Nutritional strategies for slimming overfat lambs

Abstract

Two studies were carried out to examine change in body composition of overfat lambs under different dietary slimming regimes with the objective of reducing carcass fat while maintaining or gaining carcass muscle. In the first study, changes in fleece-free empty body composition were examined over 42 days by feeding low energy diets (barley straw) supplemented with different levels of low degradable protein (fish meal) at ad libitum intakes. The second study employed the same supplemented diets but restricted feed intake. The effect of the unsupplemented diet was also examined in this study. X-ray CT imaging technology was used to study the patterns of tissue change in the second experiment.

In a comparative slaughter experiment (Experiment 1), forty overfat lambs (40 kg LW and 15 mm rib fat depth (GR)) were divided into four groups (n = 10/group) and offered one of three pelleted diets supplemented with medium-protein (MP, 101 g CP/kg DM; 6.38 MJ ME/kg DM), high-protein (HP, 189 g CP/kg DM; 7.46 MJ ME/kg DM) and super-high protein (SP, 274 g CP/kg DM; 7.84 MJ ME/kg DM) or a choice of two diets (unsupplemented diet (LP, 62 g CP/kg DM; 5.58 MJ ME/kg DM) and SP) ad libitum for 42 days.

Energy intakes were 416,393,391 and 422 KJ ME/kg LW⁰•⁷⁵/d and metabolizable protein intakes were 2.8, 4.7, 7.0 and 5.3 g/kg LW⁰•⁷⁵/d, respectively for MP, HP, SP and choice-fed lambs. In comparison with the initial slaughter group, changes in body chemical fat were -12%, -13%, -16% and -18%, while changes in body protein were -4%, -5%, -3% and -2%, respectively. Fat was preferentially depleted from the carcass compared to the non-carcass. Changes in carcass fat were -10%, -13%, -17% and -19%, while changes in non-carcass fat were -9%, -8%, -7% and -9%, respectively. Highly active metabolic organs (liver and kidneys) decreased (P<0.001) in final slaughter groups, but they increased in weight as dietary protein intake increased. Blood urea concentrations also increased (P<0.001) with dietary protein intake, but blood glucose and β-OH-butyrate levels showed no significant changes.

Experiment 2 was designed to increase the magnitude of the differential effect on carcass fat and protein seen in Experiment 1 by reducing energy intake to approximately 300 KJ ME/kg LW⁰•⁷⁵/d through restricted feeding of overfat lambs (44.5 kg and 19 mm GR) for 42 days. Groups of lambs (n = 10/group) were offered one of four diets, LP, MP, HP or SP. Animals were scanned by X-ray CT at days 0, 14 and 42 and estimates of tissue size obtained from scan images.

Energy intakes were 209, 285, 292, and 294 KJ ME/kg LW⁰•⁷⁵/d and metabolizable protein intakes were 0.9, 2.1, 3.5 and 5.3 g/kg LW⁰•⁷⁵/d, respectively. Unsupplemented lambs (LP) underwent large losses of carcass fat and carcass muscle (-39% and -26%). Relatively, supplemented lambs showed smaller carcass fat losses (-30% MP, -27% HP and -31 % SP), while maintaining or gaining carcass muscle (-0.5% MP, +5% HP and +8% SP). Liver and kidney weights, as well as urinary nitrogen were positively related (P<0.001) to dietary protein intake. Patterns of tissue change measured by X-ray CT showed a linear loss of fat over 42 days while other fat measurements (subcutaneous fat thicknesses) showed relatively greater rates of loss earlier than later in the experiment. Muscle changes showed that unsupplemented lambs (LP) progressively lost muscle, but supplemented lambs maintained or slightly gained muscle during the experiment.

Lower energy intakes in Experiment 2 were associated with greater fat loss (average -29%) and slight gains in carcass protein (+4%) compared to animals (-15% and -4%, respectively for carcass fat and protein) in Experiment 1. Both experiments showed a non-linear loss of subcutaneous fat i.e. faster for the first 14 to 21 days and slower thereafter.

The impact of tissue changes on carcass value and the costs of feeding were examined to define an optimum slimming strategy. It was concluded that MP diet (101 g CP/kg DM) fed at approximately 300 KJ ME/kg L W⁰•⁷⁵/d over 21 days would have been optimal for slimming fat while maintaining muscle. Higher levels of supplementation (eg HP and SP) resulted in relatively small effects on tissue protein accretion and fat reduction at either ad libitum or restricted intakes.