This experiment investigated the influence of plane of nutrition on mammary development in heifers. Prepubertal and postpubertal heifers were fed a ration 60:40 concentrate to forage in restricted or ad libitum amounts. Dry matter intake of the heifers on restricted feeding was 60% that of heifers fed ad libitum. Average daily gain of heifers on restricted feeding was 613 g compared with 1218 g for heifers fed ad libitum. Ad libitum feeding during prepuberty lowered mammary secretory tissue weights 23% and deoxyribonucleic acid content 32% compared to restricted feeding. In contrast, there was no difference in growth of mammary secretory tissue between postpubertal heifers fed restricted or ad libitum amounts. Composition of mammary parenchyma was not affected by plane of nutrition in prepubertal or postpubertal heifers. From these data, we suggest a critical period for total mammary cell number in heifers during which mammary growth is affected adversely by a high plane of nutrition.

Concentrations in blood serum of growth hormones, prolactin, insulin, and glucocorticoids were investigated in 22 pre- or postpubertal heifers fed restricted or ad libitum amounts of a ration. Gains for the respective feeding regimens were 613 and 1218 g daily. Growth hormone was elevated for prepubertal heifers on restricted feed, whereas there was no difference from feeding for postpubertal heifers. Concentrations of prolactin, insulin, and glucocorticoids were higher for heifers fed ad libitum than for those restricted. Mammary secretory tissue was positively correlated with blood serum concentrations of growth hormone, and extraparenchymal adipose tissue was negatively correlated with them. Prolactin was negatively related to mammary secretory tissue; however, this relationship disappeared after adjustment for concentrations of growth hormone, insulin, and glucocorticoids. Mammary tissue was not related to concentrations of insulin or glucocorticoids. The negative influence of excess feeding on mammary development of prepubertal heifers may be associated with a decrease in concentrations of growth hormone in blood.

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The influence of feeding high milk on performance, health, and absorption capability of the small intestine was studied in Holstein calves (eight males and eight females). Animals were kept in outdoor hutches bedded with straw. Treatments consisted of two quantities of milk: 1) 4.1 kg of whole milk from 3 to 48 days of age when calves were weaned and 2) gradually increasing milk from 4.1 to 7.0 kg during the first 2 wk of treatment and feeding 7.6 kg per day thereafter until day 42. Milk was reduced gradually during the 7th wk. Intake of milk averaged 4.1 and 6.7 kg per animal per day. Commercial starter, alfalfa hay, and water were offered ad libitum to all calves. Higher milk resulted in larger weight gains (615 versus 538 g/day) and less starter intake. Total dry matter intake, feed efficiency, and scour scores were not different between treatments, but rectal temperatures were greater on high milk. Female calves fed high milk showed less xylose absorption and more days medicated than females fed less milk.