Immunocastration is a new alternative to traditional surgical castration in pigs. It involves injecting a modified hormone (GnHR, gonadotropin releasing hormone), usually behind the ear, in two doses: the first about two months before market age, and the second about one month later. A third injection is recommended for heavy pigs kept longer than commonly practiced.
It is the second injection that actually depresses male sexual functions, and as a consequence, reduces the accumulation of offending aromatic compounds in carcass fat. Thus, from a nutritional point of view, we are faced with a male pig, up until the second injection, and a castrated pig right after it. How to feed such pigs has been the focus of much research lately, but in practical terms, offering a different diet is often problematic from a logistics point of view. In brief, we can follow three plans to address this issue.
Feed all pigs -- males, females and castrates -- with a single phase-feeding program designed for females from the end of the nursery period until market age. This program will cause male pigs to grow slower, and castrates to waste nutrients, but it offers benefits in terms of logistics where farms are small and pigs cannot be easily fed by sex, or where multiple diets will increase cost considerably. Here, it needs to be reminded that gilts are considered to be between males and castrates in nutritional terms.
Feed males as males, and then switch to a diet suitable for castrates, right after the second injection. Of course, gilts should be fed their normal diets as always. In this plan, three diets are required to co-exist in a farm, increasing thus the logistics complexity, but ensuring maximal growth at minimal cost. This plan is more suitable for large farms. In this plan, castrates will produce a carcass with fat percentage about the same as gilts. This might be a benefit when carcass uniformity brings a premium at the slaughterhouse.
Feed males and females using a sex-suitable diets, and then feed all pigs with the gilt diet(s) right after the onset of the castration phase. In this plan, males will continue to grow at maximal rates up until the second injection. When switched to the gilt diet, they will over-consume some nutrients, but the savings from not having to handle a new diet can offset this expense. Some careful calculations are required here to ensure the right approach is employed.
In conclusion, there is no best way to feed immunocastrated pigs as we need to accommodate a change in nutrition plan towards the end of the finishing phase. Based on farm capabilities, introducing an additional diet might be an easy proposition, but in other places it might be near impossible. In such cases, the optimum solution can be nothing but a compromise. Quite often all it takes is installing a new feed silo. In anyway, some prior thinking on how immunocastrated pigs are going to be fed will save many headaches later on.