Protein intake but not feed intake affects dietary energy for finishing pigs

The aim of this study was to evaluate the effect of the level of feed intake and dietary protein level on dietary metabolizable (ME) and net energy (NE) value of complete feeds fed to finishing pigs. Changing feeding level, i.e. feeding pigs restrictively vs. ad libitum, may alter the ratio of protein to lipid deposition. Because protein and lipid deposition differ in their energetic efficiency, the efficiency of energy retention may change and thus affect the dietary NE value.

Twenty-four castrated male pigs, in two blocks of 12 individually housed pigs, were fed either ad libitum (AL) or restrictively (RF), at twice the ME requirement for maintenance (458 kJ/kg^0.75), between 55 and 95 kg body weight. Within feeding levels, pigs received a low-protein (LP; 11.2% CP, 0.61% lysine) or high-protein (HP; 20.2% CP, 0.61% lysine) diets of similar digestible energy content, in random order following a crossover design. Dietary NE was calculated from heat production based on 24-h indirect calorimetry following a 7-day N-balance period. Nitrogen balance was conducted by quantitative collection of faeces and urine.

Feed intake was greater for LP than HP when fed for ad libitum intake (P = 0.001). Protein level did not affect daily gain but HP improved gain:feed (P = 0.003). This experiment demonstrated that dietary ME and NE were not affected by feeding level but were decreased when dietary protein content was increased. Reducing dietary protein reduced urinary energy losses and increased energy retention but did not affect heat production. The effect of dietary protein restriction was already evident on the ME level and carried over to a similar degree to the NE level because the utilization of ME was not affected by protein level. Dietary ME and NE decreased by 0.012 MJ/kg (P = 0.014) and 0.018 MJ/kg (P = 0.062), respectively, for each gram per day N intake. The results suggest that although there was an effect of protein level on NE, the greatest effect occurred at the level of ME. However, the prediction of both ME and NE may be improved by adopting energy values for dietary protein that changes with dietary protein content.