Behavior of Immunologically-Castrated Males (with IMPROVEST®) in Comparison to Gilts, and Physically-Castrated and Intact Males

Immunological castration of male pigs with Improvest® (Pfizer Animal Health) allows the production benefits of intact males to be exploited while eliminating the risk of producing pork with boar taint. One aspect that is likely to differ between physically- and immunologically-castrated males is behavior, particularly sexual and aggressive behavior; however, there has been little research carried out in this area. Therefore, the objective of this study was to compare the behavior of immunologically-castrated male pigs (with Improvest®) with that of other genders.

The study was carried out using a randomized complete block design (blocking factor date of start on test) with 4 genders: 1) Physically-castrated males (PC), 2) Intact males (IM), 3) Gilts (G), and 4) Immunologically-castrated males (IC) and involved 160 pigs in 40 pens (10 pens/gender) with 4 pigs/pen. Floor space was 1.15 m²/pig. The study was carried out over an 8-week period starting at 16 weeks of age; first and second Improvest® dose (2 ml product) were given at the start of the study (67.2 ± 2.52 kg body weight) and 4 weeks later, respectively. Diets were formulated to meet the requirements of the intact males and the pigs had ad libitum access to feed and water throughout the study. Behavior was observed over a 12-hour period from 0600 to 1800 h at the start of test (first dose) and end of week 2, 4 (second dose), 5, 6, and 8 (end of test) of the study period. General behavior (number of pigs lying, sitting, standing, eating, and drinking) was recorded every 10 minutes on all pens (10 pens/treatment) and aggressive and sexual behavior (frequency of head butts, biting, fighting, and sexual mounts) was recorded continuously during the 12-hour period on a sub-sample of 16 pens (4 pens/gender).

For the first 4 weeks of the study (prior to second dose), general behavior of IM and IC was similar (P > 0.05), however, both genders spent more time standing but less time eating compared to PC. Gilts were intermediate for these traits. Also, IM and IC had a greater (P < 0.05) number of head butts (17.0, 16.0, 7.8, and 5.8 for IM, IC, G, and PC, respectively; SEM 3.60) and sexual mounts (27.3, 25.5, 1.5, and 2.5 for IM, IC, G, and PC, respectively; SEM 4.37) compared to PC and G. During the last 4-wk of study (after second dose), IM and IC spent more (P < 0.05) time standing compared to G and PC (10.1, 9.2, 8.5, and 6.7% for IM, IC, G, and PC, respectively; SEM 0.54), and IM spent less (P < 0.05) time eating (7.1, 10.1, 8.8, and 10.0% for IM, IC, G, and PC, respectively; SEM 0.50) and had a greater frequency of all aggressive behaviors compared to the other genders, which were similar in these respects. Interestingly, differences between IC and PC in eating and drinking behavior were greatest for the observations carried out at the end of week 6 and 8 of the study period, when IC spent more (P < 0.05) time eating and drinking compared to PC.

The results of this study suggest that the behavior of immunologically-castrated males is similar to intact males prior to the second Improvest® dose, and generally similar to that of physically-castrated males in the period following the second dose.

Keywords: Behavior, Immunological castration, Pigs.