Role of Luteinizing Hormone Secretion in Split Weaning

L. Zak¹, G. Foxcroft², F. Aherne¹, and R. Kirkwood¹

Alberta Pork Research Centre: ¹Alberta Agriculture, Food and Rural Development, 9^th Floor, O. S. Longman Building, Edmonton, AB, Canada, T6H 4P2

² Dept. of Agricultural, Food & Nutritional Science, University of Alberta, Edmonton, AB, Canada, T6G 2P5

Weaning the heaviest half of a litter (split weaning) before final weaning often results in sows exhibiting a more prompt return to estrus. Split weaning is associated with a transient increase in luteinizing hormone (LH) secretion. However, whether the increase in LH secretion or an improvement in energy balance after split weaning is causally associated with ovarian development is not known. The objective of this experiment was to determine the impact of LH secretion, independent of changes in energy balance, on ovarian follicle development.

At farrowing, primiparous sows (10 piglets/sow) were allocated to treatment during a 21d lactation. Split weaned (SW) sows had all but the 4 smallest piglets weaned at d18. Control (C) and Control+GnRH (C+G) sows continued to be suckled by a full litter. From d18-21 'C+G' sows received GnRH (5 times daily, i.v; 800 ng/dose) to mimic the expected increase in LH secretion in SW sows. Sows were fed according to their energy requirements for maintenance and litter growth. Sows were heat checked twice daily and bred by AI at 12 and 24 h after standing heat. At d 28 of gestation sows were slaughtered and reproductive tracts collected. Blood samples were collected every 15 min for 10 h via a jugular catheter from all sows, at d18 before and after split weaning, and again at d21, before and after final weaning for LH analysis.

Thirty of 48 sows have completed the trial. Preliminary results show that energy balance and litter growth rates did not differ amongst treatments. Ultrasonography at 24 h after weaning showed more (P<.05) ovarian follicles ( 3mm) in SW and C+G than in C sows. Weaning to estrus interval was shorter (P <.003) in SW (4.25±.28) and C+G (4.67±.20) than for C (5.9±0.44). Ovulation rate, embryo number and embryo survival did not differ amongst treatments. LH secretion was characterized by pulse frequency and mean concentration. Split weaning and GnRH administration increased (P<.01 for all) both mean LH and LH pulse frequency at day 18. Between days 18 and 21, LH pulse frequency was reduced in SW and C+G sows (P<.03 and P<.001, respectively), but at day 21 before weaning it was still higher in SW sows than for either C or C+G. Following weaning, increased secretion of LH was observed in C and C+G sows but not in SW sows.

Implication: Split weaning improves fertility after weaning by affecting ovarian follicle development during lactation. This is achieved, in part, by an increase in LH secretion before final weaning.