LEAF GROWTH AND UTILIZATION IN 4 GRASS SPECIES UNDER STEADY-STATE CONTINUOUS GRAZING

Tiller weight, lamina length, lamina growth rate, appearance interval and lamina utilization were examined in Lolium perenne, Agrostis stolo nifera, Holcus lanatus and Poa trivialis. Marked tillers were measured in situ over 14-21 day periods in a continuously grazed permanent pas ture under steady state management in Devon, UK, in 1985, 1986 and 198 7 on plots receiving either zero (ON) or 400 kg nitrogen (400N) fertil izer/ha per annum. L. perenne was incapable of reducing its individual tiller weight or lamina length to the same extent as in the other thr ee species during the grazing season. Lamina appearance interval was l onger in L. perenne than in the other three species in most of the obs ervation periods in both the 400N and ON plots. Overall the lamina app earance intervals were similar between A. stolonifera, H. lanatus and P. trivialis. Lamina extension rates were greater in L. perenne than i n either A. stolonifera or P. trivialis in most observation periods in both plots. H. lanatus had lower lamina extension rates than L. peren ne in c. 50 % of the observation periods in both plots, at other times the extension rates of the two species were similar, with one excepti on in early spring in the 400N plot when H. lanatus had a higher exten sion rate. There was no clear pattern in seasonality as to when H. lan atus had a lower extension rate than L. perenne. H. lanatus achieved e ither similar or higher lamina extension rates than either A. stolonif era or P. trivialis in c. 50 % of the observation periods, respectivel y; the periods when higher rates were observed in H. lanatus in the 40 0N plot occurred mainly in the spring and early summer. A. stolonifera and P. trivialis achieved similar lamina extension rates in most obse rvation periods in both the ON and 400N plots. The specific difference s observed in the lamina extension rates were emphasised when converte d 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 grazi ng, was similar between the four grass species in most observation per iods in the ON plot. A. stolonifera lost more lamina length than eithe r L. perenne or P. trivialis in May and June and H. lanatus lost a gre ater percentage of lamina length than either L. perenne or P. triviali s in May in the ON plot. In the 400N plot L. perenne lost a greater pe rcentage of lamina length than any of the other species in June and mo re 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 gra zing in A. stolonifera was associated with a high incidence of tillers being grazed to stubble. H. lanatus lost more lamina length than L. p erenne in May. When the loss of lamina length through grazing was conv erted to losses in terms of weight of tissue removed, then L. perenne consistently lost more than either A. stolonifera of P. trivialis in b oth the 400N and ON plots. The implications of these differences in ti ssue production and utilization on competitive interactions between L. perenne and the other three species are discussed.