On the relation between tillering, leaf area dynamics and growth of perennial ryegrass (Lolium perennne L.)

Modern intensively managed grasslands are subject to sward deterioration as a result of urine scorching, treading, winter mortality and late mowing or grazing. The major species in Dutch grasslands is perennial ryegrass. Deterioration consists of a decreasing presence of this species through death of tillers and plant. This gives rise to open swards which are subject to weed invasion and leads to a reduced productivity and nitrogen recovery. The objectives of this study were to increase the understanding of the recovery potential of open perennial ryegrass swards and of the relation between tiller density and herbage accumulation. Tiller production, tiller mortality, leaf area dynamics and growth as affected by cutting treatment, water availability, nitrogen supply and plant density were studied in field, glasshouse and phytotron experiments. The relative tiller appearance rate was studied as the product of leaf appearance rate and site filling. Leaf appearance rate was highly positively correlated with temperature and negatively with cutting height. Cutting frequency had no effect. Leaf appearance rate was reduced by low water potential and low nitrogen supply. Site filling decreased with decreasing nitrogen supply and increasing leaf area index. Just after defoliation, site filling was lower than later after defoliation, because of low substrate availability to developing tiller buds. In studying the effect of low and high nitrogen supply, genetic variation was found in nitrogen use efficiency. Populations with a high nitrogen use efficiency and a high herbage accumulation rate, also had a high leaf weight ratio and high tillering capacity. Models were developed for the simulation of changes in tiller number, leaf area expansion and dry matter accumulation of spaced plants and swards. These models were used for sensitivity analyses of the effect of leaf area expansion rate and leaf weight ratio on regrowth after defoliation and for the analysis of the importance of substrate availability for tillering and regrowth. The field experiments and the sward model showed that even with very low seeding rates, the amount of herbage harvested in the second harvest year is not lower than at very high seeding rates. Therefore, it was concluded that only when a low tiller density coincides with a clumped distribution of tillers and with a high frequency of patches without perennial ryegrass, reseeding of perennial ryegrass swards should be considered.