Energy utilization of low-protein diets in growing pigs

Three trials were conducted to measure the effects of reducing the dietary CP content on digestive and metabolic utilization of N and energy in growing pigs. Sixty barrows weighing about 65 kg were used. In Trial 1, four semisynthetic diets with CP content decreasing from 18.9 to 12.3% were formulated. In Trials 2 and 3, two diets with 17.4 and 13.9% CP were formulated using conventional ingredients. In the three trials, diets were supplemented with variable amounts of industrial AA in order to maintain a constant standardized digestible lysine/NE ratio (0.76 g/MJ) and ratios between essential AA relative to lysine of at least 60, 65, 20, 60, and 70% for methionine + cystine, threonine, tryptophan, isoleucine, and valine, respectively. In Trials 1 and 2, feed was given in four meals per day, whereas, in Trial 3, two feeding frequencies (two and seven meals per day) were compared. Five or six N and energy balance (indirect calorimetry) measurements were conducted for each treatment, and components of heat production were estimated. Results of Trial 3 showed no effect of meal frequency on either N or energy utilization. Reduction of dietary CP content had no effect on N retention or animal performance but markedly decreased N excretion (-40% in Trials 2 and 3, and -58% in Trial 1). In the three trials, the lower N excretion with low-CP diets was accompanied by a reduction in urinary energy loss equivalent to 3.5 kJ/g of decrease in protein intake. Data of the three trials indicated that heat production was lower when CP was reduced (-7 kJ/g decrease in protein intake). This lower heat production was attributed to a reduction of the thermic effect of feed, whereas heat production associated with physical activity and maintenance were not affected. Reduction of dietary CP was associated with higher energy gain, mainly as fat. But, this effect was no longer significant when data were adjusted for similar NE intakes. These results confirm the possibility of limiting N excretion, while maintaining a high level of performance, by reducing CP level in the feed with adequate AA supplementation. This study also confirms the superiority of the NE system (in comparison with DE or ME systems) for predicting performance and energy gain of pigs and controlling carcass adiposity, especially in situations of feeds with variable CP contents.