J. Agronomy & Crop Science 160, 6-63 (1988) 0 1988 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0931-2250

Agronomy Department, Massey University, Palmerston North, N e w Zealand

Influence of Seasonal Productivity and Defoliation Frequency on the Competitive Relationships between Herbage Grasses

U. R. SANGAKKARA and E. ROBERTS

Authors’ addresses: Dr. U. R. SANGAKKARA, Faculty of Agriculture, University of Peradeniya, Sri Lanka, and Mr. E. ROBERTS, Ministry of Agriculture & Fisheries, Palmerston North, New Zealand.

With 2 tables

Received February 3 , 1987; accepted February 26, 1987

Abstract

Research has identified changes in competitive abilities and relationships between herbage grasses due to seasonal variations and defoliation pressures. Thus, a study was carried out to evaluate the effects of these variables on competitive relationships between three popular herbage grasses of New Zealand, using the replacement series principle. The competitive relationships of the established swards were monitored for an year, under two defoliation regimes. The results illustrate that defoliation intensity has a greater influence on the competitive relationships between the selected grasses, than seasonal variability. While this study suggests an increase in mixture productivity over monocultures in established swards, the need for longer term studies to consolidate such results is stressed.

Key words: herbage grasses, seasonal productivity, defoliation, competitive relationships.

Introduction

Competitive relationships between compo- nents of grass mixture have been well identified (RHODES 1970), and most studies indicate com- petition for similar environmental resources (TRENBATH 1971). This is not considered unusual, as most of the experiments collected are of insufficient duration to allow for sea- sonal shifts of dominance or are conducted in comparatively uniform environments which do not provide situations where species “crowd for dissimilar space” (HARRIS and HOG- LAND 1977). However, research has identified shifts in competitive abilities and relationships between species due to differential defoliation pressures and seasonal variations in growth habits of species in mixtures (VAN DEN BERGH 1968, HARRIS 1978, HARRIS et al. 1981).

“Grassland Nui” perennial ryegrass (Lolium perenne L.); “Grasslands Matua” prairie grass

(Byomus catharticus Vahl) and “Grasslands Apanui” cocksfoot (Dactylis glomerutu L.) are three popular pasture grasses fo New Zealand. Competitive abilities of these three species during establishment and early growth were in the order of prairie grass > rye grass > cock- sfoot under a uniform cutting regime (SANGAK- KARA and ROBERTS 1985). Thus, the present study was conducted to investigate the effects of different cutting regimes and seasonal varia- tions on the competitive relationships between these species at later stages of growth.

Methods

The experiment from which the data was obtained was arranged as three replicates in a randomized block design, at Massey University, Palmerston North. Two species mixtures were made up accord- ing ot the replacement series of D E WIT (1960), with the selected grasses in the ratios of 1 : 0; .75; .25;

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Influence of Seasonal Productivity and Defoliation Frequency 61

.50; .50; .25; .75 and 0 : 1. Thus, nine mixtures and three monocultures were obtained, and these were established in October, 1979.

This experiment was begun by cutting the swards to a height of 25 cm in September, 1980, and top dressing with a 12 : 10 : 10 NPK mixture at a rate of 200 kg/ha. Thereafter, one half of the plots were cut to the above height at monthly intervals (represent- ing a frequent defoliation regime), while the other half was cut when the average light interception measured by a photometer was 95 % acres all swards (representing an infrequent cutting regime). At each harvest, the productivity of monocultures and mix- tures measured. The plots were fertilized on the basis of the amount of herbage removed at each harvest.

The data obtained from November 1980, until November 1981, was used to analyse competition effects. Thus, data from 13 harvests were available from the frequently cut plots while six harvests were obtained from the infrequently cut sections. The separate data of the mixture components were used to fit parameters to the two species competition model of DE WIT (1960), by the use of the approach described by THOMAS (1970).

Results and Discussion

The competitive ability of each species in a given series of binary mixtures was evaluated by the Relative Crowding Coefficients (RCC), and the greater values indicate the more com- petitive species. The products of the RCC’s represent the nature of competition between the species (DE WIT 1960) where a value equal to unity represents competition for similar

environmental resources. These RCC’s and their products obtained from dry matter yields, at each harvest under the two cutting regimes are presented in Tables 1 und 2 respec- tively.

The adoption of a frequent defoliation regime induces great variations in the RCC’s of the selected grasses with seasonal changes (Table 1). With the onset of summer in December, the competitive ability of ryegrass exceeds that of prairie grass, while that of cocksfoot equals prairie grass. In the ryegrass cocksfoot mixtures, the competitive ability of cocksfoot exceeds that of ryegrass over this period. In contrast, during winter (e.g. July, August and September period), prairie grass becomes more competitive than the other species, and ryegrass indicates greater R C C than cocksfoot. These observed changes could be attributed partly to the seasonal growth habits of the selected species as ‘Nui’ ryegrass is considered a good spring and autumn pro- ducer (ARMSTRONG 1977) while ‘Matua’ prairie grass has been recognized as a superior grow- ing species (RUMBALL 1974). In contrast ‘Apanui’ cocksfoot is identified as a good sum- mer grower (LANGER 1973). Thus, with the onset of a suitable environment, the species that respond to such conditions thrive at the expense of the other species in mixture, as indicated by ENGLAND (1968) and HARRIS et al. (1981).

Table 1. The Relative Crowding Coefficients and their products of the grasses at each harvest under the frequent cutting regime

Harvest R-P mixture P-C mixture C-R mixture period K, K, K, x K, K, K, K, x K, K, K, K, xK,

November 1980 .89 December 1.01 January 1981 1.05 February 1.15 March 1.11 April 1.12 May .99

.14 1.014 1.30 .80 1.042

.94 ,996 1.24 .84 1.041

.96 1.008 1.11 .94 1.043

.95 1.092 1.98 1.03 1.009

.96 1.065 1.04 .99 1.029

.94 1.053 1.09 .96 1.046

.02 1.009 1.14 .90 1.026 June .88 1.15 1.012 1.18 .86 1.014

July .85 1.20 1.011 1.24 .81 1.004 August .89 1.17 1.041 1.28 .78 .998 September .95 1.05 ,997 1.21 .80 1.024 October .98 1.04 1.019 1.19 .85 1.011 November 1.03 .96 ,998 1.17 .88 1.038

.87 1.16 1.009 3 9 1.14 1.014 .95 1.06 1.007 .03 .96 1.009 .06 .95 1.007 .94 1.10 1.034 .88 1.16 1.038 3 2 1.24 .992 .81 1.25 1.012 .76 1.32 1.003 .82 1.24 1.016 .86 1.20 1.032 .91 1.10 1.001

62 SANGAKKARA and ROBERTS

Table 2. The Relative Crowding Coefficients and their products of the grasses at each harvest under an infrequent cutting regime

Harvest R-P mixture P-C mixture C-R mixture period K, K, K, x K, K, K, K, x K, K, K, K, X K ,

November 1980 .86 1.16 ,997 1.35 .74 .999 .88 1.14 1.003 January .98 1.02 ,996 1.31 .76 ,995 .93 1.08 1.004 April .79 1.29 1.019 1.24 .81 1.004 .75 1.35 ,997 June .81 1.23 ,996 1.38 .73 1.007 .68 1.47 .996 September .84 1.20 1.008 1.19 .84 ,999 .78 1.29 1.006 November .85 1.18 1.003 1.20 .84 1.008 .85 1.18 1.003

K,, K, and K, are the Relative Crowding Coefficients of Ryegrass (R), Prairie grass (P) and Cocksfoot (C) respectively.

The adoption of an infrequent cutting regime however, does not significantly alter the overall competitive abilities of the grasses with seasonal changes (Table 2). Although there is a shift in competitive abilities, with the onset of favourable growth conditions, the competitive abilities of these species observed during the establishment phase (i.e. Prairie grass > Ryegrass > Cocksfoot - SANGAKKARA and ROBERTS 1985) persist. This lack of change could therefore be considered an effect of the defoliation regime which has a great influence on competitive relationships (HARRIS et al. 1981), as seasonal changes may not cause suffi- cient shifts in species dominance (VAN DEN

BERGH 1968). The effect of defoliation on changes in com-

petitive abilities and relationships between pas- ture species has been well reviewed (HARRIS 1978), and the susceptibility of a species to frequent defoliation markedly depresses its yielding and competitive ability. ‘Matua’ prairie grass is such a species, which does not thrive under frequent defoliation (LANGER 1973). Thus, the adoption of a regular defolia- tion programme reduces its competitive ability when compared with an infrequent regime. This is well illustrated by the RCC’s presented in Tables 1 and 2 , where the values of prairie grass are greater under infrequent defoliation. Thus, the increased competitive abilities of ryegrass and cocksfoot over prairie grass under frequent defoliation could be attributed to both management and climatic factors. How- ever, the former could be considered more important as the RCC’s of these species d o not

increase over prairie grass with infrequent defoliations. The changes in the competitive ability of cocksfoot when combined with rye- grass under frequent cutting could also be attributed to the above factors, as cocksfoot is affected less by regular defoliation than rye- grass (IBANEZ 1963).

While most studies on pasture mixtures do not indicate yield advantages by mixing (TREN- BATH 1974), WHITTINGTON and O’BRIEN (1968) reported yield advantages in mixtures at later stages of growth especially under frequent defoliation. The results of this study also sup- port these findings, as closer evaluations of RCC’s under frequent defoliation (Table 1) show that 85 % of the values exceed unity (although some not significantly) when com- pared with 55 % of the values under infre- quent defoliation (Table 2). This suggests a slightly superior performance of the mixtures over monocultures under frequent defolia- tions, and the species at this stage may begin to compete for ‘dissimilar space’ (DE WIT 1960) under frequent cutting.

In conclusion, the results presented have demonstrated the competitive abilities of these three popular pasture grasses of New Zealand are affected by the intensity of defoliation and to a lesser extent by seasonal changes. The study also suggests the increased productivity of mixtures over monocultures of these species with time, especially under frequent defolia- tion. Thus, the need for further studies to strengthen these results and to justify the claims that such studies need to continue for longer time periods is stressed.

Influence of Seasonal Productivity and Defoliation Frequency 63

Zusammenfassung

EinfluS der jahreszeitlichen Produktivitat und der Haufigkeit einer Defoliation auf die Konkurrenzsituation von drei Futtergrasern

Es liegen Erkenntnisse vor, dafi die Konkur- renzstarke und die Beziehungen zwischen Fut- tergrasern von den jahreszeitlichen Bedingun- gen und Entblattungswirkungen abhangig sind. Es wurde daher eine Untersuchung durchgefiihrt, um die Wirkungen dieser Ein- fliisse auf die Konkurrenzsituation zwischen drei verbreiteten Futtergrasern Neuseelands zu untersuchen, wobei die Methode des Ersatzes von Serien durchgefiihrt wurde. Die Konkur- renzsituation von eingewachsenen Grassoden wurde fur die Dauer von einem Jahr iiberpriift; zwei Entblattungsbereiche wurden beriick- sichtigt. Die Ergebnisse zeigen, dafi die Inten- sitat der Entblattung einen grofleren Einflufi auf die Konkurrenzbeziehungen zwischen den Grassoden hatte als die Abhangigkeit von der Jahreszeit. Diese Untersuchung lafit eine Uberlegenheit der Leistung in Mischkulturen gegeniiber Monokulturen vermuten; eine Uberpriifung der Ergebnisse erscheint aller- dings notwendig.

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