The response of broad- and fine-wool Merino wethers to differential grazing of annual pastures during spring

Abstract

Two experiments examined the effects of manipulating grazing pressure during spring on the liveweight and wool-growth response of fine- and broad-wool genotypes of Merino wethers grazing annual pastures. One-year-old broad- and fine-wool Merino wethers (Bungaree and Peppin; mean fibre diameter 25.4 v. 20.7 m and 22.1 v. 19.8 m before years 1 and 2, respectively) were grazed on annual pastures maintained near target amounts of green feed on offer (800, 1100, 1400, 2000 and 2800 kg green DM/ha), or set-stocked at the district average of 8 sheep/ha, during the spring of 1992 and 1993. Within the control-grazed treatments, there was no significant difference in the total amount of pasture produced during the experimental periods but more (P<0.05) pasture was produced under set-stocking (7900 v. 5400 kg DM/ha and 7700 v. 5600 kg DM/ha in years 1 and 2, respectively). Liveweight change was linear for most treatments over the first 90 days or so of the spring grazing period, and in both years the average rate of liveweight change was similar for both genotypes. Liveweight change increased (P<0.001) curvilinearly with increasing feed on offer, which explained more than 80% of the variance in liveweight change in both years. A significant (P<0.05) green feed on offer × year interaction indicated a different response to feed on offer between years, with liveweight maintenance occurring at about 500 and 1060 kg DM/ha in years 1 and 2, respectively. Within genotypes, there was no significant (P>0.05) difference in liveweight change during spring for sheep grazing treatments 2000 kg DM/ha or above. Sheep from the broad-wool genotype produced more (P<0.05) wool than those from the fine-wool genotype and, as expected, the wool was also significantly broader at all feed on offer levels. For both broad- and fine-wool sheep, feed on offer and liveweight change were closely correlated with wool growth rate and fibre diameter in both years. The asymptote of the relationships with feed on offer and the intercept of the relationships with liveweight change was greater (P<0.05) for broad- than fine-wool sheep. However, there was no significant difference between genotypes in either size and shape or slope of these relationships. In other words, the wool-growth and fibre-diameter response of broad-wool sheep to decreasing feed on offer in the spring was the same as that for fine-wool sheep. For both genotypes, annual clean-wool production (P<0.001) and mean fibre diameter (P<0.05) increased withincreasing feed on offer during spring. The total amount of wool grown per hectare during spring decreased linearly (P<0.001) with increasing feed on offer. Grazing to about 2500 kg DM/ha more-than trebled the total clean wool produced per hectare compared with set-stocking. Thus, irrespective of sheep genotype, managing feed on offer in spring is a useful tool for manipulating wool characteristics, and increasing pasture utilisation and total wool production per unit area.