This study assessed pulsatile release of LH during altrenogest treatment after weaning in primiparous sows and related this to follicle development, estrus and ovulation rate. Weaned sows (n=10) received altrenogest 20mg/day from D-1 to D13 (weaning=D0) at 0800 h. On D13, blood samples were collected every 12 min from 1000 until 1900 h (1st sampling period) and from 2300 h until 0800 h (2nd sampling period). During the 1st sampling period, LH concentrations remained low and no LH pulses were detected in 8/10 sows. During the 2nd sampling period, average and basal LH concentrations (P<0.04) and frequency of pulses (P<0.0001) were higher than during the 1st sampling period. Sows with short vs. long intervals to estrus (<5 days vs. ≥5 days) had higher basal and average LH concentrations during the 2nd sampling period (P≤0.004) and showed more follicular growth during treatment (P=0.007), generating larger follicles at D14 (P=0.005). Sows with high ovulation rate (≥25) displayed more LH pulses in total than sows with low (<25) ovulation rates (P=0.03). In conclusion, this study showed that altrenogest efficiently prevented LH pulsatility during the first bleeding period and that low frequency/high amplitude LH pulses were generally present during the second bleeding period. This variability in LH release in between two altrenogest administrations (24h) may explain why follicular growth progresses to 5mm during altrenogest treatments. LH pulsatility was related to length of the follicular phase and ovulation rate, which signifies its relevance.
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