Nitrogen (N) excretion and emissions can be reduced in fattening pigs by reducing dietary crude protein (CP) levels. Effects of this strategy are well documented for moderate CP reduction, but little literature exists on further CP reduction made possible by free isoleucine, histidine, and leucine. This trial evaluated the effects of 2 levels of reduction in CP on growth performance, N balance, and gaseous emissions. Forty-eight pigs were allocated to 12 gaseous emission-measuring chambers at 28 kg live weight. Three dietary treatments (CTRL; -1.2pt; -2.4pt), with a CP content, respectively, of 18.1%, 16.9%, and 15.0% in phase 1 (28 to 48 kg) and 16.1%, 15.0%, and 13.8% in phase 2 (48 to 80 kg), were fed ad libitum. Growth performance was recorded for each phase. Body lipid and protein composition were analyzed by dual-energy X-ray absorptiometry in 2 animals per chamber at the beginning and end of the trial. These results were used to calculate the N balance. Slurry volume and composition were measured at the end of the trial. Ammonia, methane, and nitrous oxide emissions were recorded continuously. Data were analyzed with a general linear model including the linear and quadratic effect of CP reduction and phase as fixed effects. A trend for a quadratic effect of CP reduction on feed intake was observed (P = 0.085) with a decrease from CTRL to -1.2pt and then an increase with the -2.4pt treatment. Daily gain and gain to feed were improved by the low CP diets in phase 1 but were degraded in phase 2 (phase × CP interaction, P < 0.001) while body composition was not affected. Nitrogen excretion decreased with CP reduction (linear effect, P = 0.023) but tended to plateau with the low CP diets (quadratic effect, P = 0.081). Methane emissions per kg of gain were reduced with CP reduction (linear effect, P = 0.031). Ammonia emissions decreased with CP reduction, mostly in phase 1 (phase × CP interaction, P = 0.015); however, the emission factor (g N-NH3 / g urinary N) was not affected. Nitrous oxide emissions were not affected by CP reduction and its emission factor (g N-N2O / g N) tended to increase (linear effect, P = 0.07). Slurry ammonia-N decreased (linear effect, P < 0.023; quadratic effect, P = 0.049), but other slurry components were not significantly impacted. In this trial, a plateau in N excretion and emission reduction was reached with the -2.4pt treatment. Nevertheless, it is important to study such a reduction in conditions closer to commercial ones.
Keywords: crude protein; gaseous emissions; nitrogen; pig.
Dietary crude protein (CP) reduction has been proposed as a strategy to reduce the environmental impacts associated with nitrogen (N) emissions such as acidification, eutrophication, and climate change. This strategy reduces N intake while maintaining N retention through free amino acid (AA) supplementation, reducing N excretion and subsequent emissions (e.g., ammonia, nitrous oxide). The effects of reducing dietary CP have been well documented and quantified in the literature. However, few experiments have tested the effects of very low CP levels, which can be reached with isoleucine and histidine supplementation, on performance, N balance, and emissions. This experiment evaluated those effects through a trial in emission-measuring chambers, with standard low CP and very low CP treatments. While the daily gain was maintained over the treatments, feed intake and feed conversion ratio were reduced from the control to low CP treatment and then increased with the very low CP treatment. Nitrogen efficiency was also reduced from the control to low CP treatment but remained the same from the low CP to very low CP treatment. This resulted in less reduction in N excretion and ammonia emissions than expected for the very low CP treatment. Nitrous oxide emissions were not affected by CP reduction. This work showed that there is a level of CP reduction after which reduction of CP no longer reduces N excretion and emissions. Further research is needed to identify this limiting level.
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society of Animal Science.