Amino acids for mammary gland growth

The mammary glands are the key tissue organ that cannot be ignored in discussing lactating sows because of their metabolic importance in synthesizing and secreting milk that is directly related to litter weight gain and litter weaning weight. Mammary glands take up amino acids from the bloodstream, synthesize milk proteins and secrete milk to nursing pigs. Understanding the metabolism and biology of mammary glands is, therefore, a crucial initial point for improving reproductive performance of lactating sows and overall swine production.

At the onset of parturition, the mammary gland undergoes major physiological changes. Mammary epithelial cells actively synthesize and secrete milk into lumen that is then released to nursing pigs. During colostrum secretion, lactation is continuous but it becomes periodic within a couple of days after farrowing as colostrum secretion ends and milk secretion begins. As sows milk periodically, their nursing pigs establish a unique teat order which is relatively strong (McBride, 1963; De Passille and Rushen, 1989). Some mammary glands are used by nursing pigs whereas the other remaining glands are not being used. The numbers of lactating and non-lactating mammary glands depends on litter size and total number of mammary glands. Suckled mammary glands are functional and continue to lactate, whereas non-suckled mammary glands undergo substantial regression during the first 7-10 days of lactation (Kim et al., 2001a). A lactating mammary gland clearly continues to grow as lactation continues (Kim et al., 1999a).

It has been shown that 1.0 g day-1 of lysine (or 7.0 g day-1 of essential amino acids) is incorporated into mammary tissue protein for sows with ten nursing pigs (Kim et al., 1999a). However, the actual amount of amino acids needed for the tissue growth would be higher than that deposited in mammary tissue protein. Regressing non-suckled mammary glands may provide a small amount of lysine (0.4 g day-1 or 2.1 g essential amino acids day :) to other suckled lactating mammary glands (Kim et al., 2001a). Trottier et al. (1997) measured the amount of essential amino acids taken up by mammary glands (188.5 g day-1), secreted as milk proteins (139.5 g day-1), and finally retained in mammary gland (49.0 g day-1). It is, then, calculated that only 14% of essential amino acids were actually used for tissue growth (Kim et al., 1999a) and the remaining 76% of essential amino acids was either transformed to other non-essential amino acids or oxidized as energy sources (Richert et al., 1998). For individual amino acids, oxidation rates were different among essential amino acids (Table 12.4). It was shown that branched-chain amino acids are highly oxidizable relative to other essential amino acids in mammary glands. High oxidation rate in valine suggests that sows may need a higher valine requirement (11.5 g kg-1) than the requirement from NRC recommendation (Richert et al., 1996; NRC, 1998).

The amount of amino acids in lactating mammary glands increases during lactation as mammary glands grow continuously (Kim et al., 1999a). However, mammary amino acids increase at different rates depending on the stage of lactation (Kim et al., 2000a). In a

Table 12.4. Amino acids retained or accumulated in mammary gland of lactating sows3.

Retained EAA (g day-1)

Tissue EAA (g day-1)

Oxidation rate (%)

Lysine

3.2

1.01

68.2

Threonine

4.8

0.56

88.4

Tryptophan

0.8

0.15

80.0

Methionine

1.7

0.28

83.2

Valine

6.9

0.79

88,5

Leucine

12.9

1.16

91.0

Isoleucine

7.4

0.58

92.2

3Adapted from Trottier et al. (1997) and Kim et al. (1999a).

3Adapted from Trottier et al. (1997) and Kim et al. (1999a).

normal nutritional status, growth rate is maximized during the first 2 weeks of lactation, and then slows down until weaning.

The amount of dietary amino acids affects mammary gland growth. It has been shown that amino acid and energy intakes during lactation affect the growth of mammary glands (Kim et ai, 1999b). Mammary gland growth in lactating sows was maximized when a sow consumed 55 g total lysine and 70.7 MJ ME per day during lactation. Suggested energy requirement is about the same level as suggested by NRC (1998), whereas lysine (or essential amino acids) requirement for maximal mammary gland growth was about 10% higher than the NRC recommendation. One explanation for this difference is that the NRC recommendation did not consider amino acid needs for mammary gland growth in establishing requirements that were shown to be between 3.2 and 7.14 g day-1 (Trottier et ai, 1997; Kim et ai, 1999a). Jackson et ai (2000) and Hurley et ai (2000) reported that the cellular transport systems for lysine and valine should not be limiting factors in lysine and valine uptakes suggesting that increased dietary amino acids would increase amino acid uptake by mammary cells that can be used for tissue and milk protein synthesis. However, increasing intakes of specific amino acids should be achieved with a consideration of the balance with other essential amino acids.

Sows with different litter sizes had different rates of mammary gland growth during lactation. For the sows with the larger litter size, i.e. up to 12, the growth of individual suckled mammary glands were smaller than sows with smaller litter size, i.e. down to 6, whereas the total size of suckled mammary glands was definitely greater (Kim et ai, 1999c). There was a 0.13 g lysine (or 0.80 g essential amino acids) increase in mammary tissue for one pig increase to a litter during 21 day lactation. Amino acid needs for an increased litter would be greater when amino acid oxidation is considered. Nielsen et ai (1997) measured the changes of amino acid quantity taken up by mammary glands from the sows with different litter sizes. Considering Nielsen's data, amino acid needs for additional pigs in the litter would be higher than actual tissue accumulation (Table 12.5).

Lactational mammary gland growth was also affected by the anatomical location of each mammary gland in the sow. The first five pairs of suckled mammary glands grew faster than other posterior glands during lactation and pigs that suckled the first five pairs of mammary glands also grew faster than other pigs during lactation (Kim et ai, 2000b). There was a clear relationship between protein content in suckled mammary gland and the growth of pigs during lactation (Kim et ai, 2000b; Nielsen et ai, 2001) indicating benefits of encouraging mammary gland growth during lactation for improving pig production.

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