Effects of two levels of feed allocation on IGF‐I concentrations and metabolic parameters in GnRH‐immunized boars

Abstract

Immunocastration of boars leads to a maintenance of growth harmone (GH) and a loss of anabolic hormones [androgens, oestrogens, insulin-like growth factor (IGF-I)] but an increase of voluntary feed intake. The aim of the experiment was to clarify whether IGF-I is increased by increasing feed supply in immunocastrated boars leading to improved anabolism. Two groups of six boars were given 2 or 3 kg of feed (13.5 MJ ME/kg) daily from 18–28 weeks of age. Because in boars feed intake is limited by gonadal hormones, a group with further increased feed supply could not be included. Until week 22 (second vaccination) gonadal steroids in blood were normal but dropped rapidly thereafter. Growth harmone levels did not change following vaccination. Pigs allocated 3 kg feed had 28% higher circulating IGF-I after the second immunization compared with pigs fed 2 kg feed daily. Higher IGF-I was associated with increased weight gain (682.4 g/day vs. 466.7 g/day; p < 0.01) and protein synthesis ((13)C-leucine infusion; 405 g/day vs. 247 g/day, p < 0.01). Protein breakdown (urea) was not different. Body fat (D(2)O) decreased in the low feed group from 15.2% (week 19) to 6.1% (week 25). In the high feed group it remained at the level found before second vaccination (13.7% vs. 15.0%). It is concluded that in the phase of reduced testicular steroids which inhibit appetite it is possible to increase feed intake which in turn increases IGF-I and protein deposition without accumulating excessive fat.