Female lambs were given a high energy and high protein diet in varying amounts to achieve either high (H; 220 g/day) or low (L; 110 g/day) rates of gain during two consecutive periods between 4 and 20 and 20 and 36 weeks of age. The effects on body growth and mammary gland development were compared by slaughtering an initial group of four lambs at 4 weeks of age (mean live weight 11 kg), eight L lambs (24 kg) and eight H lambs (33 kg) at 20 weeks of age, and eight LL lambs (36 kg), eight LH lambs (49 kg) and eight HL lambs (48 kg) at about 36 weeks of age.
At slaughter, a single mammary fat pad from each lamb was trimmed and weighed, and the deoxyribonucleic acid (DNA) content of the portion containing parenchyma determined. The mean weight (g) and DNA content (mg) were: 4-week-old lambs, 8·6 g, 1·5 mg; L, 14·7 g, 32·3 mg; H, 30·0 g, 25·9 mg; LL, 46·0 g, 61·2 mg; LH, 86·7 g, 91·0 mg; HL, 70·3 g, 73·0 mg respectively. Relative growth coefficients for mammary parenchyma, estimated from the increase in DNA relative to that for live weight, were 3·7 and 2·4 for L and H lambs respectively, between 4 and 20 weeks, and 1·6, 1·4 and 2·6 for LL, LH and HL lambs respectively, between 20 and 36 weeks of age.
These results are consistent with the hypothesis that a high plane of nutrition can decrease the rate of allometric growth of mammary parenchyma before puberty. However, the cessation of this growth phase was not related to the attainment of puberty, or of a specific live weight or age, but may be related to the availability of mammary fat pad tissue limiting further parenchymal development after 20 weeks of age.