Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 19 APPROPRIATE ANIMAL STOCKING REGIMES FOR PASTURE COMPARISONS G. R. SAUL and J. W. D. CAYLEY Department of Food and Agriculture, Pastoral and Veterinary Institute, Hamilton, Vic. 3300. Pasture management systems, pasture species or fertilisers should be evaluated under grazing. Usually animals are grazed at a fixed range of stocking rates (SR) over all treatments; for example, Cayley et al. (1987) compared the productivity of wethers stocked at 10, 14 or 18 per ha on pastures receiving from 1.2 to 32.1 kg phosphorus (P) per ha annually from 1979 (Table 1). Table 1. Effect of phosphorus treatment on peak liveweights (kg/sheep) and average herbage masses (t DM/ha) in 1987 The high P treatments were under much less stress than the low P treatments, indicating that a SR greater than 18 sheep/ha would be possible (optimum SRs should be associated with some reduction in production per head); in contrast, the 2 lowest P treatments were overgrazed at 18 sheep/ha, and the plots had to be de-stocked for several weeks each year. The SRs were altered 4 years ago, and plots are now stocked so that spring-lambing ewes are of similar liveweights within each SR treatment. Low, medium or high SRs are used but the actual number of ewes/ha varies with fertiliser rate, and could be regarded as a result (Table 2). Table 2. Effects of incremental stocking and fertiliser rates on animals and pastures The new stocking policy results in less variation between liveweight and pasture on offer within SR treatments. Liveweight and herbage mass at the 2 lowest P rates is still less than in other P treatments; absolute SRs have since been reduced to increase the herbage mass and liveweight of the ewes in these treatments. Stocking plots so that the animals are of similar liveweight provides a more realistic assessment of carrying capacity than rigid SR regimes, particularly where there are large differences between treatments. Maxwell (1986) proposed that pasture treatments grazed to a common height provide the most objective comparison of a pasture or animal management system. This is inappropriate where there is a likelihood of large differences in nutritive value of pasture between the treatments (e.g. due to fertiliser). In such cases animal production per head is the most objective indicator of equivalent grazing pressure. CAYLEY, J. W. D., CUMMING, K. N. and FLINN, I? C. (1987). Proc. Aust. Sot. Agron. 4: 250. MAXWELL, T. J. ( 1986). HFRO Biennial Report 1984-85, Pentlandfield, Midlothian, Scotland. 235