The regression of unsuckled mammary glands during lactation in sows: The influence of lactation stage, dietary nutrients, and litter size

During lactation in the sow, mammary glands that are not regularly suckled undergo regression. This study characterizes the regression of unsuckled ma mmary glands and how that regression is affected by dietary nutrients and l itter size. Sixty-nine primiparous sows were fed one of four diets containi ng combinations of two protein levels (32 or 65 g lysine/d) and two energy levels (12 or 17.5 Mcal ME/d) during lactation. Litter size was adjusted to 10. Sows were killed on d 0, 5, 10, 14, 21, or 28 of lactation. In another experiment, twenty-eight primiparous sows were allotted to have different litter sizes and were killed on d 21 of lactation. The day before slaughter , teat order of each litter was observed. After death, mammary glands were removed and dissected. Skin and extraneous fat pads were removed from the m ammary glands and individual glands were separated. Each gland was weighed, cut in half to measure cross-sectional area, and ground for chemical analy sis. The amounts of dry tissue, protein, fat, ash, and DNA were measured. O nly glands observed to be unsuckled were included in the results. Regressio n of unsuckled mammary glands occurred rapidly during the first 7 to 10 d o f lactation, as indicated by a decline in wet weight, dry weight, protein, fat, DNA, and cross-sectional area. The rate of regression was slowed after the early lactation period. The rate of regression of unsuckled glands was affected by dietary nutrient levels. Dietary energy level affected (P < 0. 05) the decline in wet and dry weights, protein, fat and DNA content, and c ross-sectional area, whereas dietary protein level affected (P < 0.05) the decline in dry weight and fat content. At d 5 of lactation, the wet weight of unsuckled mammary glands in sows fed the high-energy high-protein diet w as 91% greater (P < 0.05) than in sows fed the low-energy low-protein diet. Effects of litter size on size and composition of unsuckled glands were no t significant by d 21 of lactation. Unsuckled mammary glands regress rapidl y during early lactation, and the rate of regression is affected by dietary nutrient intake.