Characterizing the feed intake pattern of immunologically castrated male pigs following administration of the second anti-GnRF injection

Abstract Text: The commercial introduction of an anti-GnRF product (Improvest, Zoetis Inc., Florham Park, NJ) has prompted increased interest in developing optimum feeding programs for immunologically castrated male pigs (IC). One characteristic of IC is a rapid increase in feed intake following administration of the second immunization injection. Understanding the nature of the change in feed intake is central to the development of optimum feeding programs for IC. The objective of this experiment was to compare the feed intake of IC versus physical castrated male pigs (PC) and characterize the dynamic changes in feed intake of IC following the second injection.  One hundred fifty pigs (PIC 359 x C29) of each of the two genders were housed in pens of 5 pigs providing 1.0 square meter per pig on partially slatted floors. Feed and water were available ad libitum. Per pen feed intake was recorded for 2-d intervals starting on the day of the second injection and continued for 14 d.  Weekly feed intake was recorded for the following 2 weeks. Initial body weight for the IC and PC pigs was 97.2 ± 1.8 kg and 101.2 ± 1.2 kg, respectively. During the week immediately prior to the second injection, feed intake of IC averaged 2.91 kg/d compared to 3.52 kg/d for PC (P<0.05). There was little change in IC feed intake for d1-2 (2.83 kg/d) and d3-4 (2.89 kg/d), or in PC feed intake (3.36 and 3.50 kg/d) for the same periods, such that IC feed intake remained at 83-84% of PC (P<0.05). However, IC daily feed intake increased rapidly commencing at d5-6 (86% of PC). This increase continued, such that IC intake was similar to that of PC intake during d9-10 (3.36 vs. 3.46 kg/d; P>0.05) and d11-12 (3.66 vs 3.52 kg/d; P>0.05), but exceeded that of PC starting on d13-14 (3.55 vs. 3.31 kg/d; P<0.05). Interestingly, average daily feed intake of IC relative to that of PC continued to increase during week 3 (3.74 vs. 3.39; P<0.05) and  week 4 (3.86 vs. 3.37 kg/d; P<0.05) following injection. In conclusion, daily feed intake of IC remained constant for the first 4 days following the second injection but then increased relative to PC and continued to do so for the remainder of the 4 week recording period. By 2 weeks post injection, the intake of IC exceeded that of PC.

Keywords: pigs, feed intake, immunological castration