Measurements of wool production and its physiological components in a group of South Australian Merino sheep.

Abstract

Data for wool production, feed intake, and body weight were recorded for a group of 31 rams selected from the South Australian strong-woolled Merino flock at Roseworthy. Measurements were made during (a) grazing, and (b) ad libitum pen feeding. A locally developed faecal nitrogen method was used to estimate intakes at pasture. For the pen diet, which contained 15% crude protein, two measurements of apparent dry matter and nitrogen digestibility were made for each sheep. The repeatability of gross conversion efficiency of feed into wool under grazing versus pen feeding was highly significant, viz. r = +0.79. While the repeatability of wool production was also highly significant, viz. r = +0 77, the repeatability of feed intake was not significant in these data, viz. r = +0 12 In pens, voluntary feed intake was found to vary with body weight to the power of + 1 09, the 99% fiducial limits extending from +0.81 to + 1 37 and thus excluding fractional powers conventionally assigned to basal metabolic nee&, e.g. +0.7. The physiological components of wool production were studied with the use of' the pen data. An identity was used which involved three terms, viz. gross conversion efficiency of feed into wool, gross feed intake per unit body weight, and body weight. Of these, gross conversion efficiency was the prime determinant of wool production. Gross feed intake per unit body weight was significantly associated with wool production (P<0.05), while the relationship between wool production and body weight was not significant in these data. In the pen experiment 'the regression relationship between voluntary feed intake and body weight accounted for approximately one-half of the variation in voluntary feed intake. A negative association between body weight and efficiency was also revealed. However, the general significance of this latter finding is doubtful, particularly in view of the variable relationships recorded between body weight and clean fleece weight for rams in 5 years' data from the general Roseworthy flock. For the experimental group of 31 rams, substantial differences were recorded in individual apparent digestive efficiencies for both dry matter and nitrogen. The ranges of individual values, expressed as four standard deviations, were 9.2 and 12.2% respectively. However, these differences did not contribute to differences in gross conversion efficiency of feed into wool, which suggests that the latter resides in either general or special aspects of metabolism.