Mammary glands undergo physiological and morphological changes at the onset of pregnancy and grow substantially during gestation. The size of mammary glands increases almost fourfold during gestation as indicated by DNA content (Kensinger et al., 1982), which involves 148 g protein gain per gland. Based on the assumption that sows normally have 14 functional glands, that is 2066 g mammary protein gain during gestation which is calculated as 18 g day-1 protein. It is then easily calculated that 1.4 g of lysine day-1 (or 8.1 g of essential amino acids day-1) accumulated to mammary tissue during gestation based on the amino acid pattern in mammary tissue from Kim et al. (1999a).
Hacker and Hill (1972) and Kensinger et al. (1982) indicate that mammary gland growth occurs mostly during days 75-90 of gestation. This suggests that additional amino acid provision during this period may help an optimal mammary gland growth. However, Weldon et al. (1990) showed that increased dietary amino acid level during days 75-90 did not affect mammary gland growth.
Kusina et al. (1999) fed gilts different levels of amino acids during gestation. Daily lysine intakes were 4, 8 or 16 g day-1 during gestation. Additional amino acid provision during gestation also failed to improve gestational mammary gland growth (Kusina et al., 1999). Restricting amino acid intake, however, reduced the mammary gland of gilts during gestation (Head and Williams, 1991).
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