PLASMA-HORMONE AND METABOLITE CONCENTRATIONS AND THE INTERVAL FROM WEANING TO ESTRUS IN PRIMIPAROUS SOWS

The hypothesis that the interval from weaning to oestrus in first-litt er sows is controlled by luteinizing hormone (LH) secretion at or arou nd the time of weaning was examined using 16 primiparous Large White-L andrace sows weaned after 28 day lactations. Blood samples for LH, ins ulin, glucose and non-esterified fatty acids (NEFA) were collected usi ng indwelling jugular vein catheters every 15 min for 7 h on Days -3, 0, +1 and +3 and for 9.25 h on Day -1 with respect to weaning (Day 0). On Day -1, each sow received 200 ml of 5% dextrose in sterile saline via the cannula immediately after 7 h of sampling. There was no signif icant difference between the nine sows that returned early (less than 14 days) and the seven sows that returned late (more than 32 days) in mean daily feed intake during lactation, mean body weight loss, body w eight loss as a percentage of post-farrowing weight, mean P2 backfat t hickness loss or P2 backfat thickness loss as a percentage of post-far rowing thickness. In the early returning sows, LH pulse frequency (P<0 .05), mean LH concentration (P<0.05) and interpulse nadir (P<0.05) inc reased and interpulse interval (P<0.05) decreased on the day of weanin g (Day 0), and these differences persisted on Day 1 and Day 3. Pulse a mplitude remained stable in the immediate postweaning period and then decreased (P<0.05) on Day 3. Pulse area decreased after weaning, and w as lower on Day 0 (P<0.05), Day 1 (P<0.01) and Day 3 (P<0.01) than in the preweaning period. In the late returning sows, no characteristic o f LH secretion on Day 0 or Day 1 differed significantly from that in t he preweaning period, but on Day 3 pulse frequency (P<0.05) and mean L H concentration (P<0.05) were higher and the interpulse interval (P<0. 05) was shorter than on any day before weaning. Among the late returni ng sows mean plasma insulin was higher (P<0.05) and mean plasma NEFA w as lower (P<0.05) on Day 1 than on Day -1. Mean plasma glucose was low er in the late returning sows on every day of sampling, but these diff erences did not reach statistical significance. In both early and late returning sows plasma insulin and plasma glucose were elevated (P<0.0 1) in the first sample taken after the challenge with dextrose but had returned to basal concentrations 15 min later. There was no significa nt difference in either the peak concentration or the duration of the increase between the two groups of sows. In both the early and late re turning sows plasma NEFA was unaffected by the challenge with dextrose . Regression analysis of the data for all 16 sows revealed significant relationships between the interval from weaning until oestrus and (a) the number of LH peaks in the 7 h window on Day 0 (r=-0.58, P=0.018) and Day 1 (r=-0.54, P=0.033); (b) the mean LH concentration in the 7 h window on Day 0 (r=-0.54, P=0.032) and Day 1 (r=-0.62, P=0.011); (c) the mean LH interpulse interval in the 7 h window on Day 0 (r=0.58, P= 0.018) and Day 1 (r=0.54, P=0.033). No significant relationship betwee n the interval from weaning to remating and any measure of LH secretio n was revealed on Day -3 or Day -1 (r<0.468, P>0.05), and there were n o significant relationships between mean plasma insulin, plasma glucos e or plasma NEFA concentrations on any day and the weaning to remating interval or any of the six characteristics of LH secretion on the sam e day for the 16 sows. The data indicate that changes in LH associated with weaning are critical to the resumption of cyclic activity, but i t appears that simple plasma concentrations of insulin, glucose and NE FA are not involved in the controlling this LH response.