Immunisation against gonadotrophin releasing factor increases fat deposition in finisher pigs

Immunisation against gonadotrophin releasing factor (GnRF) is associated with an increase in backfat and carcass fatness and a decrease in lean meat content (Batorek et al. 2012; Dunshea et al. 2013). However, the timing of the increase in fatness and how it is impacted by feed intake and the weight at which the second immunisation of GnRF is given needs to be further explored to enable strategies to be developed to minimise the increase in carcass fatness. The hypothesis was that pigs that have been immunised against GnRF at heavy live weights (80 kg) and fed ad libitum would deposit more fat than those immunised at light live weights (50 kg) or fed restrictively.

Sixty-four individually-housed male Large White × Landrace × Duroc pigs were used in a 2 × 2 × 2 factorial experiment with the main factors being: sex (entire male pigs and immunised male pigs); live weight (LW) at second immunisation against GnRF [50 kg (light) and 80 kg (heavy), Improvac^® (Zoetis Australia, Rhodes NSW)]; and feeding regime [2.5 times maintenance (restricted E_m (kJ/d) = 444 kL × LW^0.75, where E_m = energy maintenance) and ad libitum]. Diets were formulated to contain 13.5 MJ digestible energy (DE)/kg and 0.59 g standardised ileal digestible lysine/MJ DE. The experimental treatments were implemented 28 days before slaughter (68.4 kg LW for light pigs and 106 kg LW for heavy pigs). Pigs were scanned for body composition using dual energy X-ray absorptiometry (Suster et al. 2004) on d 0, 14 and 28 after the second immunisation against GnRF. Data were analysed with ANOVA (Genstat, 15th Edition; UK).

The heavy immunised male pigs deposited 135 g/d less lean tissue than entire male pigs during days 15–28 (P = 0.022) with no difference between sex in the light pigs (Table 1). Fat deposition was not affected by sex during d 0–14 (P > 0.05) but during d 15–28, the immunised male pigs deposited nearly 50% more fat than entire male pigs (P = 0.025). Immunised male pigs fed ad libitum deposited 87.1 g/d more fat during d 15–28 compared to entire male pigs (P = 0.036) with no difference between sex when fed restrictively. The majority of fat deposition occurred during the second 2-week period after the second immunisation against GnRF. However, the increase in fat deposition did not occur in those fed the diet restrictedly. Future research should target ways to decrease carcass fatness in immunised male pigs, particularly during the two to three weeks after the second immunisation.

Table 1.  The effects of sex (S), live weight (LW) and feeding regime (F) on lean and fat deposition for the periods d 0-14 and d 15-28 after the second immunisation against GnRF (n = 8)