Effect of duration of altrenogest treatment on farrowing rate and litter size of gilts

The objective of the present study was to compare two common durations of altrenogest (ALT) feeding during different periods of the year on the fertility of gilts after ALT withdrawal. During a 12-month period gilt replacements that were assumed to be cyclic were subjected to oestrus synchronisation with 15 mg/day ALT administered for 18 days (ALT-18; n = 268) or 14 days (ALT-14; n = 153) whereas 275 non-treated gilts served as controls. Fewer ALT-14 than ALT-18 gilts expressed oestrus by 7 days after last ALT treatment (79.1% vs 88.8%; P < 0.05). Farrowing rate was lower (P < 0.05) for ALT-14 than for the other groups (81%, 91% and 92% for ALT-14, ALT-18, and Control, respectively) but farrowing rates were not affected by time of year. Control litter sizes were not different from ALT-14 except during September to November when Control litter sizes were larger than either ALT treatments (13.6 ` 0.33, 12.3 ` 0.65 and 12.7 ` 0.39 for Control, ALT- 14 and ALT-18, respectively; P < 0.05). The ALT-18 gilts had larger litter sizes during January to August. The present data suggest that the appropriate duration of ALT feeding to synchronise oestrus in gilts is 18 days.