JX Xia
Abstract
Four pairs of paddocks of prairie grass (Bromus willdenowii Kunth cv. Grasslands Matua) and white clover (Trifolium repens L. cv. Grasslands Pitau) were grazed by lactating cows 3 times in rotation between September 1988 and February 1989 to residuals of either 6 cm height and 1.5-2.5 t DM/ha residual mass (hard grazing, Treatment H) or 12 cm height and 2.5-3.5 t DM/ha (lax grazing, Treatment L). Detailed measurements were made of the effects of grazing management and season upon plant and tiller populations, and upon prairie grass plant tissue dynamics. Treatment H reduced the leaf appearance interval by 35% and the number of live leaves per tiller by 17%, compared with Treatment L. Leaf weight and total dry weight per tiller were also substantially reduced by hard grazing, though the reductions were greater after the first (39 and 53%) and second grazings (53 and 74%) than after the third grazing (22 and 22%). Hard grazing reduced tiller numbers per plant and per unit area by 22 and 37% respectively compared with lax grazing. Treatment x period interactions were not significant for any of the above parameters. Numbers of plants per unit area were 33% greater in Treatment H, and this treatment had a higher proportion of small plants (< 26 tillers) and a smaller proportion of large plants (> 45 tillers) than did Treatment L. Hard-grazed plots had lower proportions of prairie grass, higher proportions of clover and other grass species, and higher proportions of leaf and other live plant components than did lax-grazed plots. Rates of prairie grass growth on Treatment L were approximately twice those on Treatment H, both per tiller and per unit area. These differences were only marginally offset by lower senescence losses in Treatment H, so that rates of net herbage production from prairie grass were, on average, 119% greater per tiller and 147% greater per unit area on Treatment L. These differences were attributable largely to greater rates of gross and net production for stem and daughter tiller tissue in Treatment L; differences between treatments in leaf tissue production were small. These results clearly identify the very limited tiller size/density compensation in prairie grass swards in response to increased grazing pressure. It is suggested that this provides an explanation for the limited flexibility of the plant, and its poor persistence, in many grazing situations.
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