Immunization against gonadotropin-releasing factor (GnRF) in market gilts: Effect on growth and carcass parameters, and impact of immunization timing.

Reproductive cycling in fattening gilts can be associated with undesirable effects, such as estrus-related aggressive behavior, reduced feed intake and, in production systems where gilts are co-housed with entire males, unwanted pregnancy. Immunization against Gonadotrophin Releasing Factor (IM) can temporarily suppress ovarian activity, including related negative consequences on animal welfare and productivity. Feed intake has been shown to be higher after IM, resulting in both increased growth and increased carcass fat. A series of studies was conducted to confirm these effects on production and look at their dynamics over time. Three trials were performed to a similar design, each involving 240 gilts divided into 4 experimental groups at 12 weeks of age. One group remained untreated while the others had the two dose, IM course completed 8, 6 or 4 weeks before harvest, which was on a single day at 24, 25 or 26 weeks of age depending on the study. Feed intake was measured daily and bodyweight weekly, allowing growth parameters to be calculated on a weekly basis and for specific longer periods. Carcass weight, backfat depth and lean meat percentage were recorded at harvest. No effects were observed before the second application of the immunological product (V2) and completion of the IM course. Starting in the second week after V2 all IM groups showed a marked and consistent increase in Average Daily Feed Intake (ADFI), typically peaking at over 120% of the control group 3 to 4 weeks after V2 and then slowly declining, but still remaining elevated at 8 weeks. Weekly Average Daily Gain (ADG) showed a similar pattern but with a faster decline, resulting in the initially favorable impact on feed efficiency becoming less favorable as the V2 to harvest interval (V2H) progressed. Carcass weights were higher in IM gilts and backfat depths were greater, with the effects increasing with increasing V2H. Correspondingly, carcass lean meat percentage tended to decrease, although the higher carcass weights meant that the absolute weight of lean meat remained similar or higher. Carcass yield was generally unaffected by IM, but some between-group differences were statistically significant, and it is possible that different factors predominated at different times after V2, creating a complex relationship with V2H duration. The optimum IM protocol will depend on local conditions and production objectives but, as a generalization and assuming ad libitum feeding, a shorter V2H will favor efficient growth, while a longer duration will maximize carcass changes, such as increased fat coverage. It is suggested that the growth performance changes seen after IM in gilts might be viewed as a process of adjustment to a heavier and fatter target body type.