Studies on the growth and carcass composition in Daldale wether lambs. 2. The effect of dietary protein/energy ratio

Abstract

Two experiments on the effect of dietary protein/energy ration on growth and carcass composition in Daldale wether lambs were carried out.In experiment 1, two groups of eight lambs in each group were fed in pens at a restricted intake (30 g air-dry feed per kg liveweight) either on a high protein/high energy diet (HP/HER) (20.7% crude protein) or on a low protein/high energy diet (LP/HER) (10.3% crude protein). The lambs were grown from 24.8 kg liveweight until slaughter at 37.5 kg. In experiment 2, two groups of eight lambs in each group were fed in pens a pelleted diet ad libitum, either on a high protein/high energy ration (HP/HEA) or on a low protein/high energy ratio (LP/HEA). The lambs were grown from 22.0 � 0.8 kg liveweight until slaughter at 37.5 kg. Those lambs on the HP/HE diets grew significantly faster than their counterparts on the LP/HE diets. Accordingly, animals given the HP/HE diets showed a superior feed conversion ratio than animals given the LP/HE diets. Despite these differences, there was no overall differences between treatments either at restricted or at ad libitum intake in the weight of carcass, the dressing percentage, and in the chemical composition of the carcass. In general, the carcasses produced contained a similar amount of fat (30.6%-32.9% of carcass). It was concluded that varying protein to energy ratio in diets affected liveweight gain and feed conversion ratio, but did not affect carcass composition.An experiment was conducted on the diets to investigate some aspects of digestion in the stomach and intestines at the two levels of crude protein.Dietary crude protein level had little effect upon the apparent digestibility of organic matter (OM) and neutral detergent fibre in the whole tract, the proportion of NDF digested in the stomach, the total volatile fatty acid concentration (VFA) in the rumen and the proportions of individual VFA.The flow of rates of digesta from the rumen and abomasum were also similar for the two diets. However, the proportion of the total digestion of OM occurring in the stomach decreased, while the flow of total N and crude protein (CP) (total N minus ammonia N x 6.25) to the intestines and the amounts of N and CP digested therein was higher on the HP diet.The data from the digestion experiment showed that the amount of digestible CP in the intestines per unit of digestible OM intake did not affect carcass composition.