Competition among pasture plants

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Abstract
Competition of white clover with the ryegrass cultivars ‘Grasslands Manawa’ and ‘Grasslands Ruanui’ was studied under four cutting regimes which initially involved a 2 X 2 application of cutting height and frequency. Cutting height and frequency treatments were discontinued after 6 and 17 months respectively so that for the final year all plots were cut uniformly. Competition effects were quantified by the de Wit (1960) model of competition with modifications to account for the effect of nitrogen transfer from legume to grass. Initially white clover was suppressed under all treatments. Clover suppression was greatest with Manawa ryegrass and with infrequent cutting. Plant arrangement effects on competition also occurred. Overall competition effects indicated that in the first year ryegrass and clover were mutually exclusive, but there was some evidence of effect of nitrogen transfer on ryegrass growth at the end of the first year. In the second year relative crowding coefficients of white clover were high under infrequently and frequently cut treatments of the Ruanui and Manawa series respectively. This contrast was attributable to effects of between-ryegrass competition, the direct susceptibility of Manawa to frequent cutting, over-dominance of Manawa with infrequent cutting in the first year, and the vegetative spread by stolons of white clover. Where clover contributed more than 30% of the yield of the mixtures an equilibrium was attained. Dynamic aspects of this equilibrium showed seasonal changes of ryegrass and clover content without marked effects of dominance and suppression, and within-sward oscillation of grass and clover dominance related to nitrogen transfer. These effects are discussed in relation to management systems and plant breeding objectives which will maintain the efficiency of the grass-legume association.