Mammary Growth and Milk Production Capacity of Replacement Heifers in Relation to Diet Energy Concentration and Plasma Hormone Levels

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Abstract
The objective was to investigate the influence of ration energy concentration at the same feeding level on mammary growth, hormone secretion and subsequent milk production capacity of dairy heifers. Two groups of 19 Red Danish heifers were given the same amount of net energy in diets with either low or high energy concentration from 3 months of age. The average daily gain (ca. 500 g) was similar at the two diets. Nine heifers from each group were designated for slaughter at different live weights (range 139–383 kg) for estimation of mammary growth. Compared at the average body weight at slaughter (250 kg) the least-square means of mammary gland weights, extra-parenchymal tissue weights and parenchymal tissue weights of the groups fed low and high energy density rations were 1201±79 vs. 1162±84 (p>0.20), 789±63 vs. 698±67 (p>0.20) and 412±44 vs. 464±47 (p>0.20), respectively. In line with this there was no significant difference in the amount of milk produced by the cows raised on a low or high energy diet (16.7±1.0 vs. 16.8±1.0 kg 4% fat corrected milk; p>0.20). The amount of mammary parenchyma increased linearly with live weight (b = 3.08 g; Pr=0.61; p<0.01). From the results it can be concluded that mammary growth and milk production capacity in heifers is unaffected by ration energy density when the heifers are raised at a low feeding level.