EFFECTS OF STOCKING RATE IN SPRING ON LIVEWEIGHT AND WOOL PRODUCTION OF SHEEP GRAZING ANNUAL PASTURES

Two experiments examined the effects of different stocking rates in sp ring, and hence the availability of annual pastures, on changes in liv eweight and wool production in Merino wethers (Experiments 1 and 2 res pectively: age 5 and 2 1/2-year-old; liveweight 63.8+/-0.64 (s.e.m.) k g and 43.8+/-0.34 kg; condition score 3.9+/-0.14 and 3.1+/-0.08). In E xperiment 1, stocking rates were 8, 16, 24, 32 and 40 sheep/ha from 8 August, 1989 for 122 days; Experiment 2 involved an additional stockin g rate of 48 sheep/ha from 23 August, 1990 for 98 days. Feed on offer (FOO kg DM/ha) declined (P < 0.01) lineraly as stocking rate increased . Stocking rate and initial FOO (ranging between 1100 and 7000 kg DM/h a) had no significant effects on pasture growth rate (PGR) through mos t of spring. Late in spring, increased stocking rates resulted in grea ter (P < 0.05) PGR. The total amount of pasture produced in the grazin g period was not significantly affected by stocking rate (Expt 1, 7530 to 8200 kg DM/ha; Expt 2, 6390 to 6860 kg DM/ha). The relationships b etween liveweight change (LWC) or wool growth rates (WGR) and FO, duri ng the period until pasture wilting at the lowest stocking rate (83 da ys in Expt 1; 76 days in Expt 2), were described by Mitscherlich equat ions. More than 74% of the variation in LWC or WGR wax explained by di fferences in green FOO. In Expts 1 and 2 respectively, more than 90% o f the maximum liveweight gain (66 and 192 g/day) was achieved at a FOO of 4000 or 3000 kg DM/ha, and sheep maintained weight at 2000 or 1000 kg DM/ha. More than 90% of the maximum WGR (22.3 and 19.0 g/day) was achieved at a FOO of 3000 or 2000 kg DM/ha. More than 70% of the varia tion in WGR was explained by LWC in both experiments. The slopes of th e linear relationships were 0.047 g wool/g LWC in Expt 1, and 0.024 g wool/g LWC in Expt 2. At liveweight maintenance, sheep produced 15% le ss (Expt 1) or 25% less (Expt 2) wool than those grazed under conditio ns which allowed maximum rates of liveweight gain. Fibre diameter (FD) and length of wool grown were affected in the same manner as WGR by i ncreases in FOO and hence LWC. In Expts 1 and 2 respectively, total cl ean wool weights were reduced by 17 and 9 g, mean FD by 0.05 and 0.02 microns and staple length by 0.35 and 0.13 mm, for each increase of on e sheep/ha during the spring treatment periods. The effects of stockin g rate in spring on annual wool productoin, mean FD and staple length were described by linear (P < 0.04 to P < 0.01) relationships. Standar d deviation of midside FD (Expt 2), staple strength and position of br eak (both experiments) did not change significantly with stocking rate . These results indicate that grazing to a lower FOO during spring can be used to manipulate the amount and characteristics of wool produced by Merion wethers grazing annual pastures in Mediterranean climates w ith 600-700 mm rainfall.