values for Russet Burbank pressed vines were almost identical (11.0070 vs. 11.4%) for both analyses. Protein levels for alfalfa silage de- creased f rom 18.50/0 to 15.2% with pressing. Thus, significant amounts of protein were lost f rom the forage during the water removal procedure.

The F values and means for vine glycoalkaloid content (TGA) from clones at 135 days f rom planting are presented in Tables 3 and 5. The last harvest date was selected for testing because that was the harvest date that

TABLE 5. - - Mean total glycoatkaloM (TGA) concentrations for potato vines

harvested 135 days from planting in 1976.

Clone T G A mg/100 g dry vine

Kennebec 105 Russet Burbank 246 Superior 163 W729R 207 PSDT 5 88 PSDT 17 100 PSDT 24 286 PSDT 41 295

5% LSD = 145

was considered to be the most likely date for economical pota to vine harvesting. Considerable variation in T G A content was present among clones. PSDT 5, PSDT 17, and Kennebec had the lowest T G A concentrations. PSDT 41, PSDT 24, and Russet Burbank had the highest TGA concentrations.

A significant positive correlation was observed between in vitro digestible dry matter and vine yields (dry weight). The unbiased correlation coef-


ficient was 0.38_+ (0.20 to 0.53) for P=0 .05 . The correlation between in vitro digestibility and TGA content was nonsignificant. The unbiased correlation coefficient was - 0 . 3 8 + ( - 0 . 6 7 to 0.00) for P = 0.05.

Discussion

The feeding study demonstrated that potato vine silage was an acceptable feed for ruminant animals. The study was not a test of palatability, since the animals were not offered a choice of feeds. The test indicated that voluntary intake levels for pota to vine silage were similar to voluntary intake levels for alfalfa silage. Pota to vines are quite digestible. Although the in vivo and in vitro assays gave different results for pota to vine vs. alfalfa digestibility, the high digestibility of potato vines was adequately demonstrated. Potato vine digestibility declined late in the growing season, but was high enough to provide good nutritive value at the later harvest dates. Potato glycoalkaloids did not appear to be detrimental to the animals. Similar re- suits were reported by Nicholson, et al. (6) for sheep. The glycoalkaloids were apparently degraded during the ensiling process or in the animals ' rumens. Acidic systems are known to cleave the glycosides f rom the aglycones. Without attached sugars, the alkaloids are much less soluble and may be less biologically active. Rumen bacteria may also be capable of de- grading the aglycones. Nicholson's study provided evidence that ensiling also reduced the amount of glycoalkaloid in the vines.-Glycoalkaloid concentrations were reduced by 30% to 50°-/o in that study.

A significant amount of glycoalkaloid was present in the potato vines. Nicholson, et al. (6) found T G A levels in Kennebec and Russet Burbank vines (184 mg and 224 mg/100 g of dry vine) that were similar to the values reported in the present study. Street, et al. (9) found T G A levels of 13 mg / 100 g of fresh leaves (equal to 130 mg/100 g of dry leaves) for King Edward potatoes in late August. Wolf and Duggar (13) observed T G A levels of about 200 mg/100 g of dry leaves and stems for Russet Burbank. The results f rom these potato vine glycoalkaloid studies should be regarded as approxi- mations of the true TGA levels in potato vines. These T G A values and values presented in Table 5 may have systematic errors due to incomplete recovery of the glycoalkaloids. Therefore, T G A comparisons between specific clones should be made with caution. T G A levels declined during the growing season in all of the cited studies. Harvesting potato vines later in the season could minimize the amount of glycoalkaloid in the vines. Economic and yield con- siderations also favor late harvest dates. Glycoalkaloids do not appear to be a problem when potato vines are ensiled and used as feed for ruminant animals.

Significant toxic, carcinogenic, or tetratagenic effects, f rom fungicides or pesticides that are applied to the crop, could be a problem when potato vines are fed. Research pertaining to the effects of agricultural chemicals on ruminant animals has not been reported for potatoes.


P r o t e i n concen t ra t ion in p o t a t o vines decl ined to marg ina l l y accep tab le levels by 135 days f rom plan t ing . P ro t e in concen t ra t ion grea ter t han ten percent is cons ide red to be a d e q u a t e for r u m i n a n t forages (4). Vine p ro te in conten t was reduced by press ing a l f a l f a or p o t a t o vines. There fo re , a n i t rogen supp lemen t may be requ i red with pressed p o t a t o vines to suppo r t high levels o f an ima l pe r fo rmance .

P o t a t o vines should be a g o o d fo rage , especial ly i f supp l e me n ta l n i t rogen is a d d e d to the silage. A l f a l f a m a y be more nu t r i t ious than p o t a t o vines, but does n o t p rov ide a high va lue vegetable c rop (po ta toes ) at the same t ime. Breed ing for nu t r i t iona l qua l i ty could be l imi ted to test ing the best clones f rom yield tr ials for in v i t ro digest ibi l i ty . Clones wi th d iges t ib le vines will p r o b a b l y be f o u n d if this p r o c e d u r e is fo l lowed, since vine yield (dry wt.) and diges t ib i l i ty are pos i t ive ly cor re la ted . Breeding for p ro t e in quan t i t y is not r e c o m m e n d e d , as no genet ic va r iab i l i ty was observed . G l y c o a l k a l o i d screening is no t feasible due to the l imi ted re l iabi l i ty o f the r a p i d assay procedures tha t a re present ly ava i lab le and the ques t ionab le s ignif icance o f g lycoa lka lo ids as qual i ty fac tors in silage. The f inal test for any p o t a t o clone tha t is se lected for release to growers should be a large scale feeding tr ial to es t imate the forage pa rame te r s o f d igest ibi l i ty , vo lun t a ry in take , pa la t ab i l - ity, and long te rm toxic effects under commerc ia l cond i t ions .

Acknowledgments

I wish to t hank Dr. R icha rd Kogel for pressing the p o t a t o vines and Pau l Fr i t sche l for helping set up the d iges t ib i l i ty assays.

Literature Cited

1. Barnes, R.F. 1973. Laboratory methods of evaluating feeding value of herbage, p. 179-214. In G.W. Butler and R.W. Bailey (ed.), Chemistry and Biochemistry of Herbage, Vol. 3. Academic Press, New York.

2. Fitzpatrick, T.J., and S.F. Osman. 1974. A comprehensive method for the determination of total potato glycoalkaloids. Am Potato J 51:318-323.

3. Heaney, D.P. 1970. Voluntary intake as a component of an index to forage quality. Proc. Nat. Conf. Forage Qual. Eval. Util., Sept. 3 & 4, 1969, Lincoln, Neb. p. CI-10.

4. Hughes, H.D., M.E. Heath, and D.S. Metcalfe. 1962. Forages. Iowa State U. Press, Ames, Iowa. 707 p.

5. Marten, G.C. 1970. Measurement and significance of forage palatibility. Proc. Nat. Conf. Forage Qual. Eval. Util., Sept. 3 & 4, 1969, Lincoln, Neb. p. D1-D55.

6. Nicholson, J.W.G., D.A. Young, R.E. McQueen, H. De Jong, and F.A. Wood. 1978. The feeding value potential of potato vines. Can J Anim Sci 58:559-570.

7. Parfitt, D.E., and S.J. Peloquin. 1977. Variation of vine and tuber yield as a function of harvest date and cultivar. Am Potato J 54:411-417.

8. Russell, J.R., J.P. Hurst, N.A. Jorgensen, and G.P. Barrington. 1978. Wet plant fractiona- tion: utilization of pressed alfalfa silage. J Anita Sci 46:278-287.

9. Street, H.E., A.E. Kenyon, and G.M. Watson. 1946. The nature and distribution of various forms of nitrogen in the potato. Ann Appl Biol 33:1-12.


10. Waldo, D.R. 1970. Factors influencing voluntary intake of forages. Proc. Nat. Conf. For- age Qual. Eval. Util., Sept. 3 & 4, 1969, Lincoln, Neb. p. El-E22.

11. Waldo, D.R. and N.A. Jorgensen. 1980. Forages for high animal production: nutritional factors and effects of conservation. (In press).

12. Wang, S.L., C.L. Bedford, and N.R. Thompson. 1972. Determination of glycoalkaloids in potatoes (S. tuberosum) with'a bisolvent extraction method. Am Potato J 49:302-308.

13. Wolf, M.J., and B.M. Dugger. 1946. Estimation and physiological role of solanine in the potato. J Agric Res 13:1-32.

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The nutritional value of pressed potato vine silage