Leaf growth and utilization in four grass species under steady state continuous grazing

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Abstract
SUMMARY: Tiller weight, lamina length, lamina growth rate, appearance interval and lamina utilization were examined in Lolium perenne, Agrostis stolonifera, Holcus lanatus and Poa trivialis. Marked tillers were measured in situ over 14–21 day periods in a continuously grazed permanent pasture under steady state management in Devon, UK, in 1985, 1986 and 1987 on plots receiving either zero (0N) or 400 kg nitrogen (400N) fertilizer/ha per annum.L. perenne was incapable of reducing its individual tiller weight or lamina length to the same extent as in the other three species during the grazing season. Lamina appearance interval was longer in L. perenne than in the other three species in most of the observation periods in both the 400N and 0N plots. Overall the lamina appearance intervals were similar between A. stolonifera, H. lanatus and P. trivialis. Lamina extension rates were greater in L. perenne than in either A. stolonifera or P. trivialis in most observation periods in both plots. H. lanatus had lower lamina extension rates than L. perenne in c. 50% of the observation periods in both plots, at other times the extension rates of the two species were similar, with one exception in early spring in the 400N plot when H. lanatus had a higher extension rate. There was no clear pattern in seasonality as to when H. lanatus had a lower extension rate than L. perenne. H. lanatusachieved either similar or higher lamina extension rates than either A. stoloniferaor P. trivialis in c. 50% of the observation periods, respectively; the periods when higher rates were observed in H. lanatus in the 400N plot occurred mainly in the spring and early summer. A. stolonifera and P. trivialis achieved similar lamina extension rates in most observation periods in both the ON and 400N plots. The specific differences observed in the lamina extension rates were emphasised when converted to growth rates by multiplying the length increments by the average weight per unit length of the expanded lamina for each species.Lamina utilization, in terms of percentage of lamina length removed by grazing, was similar between the four grass species in most observation periods in the ON plot. A. stolonifera lost more lamina length than either L. perenne or P. trivialis in May and June and H. lanatus lost a greater percentage of lamina length than either L. perenne or P. trivialis in May in the ON plot. In the 400N plot L. perenne lost a greater percentage of lamina length than any of the other species in June and more than P. trivialis in May. A. stolonifera lost a greater percentage of lamina length than any of the other species in July, and more than either L. perenne or P. trivialis in May. This greater severity of grazing in A. stolonifera was associated with a high incidence of tillers being grazed to stubble. H. lanatus lost more lamina length than L. perenne in May. When the loss of lamina length through grazing was converted to losses in terms of weight of tissue removed, then L. perenne consistently lost more than either A. stolonifera or P. trivialis in both the 400N and 0N plots.The implications of these differences in tissue production and utilization on competitive interactions between L. perenne and the other three species are discussed.