Competition among pasture plants

Abstract

Yield, plant numbers, and tiller numbers were measured on broadcast swards of a mixture of ‘Grasslands Manawa’ and ‘Grasslands Ruanui’ ryegrass and on simulated swards of monocultures and a mixture of the ryegrasses grown at contrasting levels of nitrogen and phosphorus and under two cutting frequencies.

Greatest yield responses were to applied nitrogen, but this response varied with season, and interacted with the other treatment effects. Overall, small yield increases were obtained by applying phosphorus, and greater yield was obtained with infrequent cutting. Response to phosphorus was relatively greater under frequent cutting. During flowering, response to nitrogen was relatively greater where cutting was infrequent.

Initially Manawa grew more than Ruanui, giving Manawa a competitive advantage in all treatments. Manawa dominance was increased by applied nitrogen, and in broadcast swards this dominance was greater with infrequent cutting. In simulated swards, low seeding density and sub-optimal availability of nitrogen and probably water, by reducing light competition, restricted effects of cutting frequency on Manawa dominance over Ruanui. In summer direct susceptibility of Manawa to intensive defoliation led to dominance of Ruanui in the frequent cutting treatment. Where nitrogen application and frequent cutting increased the effect of phosphorus as a limiting factor, competition for phosphorus was indicated by further suppression of the cultivar already suppressed by other effects.

From widely different seeding densities tiller densities of broadcast and simulated swards converged, increases in tiller numbers per plant being greater and plant mortality less in simulated swards. Overall, applied nitrogen and phosphorus and frequent cutting increased tiller numbers, but these effects varied with season and cultivar. Response of Ruanui to nitrogen was relatively greater than that of Manawa under frequent cutting. Greater yield response to phosphorus under frequent cutting was associated with larger tillers during flowering. Frequent cutting caused most plant death and reduced survival of Manawa in summer, but other effects on plant survival involving all factors were indicated.

The results allow definition of situations where mixing of Ruanui and Manawa may provide production advantages.