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Abstract
The effect of feeding level and nitrogen source on mammary development and plasma hormone levels of pre-pubertal (86 to 220 kg) Finnish Ayrshire heifers was studied. The treatments were: (1) LU, low feeding level + urea; (2) LR, low feeding level + rapeseed meal; (3) HU, high feeding level + urea; (4) HR, high feeding level + rapeseed meal. The average daily gains of the heifers on LU, LR, HU and HR diets were 692, 655, 805 and 890 g, respectively. There was a significantly higher amount of mammary parenchymal tissue and parenchymal DNA at low (L) compared with high (H) feeding levels, but no difference in mammary weights between nitrogen sources was found. Plasma insulin concentration was greater at H compared with L feeding levels, but plasma growth hormone (GH), insulin-like growth factor-I (IGF-I) and prolactin concentrations were not significantly affected by the feeding level. The amount of mammary parenchymal tissue did not correlate with the mean plasma concentration of prolactin and insulin. Also, no significant correlation between plasma GH (r = 0.32; P < 0.15) or IGF-I (r = 0.37; P < 0.09) and mammary growth was found. However, when GH measurements around feeding (2 h) were excluded, mammary development was positively correlated to plasma GH (r = 0.44; P < 0.05).
