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 growin g pigs. Sixty barrows weighing about 65 kg were used. In Trial 1, four semi synthetic diets with CP content decreasing from 18.9 to 12.3% were formulat ed. In Trials 2 and 3, two diets with 17.4 and 13.9% CP were formulated usi ng conventional ingredients. In the three trials, diets were supplemented w ith variable amounts of industrial AA in order to maintain a constant stand ardized 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 T rials 1 and 2, feed was given in four meals per day, whereas, in Trial 3, t wo feeding frequencies (two and seven meals per day) were compared. Five or six N and energy balance (indirect calorimetry) measurements were conducte d for each treatment, and components of heat production were estimated. Res ults of Trial 3 showed no effect of meal frequency on either N or energy ut ilization. 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 l ow-CP diets was accompanied by a reduction in urinary energy loss equivalen t to 3.5 kJ/g of decrease in protein intake. Data of the three trials indic ated that heat production was lower when CP was reduced (-7 kJ/g decrease i n protein intake). This lower heat production was attributed to a reduction of the thermic effect of feed, whereas heat production associated with phy sical activity and maintenance were not affected. Reduction of dietary CP w as 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 adequat e AA supplementation. This study also confirms the superiority of the NE sy stem tin comparison with DE or ME systems) for predicting performance and e nergy gain of pigs and controlling carcass adiposity, especially in situati ons of feeds with variable CP contents.