C.V. Rikard-Bell

Dietary ractopamine promotes growth, feed efficiency and carcass responses over a wide range of available lysine levels in finisher boars and gilts

The aim of this study was to investigate the performance and carcass responses of finisher boars and gilts offered a range of dietary lysine levels and three levels of dietary ractopamine hydrochloride (RAC). The study involved three experiments of 90 pigs each, totalling 270 individually penned pigs in a 2 by 5 by 3 factorial design comprising two sexes (gilts, boars), five levels of dietary lysine [0.40, 0.48, 0.56, 0.64, and 0.72 g available lysine per MJ of of digestible energy (DE), respectively] and three RAC dose regimes (0, 5 and 10 mg/kg) for 28 days. An outbreak of pneumonia (Actinobaccilus pleuropneumonia) at Day 26 in Experiment 1 compromised Day 28 data; however, Day 21 data was considered suitable across all three experiments. The results indicate that 0.56 g available lysine/MJ DE is sufficient to maximise average daily gain (ADG), feed conversion ratio (FCR) and carcass weight in gilts. Control boars indicated that ADG and FCR were not limited by the lysine : energy ratios fed in this study. Increasing levels of dietary lysine linearly increased ADG (P < 0.001), improved FCR (P < 0.001) and increased carcass weight (P = 0.001). Likewise, increasing dietary RAC further improved ADG(P = 0.001), FCR (P = 0.002) and carcass weight (P = 0.075) linearly. The critical lysine levels calculated for ADG and FCR in gilts fed diets supplemented with RAC were less than required for controls. Boars had higher critical lysine levels than gilts when supplemented with dietary RAC, and increasing dietary RAC increased critical lysine levels for ADG and FCR in gilts and boars. An interaction (P = 0.016) between dietary lysine and RAC occurred for FCR, such that the response to 5 mg/kg dietary RAC diminished in diets containing 0.64 g and 0.72 g available lysine/MJ DE; however, these diets elicited a response when supplemented with 10 mg/kg RAC. Responses in ADG, FCR and carcass weight to dietary RAC were noted when dietary lysine was at or below the current recommendations for RAC diets, and it was suggested that this may have been due to reduced efficiency of lysine utilisation due to chronic disease challenge. A Sex X RAC interaction (P = 0.027) occurred for carcass P2, indicating the higher RAC dose reduced carcass P2 in boars but not in gilts. When formulating finisher pig diets between 60 and 90 kg liveweight, consideration of the lysine : energy requirements for boars and gilts is needed in order to maximise ADG, FCR and carcass characteristics. When supplementing pigs with dietary RAC, a wide range of lysine : energy levels maybe employed; however, this is dependent on RAC inclusion level and probably herd health.

Current recommended levels of dietary lysine in finisher pig diets are sufficient to maximise the response to ractopamine over 28 days but are insufficient in the first 7 days

Dietary ractopamine increases lean tissue deposition and responses increase as dose is increased provided sufficient dietary lysine is supplied. In Australia, diets supplemented with ractopamine (RAC) are formulated with 0.56 g available lysine per MJ digestible energy. The present study was conducted to investigate the interactions between dietary RAC and lysine on growth and carcass characteristics in ad libitum fed (13.8 MJ/kg) boars and gilts. The study involved 108 individually penned pigs at 17 weeks of age (64.1 ± 0.57 kg) in a 2 by 2 by 3 factorial design, with the respective factors being sex (gilt or boar), dietary lysine (low and high, i.e. 0.56 or 0.65 g available lysine/MJ digestible energy, respectively) and dietary RAC (0, 5 or 20 mg/kg) for 28 days. Over the 28-day study duration, both lysine diets containing dietary RAC were sufficient to elicit a response in average daily gain (ADG) (+5.8%, P = 0.026) and carcass weight (3%, P = 0.045), but not in feed efficiency (FE) (P = 0.555). However, over the period of the first 7 days, there were interactions between the effects of RAC and lysine for FE (P = 0.025) and ADG (P = 0.023), with both traits being responsive only to dietary RAC containing the high lysine, which increased FE (+9.1%, P = 0.002) and ADG (+7.2%, P = 0.068). Dietary RAC improved FE in the latter stages of the study, namely Days 15–21 (+5.7%, P = 0.031) and Days 22–28 (+4.9%, P = 0.040). The high RAC diet reduced carcass P2 backfat (–16.5%, P < 0.001) and fat tissue deposition (–6.2%, P = 0.074) and high lysine tended to reduce fat tissue deposition (–13.3%, P = 0.072). A sex by lysine interaction (P = 0.043) was observed for lean tissue deposition at 28 days, such that only the high-lysine diet increased lean deposition in boars (+11%, P < 0.05) but not in gilts. Dietary RAC tended to increase lean deposition (+14.0%, P = 0.067) in the first 14 days; however, only the high RAC diet increased lean deposition (+9.6%, P < 0.05) over 28 days. In conclusion, the current recommended supplementation levels of lysine for commercial gilts and boars fed RAC may limit the response to dietary RAC if the feeding regime is for short durations and boars will not maximise their lean tissue deposition rates.