Estrous Synchronization and Fertility in Gilts after 14- or 18-Day Feeding of Altrenogest Beginning at Estrus or Diestrus2

A synthetic progestogen (altrenogest) was used to synchronize estrus in 160 Yorkshire and Duroc x Yorkshire gilts (6 to 11 mo old) in four trials. Gilts were fed 15 mg altrenogest for 14 or 18 d beginning either at or near estrus (d -1, 0, +1 or +2) or at diestrus (d +3 to +21). Mean intervals to estrus after treatment did not differ between 14- and 18-d treatments (5.4 +/- .1 vs 5.3 +/- .1 d, respectively), but were slightly longer (P less than .01) in gilts beginning treatment at or near estrus than in diestrus (5.6 +/- .1 vs 5.2 +/- .1, respectively). Average intervals to estrus were similar between treatment groups and among stages of the cycle even though more gilts (P less than .01) beginning treatment at estrus had serum progesterone concentrations greater than 2 ng/ml at the end of progestogen treatment. Although more 18- than 14-d treated gilts (P less than .05) were in estrus on d 5 post-treatment, proportions in estrus from 3 to 10 d post-treatment were similar (greater than 98%). Neither stage of estrous cycle at onset of treatment nor duration of progestogen treatment affected percentages of gilts farrowing (greater than 73%), average gestation length (116.5 d), or total (10.3), live (9.7) and dead (.6) pigs at birth after artificial insemination at the post-treatment estrus. We concluded that 14-d feeding of altrenogest effectively synchronized fertile estrus in gilts regardless of stage of estrous cycle at the onset of progestogen treatment. Although 18-d treatment of gilts with altrenogest improved estrous synchronization precision compared to 14-d treatment, there was no advantage of the 18-d treatment for subsequent farrowing responses.