Immunisation against gonadotrophin-releasing hormone (GnRH) increases growth and reduces variability in group-housed boars

Abstract

It is generally accepted that entire boars are leaner and more efficient than barrows but that they have poorer meat quality with an increased risk of boar taint. An additional issue in producing boars to meet market specifications is variation in growth performance, which may be exacerbated by high levels of aggression and mounting behaviour in group-housed animals. Immunisation against gonadotrophin-releasing hormone (GnRH), leading to immunological castration, offers a means of reducing aggression and mounting behaviour while maintaining most of the production efficiencies associated with entire boars. A total of 120 control boars, 120 immunised boars and 60 barrows (physically castrated at 5 days of age) were used in a study conducted in two replicates. Each replicate consisted of two pens each of boars, immunised boars and barrows, each pen containing 15 pigs, and two blocks of 15 individually-housed boars and immunised boars. The group-housed pigs had access to two single place electronic feeders per pen. Immunised boars were given the anti-GnRH vaccine (Improvac) at 14 and 18 weeks of age. In the 2 weeks immediately after the secondary immunisation at 18 weeks, average daily gain was greater in the immunised boars than the boars or barrows, particularly in those that were group housed, as indicated by the interaction (P = 0.035). From 18 to 23 weeks of age average daily gain was greater (P = 0.005) in immunised boars than in control boars and barrows and was greater (P < 0.001) in individually- than in group-housed pigs. The average daily gain of group-housed immunised boars and individually-housed control boars were similar over the 5-week period after secondary immunisation (1090 vs 1099 g/day), being ~20 and 15% higher than the group-housed control boars and barrows, respectively. The average daily feed intake was greater (P = 0.011) in immunised boars than in control boars with barrows intermediate. The standard deviation of bodyweight increased with age but was lower (P from 0.032 to 0.09) in immunised boars than in control boars and barrows between 18 and 22 and 17 and 21 weeks of age. Carcass fighting damage and pork pH were higher (P < 0.05) for control boars than for immunised boars or barrows. In conclusion, immunisation against GnRH increased average daily gain and average daily feed intake while decreasing variation in bodyweight and improving carcass and pork quality.