Influence of cumulative feed intake during early and mid-lactation on luteinizing hormone secretion and weaning-to-estrus interval in primiparous sows.

Twelve pregnant gilts were assigned to a completely randomized block design with two treatments in two blocks (2 farrowing groups). The treatments were a feeding amount of 6 kg or 2 kg/day provided during lactation. The lactation diet contained 18.6% crude protein, 1.0% lysine, and 3.27 Mcal/kg metabolizable energy (as-fed basis). Litters were weaned at 2100 on day 21 after farrowing. Blood samples for luteinizing hormone (LH) measurements were taken at 15-min intervals for 8 hr on day 12 of lactation, and samples for glucose and insulin were collected at 1-hr intervals for 3 hr on day 12. The effects of feed intake treatments on LH pulse frequencies (2.9 vs 0.7) and insulin concentrations (15.0 vs 8.9 IU/mL) were found (P < 0.05) on day 12 of lactation. In regression analysis, greater cumulative feed intake from 1 to 12 days was associated with higher insulin concentrations (P = 0.04), greater LH pulse frequencies (P = 0.01) on day 12 of lactation, and shorter weaning-to-estrus intervals (WEI) (P = 0.03). Furthermore, an association between insulin concentrations and LH pulse frequencies was found on day 12 of lactation (P = 0.01). Using regression models for weaning-to-estrus interval, when each cumulative feed intake from 4 to 21 days was used as an independent variable, the R2 values increased from 0.24 to 0.37. These results suggest that feed intake during early and mid-lactation influences LH secretion as early as day 12 after farrowing, and is associated with shorter WEI. This research also indicates that feed intake from 4 to 12 days of lactation is more important than that during the first few days after farrowing.