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Dietary Nutrient Affects Mammary Gland Growth in Lactating Sows S. W. Kim, W. L. Hurley, and R. A. Easter1 Department of Animal Sciences University of Illinois at Urbana-Champaign There are many factors that can potentially affect mammary gland growth. Estrogen and progesterone are known as essential hormonal factors for the growth of mammary glands during lactation. Insulin, cortisol, thyroid hormone, and prolactin also support mammary gland growth. Nutrient intake may also affect the growth of mammary glands. During gestation, dietary protein level does not affect mammary gland growth of gilts (Weldon et al., 1991; Kusina et al., 1999). However, excessive energy intake during gestation reduces mammary gland growth of gilts (Weldon et al., 1991). The lactation period is a challenge for the sow. Milk production results in a high demand for nutrients by the sow and sows will mobilize their body tissues when dietary nutrients do not meet the demands for milk production. However, the mammary glands are also growing during lactation (Kim et al., 1999a), even though sows are in a catabolic state. It may be expected that level of dietary nutrient intake during lactation may impact mammary gland growth, although this relationship has not been demonstrated previously. Protein and energy intake affect mammary gland growth during lactation Sixty-one primiparous sows were fed four different diets containing two levels of protein and two levels of energy (Kim et al., 1999b). Teat order was observed from each litter one day prior to killing of sows to know which glands were nursed during lactation. Sows were killed at times up to 30 d of lactation. Mammary glands were dissected into individual glands and only glands known to have been nursed by pigs during lactation were used for these analysis. Wet weight of individual glands was recorded. Both protein intake and energy intake of lactating sows affected the growth of the mammary gland during lactation. Growth of mammary glands was stimulated by increased dietary protein and energy during lactation. We generated three-dimensional response surface graphs to show the effects of protein and energy on mammary gland growth as lactation progressed (Figure 1). The x-axis represents total or accumulative energy intake of the sow as lactation progresses, the y-axis represents total protein intake of the sow, and the z-axis represents response variables, such as wet weight and DNA content. The highest point of the graph indicates the combination of dietary protein and energy when mammary gland 1 Sungwoo Kim is a postdoctoral research associate of swine nutrition, Walter Hurley is a Professor of lactation biology, and Robert Easter is a Head of Department and Professor of swine nutrition. growth was maximized. Thus, the day of lactation and dietary protein and energy levels when sows had maximum mammary gland growth during lactation can be estimated. Mammary gland growth of lactating sows was maximized when sows consumed daily 16.9 Mcal ME and 55 g lysine (990 g crude protein). Maximal mammary gland size occurred between d 25 and 28 of lactation. Feeding sows adequately for maximal mammary gland growth should be emphasized for improving weaning weight of pigs. Both protein intake and energy intake during lactation should be considered when accounting for mammary growth. References Kim, S. W., W. L. Hurley, I. K. Han, and R. A. Easter. 1999a. Changes in tissue composition associated with mammary gland growth during lactation in the sow. J. Anim. Sci. 77:2510. Kim, S. W., W. L. Hurley, I. K. Han, H. H. Stein, and R. A. Easter. 1999b. Effect of nutrient intake on mammary gland growth in lactating sows. J. Anim. Sci. 77 (In press). Kusina, J., J. E. Pettigrew, A. F. Sower, M. R. Hathaway, M. E. White, and B. A. Crooker. 1999. Effect of protein intake during gestation on mammary development of primiparous sows. J. Anim. Sci. 77:925. Weldon, W. C., A. J. Thulin, O. A. MacDougald, L. J. Johnston, E. R. Miller, and H. A. Tucker. 1991. Effects of increased dietary energy and protein during late gestation on mammary development in gilts. J. Anim. Sci. 69:194.