Competition effects on yield and plant and tiller density in mixtures of ryegrass cultivars
- Grasslands Division, D.S.I.R., Palmerston North
The ryegrass cultivars, Grasslands Manawa and Grasslands Ruanui, were grown ill simulated swards in replacement series under systems involving a 2X2 arrangement of height and frequency of cutting.
Estimates of plant survival at two dates indicate greater overall mortality in the Manawa population. In the earlier part of the experiment the death of Manawa, and of Ruanui plants—particularly when mixed with Manawa—was greater under infrequent than frequent cutting. This effect is mainly attributed to competition for light excluding more prostrate individuals. In the later period drought was probably the important factor determining the survival of plants, with Manawa plants of small size and at high densities being most susceptible.
Examination of the frequency distribution of tiller number per plant at three sampling dates showed trends similar to those obtained by Koyama and Kira (1956). The frequency distributions were approximately normal at the earliest sampling date but became progressively more skewed towards L-type distributions later. The skew was more pronounced for Manawa and was emphasised for Ruanui when it was the minority component in mixture with Manawa. Infrequent cutting promoted the development of a skewed distribution.
Changes in the relative reproductive rate of the cultivars with change in the proportion of the cultivars in the mixtures were examined by the use of ratio diagrams. These indicated a general trend of greater relative reproductive rate of Manawa when it was sown at a ratio low in comparison with the intermediate seeding ratio. Thereafter the relative reproductive rate remained constant or increased at higher seeding ratios of Manawa.