Follicular heterogeneity of sows at weaning in relation to within-litter variation in piglet birth weight

In pig husbandry, relations have been found between lactational body weight losses,subsequent follicle development and birth weight uniformity, and these relations seem tobe mediated by changes in insulin-like-growth factor (IGF-1) levels during lactation. We aimto investigate follicle size and follicular and serum IGF-1 levels and relate these tolactational body weight losses. In total 31 multiparous GY× DL crossbred sows (parity 4.7 ±2.5), equally distributed over parity, lactation weight loss and weaned litter size, wereslaughtered at day 0 or day 4 after weaning. Blood was collected to obtain plasma andserum samples. Ovaries were photographed for follicle size determination (5 largest oneach ovary) and follicular fluid was collected from the 15 largest follicles on one ovary, andstored separately for follicles 1-5 (Large), 6-10 (Medium) and 11-15 (Small). Sow bodyweight losses during lactation were assessed and IGF-1 levels were determined in the Largefollicular fluid pool.Parity 2-3 (n=15) vs. 4-10 (n=16) differed in body weight at parturition (269 ± 5 vs. 228 ± 5kg; P<0.0001) and backfat loss (5.1 ± 0.52 vs. 3.4 ± 0.59 mm; P=0.03) but not body weightloss (9.8 ± 3.1 vs. 7.0 ± 3.0 kg, P=0.53; 4.0 ± 1.2 vs. 2.6 ± 1.2 %, P=0.42). Average follicle sizeof the Large follicles increased from day 0 to 4 (0.30 ± 0.03 vs. 0.65 ± 0.03; P<0.0001). Inthe Large follicles, average IGF-1 levels increased from day 0 to day 4 (153 ± 44 vs. 217 ± 32μl; P=0.0015). Not at day 0, but at day 4, the size of the Large follicles was negativelyrelated to follicular fluid IGF-1 levels (r=-0.58; P=0.049). The percentage of body weightloss did not affect the size of the Large, Medium and Small follicles on day 0 and 4 and wasalso not related to follicular IGF-1 levels in the Large follicles at day 0 (P=0.20) or at day 4(P=0.89). IGF-1 levels of serum and of the Small follicle fluid pools will become available,but so far, there was no major role for IGF-1 in post-weaning follicle development inmultiparous sows, which is in contrast to young sows that showed a larger body weightloss.