The body composition and herbage utilization of grazing Merino and crossbred lambs during periods of growth and summer undernutrition

Abstract

A grazing study was undertaken (i) to determine the changes in body energy and body composition of Merino and Dorset Horn x Merino lambs during their first year of life with the object of assessing the nature of body tissue reserves during periods of summer undernutrition, and (ii) to examine the nutritional basis of growth differences between the two genotypes in terms of herbage intake and feed utilization for weight gain. The relations between body weight and (i) total body energy, (ii) body protein, (iii) body fat, and (iv) body water were of linear form and similar for both groups. Each 1 kg of wool-free body weight increment by the unfasted grazing animal was associated with the retention of 3.17 megacalories (mcal) of body energy, whereas on an empty body weight basis 1 kg gain was associated with the retention of 4.18 mcal. Empty body weight was no better a predictor of body energy than the unfasted wool-free body weight of the grazing sheep; by either method more than 97% of the variance in body energy could be accounted for by differences in body weight. Dorset Horn x Merino lambs were significantly heavier at birth (4 5 v. 3.5 kg). They grew more rapidly than the Merinos throughout the 300 day experiment although during the first 200 days relative growth rates were similar. The intake of digestible energy WE) of the two groups was estimated for 112 days from weaning at c. 7 weeks. During this period the crossbreds weighed 20-23 % more than the Merinos; they consumed 23 % more herbage at the beginning and 11 % more at the end (P < 0.01). The efficiency of energy retention was similar for both breeds, 42.5 % of metabolizable energy above the maintenance being stored as wool-free body energy. During the summer drought period the crossbreds used their feed more efficiently than the Merinos; they consumed up to 25 % less feed per kilogram weight and at the same time increased their weight advantage. Differences in body weight underestimated differences in body energy. Crossbreds weighed 30% more than the Merinos at the beginning of the summer drought period, and 36 % more at the end. Corresponding differences for body energy were 40 and 53 %. There was no evidence to suggest that prolonged periods of arrested growth influenced the composition of the body at a given weight.