Effects of feeding level and diet composition on mammary growth in prepubertal lambs and mice
Forty ewe lambs were grouped into four treatment groups: A) fed a standard, high-energy diet, ad libitum; G) fed as group A, but treated with GH (.1 mg/kg bodyweight/d); R) fed the standard diet in restricted amounts to a target weight gain of 120 g/d; S) fed a ration including 30% of a protected fat supplement, ad libitum. Rations were formulated to be isonitrogenous and isocaloric and were fed from approximately seven to 22 weeks of age. Growth rates differed in the order S>A = G> R, although final weights did not differ among ad libitum fed groups. Lambs in group S had heavier mammary glands, with greater amounts of parenchyma and fat pad and higher content of dry, fat-free parenchymal tissue compared to the mean of the remaining groups. Total gland weight was lower in group R, although weight of parenchyma was similar to groups A and G. Parenchyma made up a higher percent of total udder weight in lambs of group R compared to any other group. Parenchymal DNA content was not different by treatment, but glands from group G had twice the total DNA of groups A and R, and group S had 50% more than the latter groups. Volume of mammary glands occupied by parenchyma was increased by more than 50% in group S, compared to the other groups which were similar. Concentrations of prolactin receptors in mammary parenchyma and of GH receptors in liver were increased in lambs of group S. Percent Iinoleic acid in mammary parenchymal lipid of lambs in group S was increased relative to other groups. Unsaturated acids also made up a greater percentage of total fatty acids in group S. Feeding the protected fat supplement resulted in increased unsaturated fatty acid, especially linoleic acid, percentage in mammary fat. This effect was associated with increased mammary growth compared to lambs fed a standard ration. Lambs treated with GH showed some indications of increased mammary growth, but groups A and R were similar except for the increase in percent of gland occupied by parenchyma in group R.
In a second study, mammary growth in prepubertal mice increased with increasing dietary energy intake. Differences in ductal growth persisted at 18 weeks of age, and effects of exogenous steroids at this time were not significant. Prepubertal mammary growth in mice is not sensitive to inhibition by high plane of nutrition as is the case in ruminants.