Lactation is physiologically and nutritionally unique and distinct from other stages of a pig's life. During lactation, amino acid metabolism is directed favourably to milk production through mammary glands. When there are not enough amino acids available from dietary sources, a sow starts mobilizing her body protein to provide amino acids for synthesizing milk protein. Excessive tissue protein mobilization during lactation reduces the sow's body condition that impairs the normal reproductive performance for the subsequent parities, such as delayed return to oestrus, reduced litter size in subsequent litters, or even reproduction failure (Reese et ai, 1982; King and Williams, 1984; Kirkwood et ai, 1987). Focusing lactation feeding strategies on minimizing weight loss is, therefore, important.
During lactation, a common problem in sow management is low voluntary feed intake, especially for the primiparous sows. In addition, genetic selection for high milk production
Table 12.3. Amino acid uptake by uterus and the amount of amino acids used for tissue accretion.
Uterine amino acid uptake3 Amino acid pattern %b of amino acid --from tissue accretion mg day 1 fetus 1 %b accretion0 (%b) (uptake/accretion)
Lysine 762 100 100 100
Threonine 482 63 55 115
Tryptophan 204 27 19 142
Methionine 245 32 27 119
Valine 585 77 71 108
Isoleucine 416 55 51 108
Leucine 1031 135 105 129
3Adapted from Wu etal. (1998b). bRelative ratio to lysine.
0Adapted from Table 12.2 and Fig. 12.1. Tissues include fetus, uterus and placenta.
and large litter size has burdened sows to deplete amino acids to support increased milk protein synthesis. Low dietary amino acid intake in relation to the increased milk protein synthesis causes a massive tissue protein mobilization during lactation. Lactating sows are, therefore, often in a catabolic status until weaning. To prevent excessive and extensive amino acid catabolism during lactation, one strategy will be to increase amino acid intake. Designing a nutritionally well-balanced 'ideal' diet for lactating sows would be another important strategy. The goals of these strategies should not be limited to maximizing milk production for nursing pigs but also should be extended to maintaining optimum body condition for normal or improved reproductive performance. In designing a diet with an ideal amino acid pattern for lactating sows, clear understanding of amino acid requirements is a critical starting point. In this section, the role of amino acids in the metabolism of lactating sows is discussed, especially focusing on reproductive performance. A concept in designing an ideal lactation diet is also discussed.
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