Impacts of diet on nutrient excretion and gas emissions of grow-finish pigs

Abstract

Swine producers in the United States are tasked with meeting the increasing demands for pork while remaining economically efficient and minimizing the environmental impacts of production. From an environmental perspective, reducing nutrient excretion is of particular interest because nutrients in the manure, specifically excess nitrogen and phosphorus, can contribute to non-point source pollution of ground and surface water through runoff and leaching and can impact air quality through gaseous emissions. Diet manipulation can be used as a strategy to improve both economic and environmental efficiency. Precise formulation to meet the requirements of the pigs will minimize the excess nutrients, which are ultimately excreted in the manure. Furthermore, combined use of non-nutritive feed additives, such as organic acids, specifically benzoic acid (BA), and direct-fed microbials (DFM), may further improve the efficiency of nutrient utilization and performance. Therefore, the objective of this thesis was to investigate the effect of feeding BA to grow-finish pigs, alone or in combination with a DFM, on growth performance, nutrient metabolism, and manure emissions compared to a low protein feeding strategy. To accomplish this objective, three experiments were conducted. In Exp. 1, reducing dietary crude protein level by 3% resulted in no differences in average daily gain, feed intake, feed efficiency, or loin eye area, but increased 10th rib backfat compared to pigs fed the control diet. The differences in backfat may be a result of unequal net energy between diets, supporting the concept that low crude protein diets should be formulated on a net energy basis to mitigate impacts on carcass quality. Experiment 2 (Chapter 2) investigated the effects of feeding BA with or without a Bacillus-based DFM on the growth performance of growing and finishing pigs. Feeding BA alone increased average daily feed intake compared to the control diet and improved daily gain compared to a low protein diet. Similar to Exp. 1, pigs fed the low protein diet had increased backfat compared to pigs fed the control diet or BA. Experiment 3 (Chapter 3) sought to understand the effects of BA with or without a Bacillus-based DFM on the nutrient metabolism and gas emissions of growing pigs. Supporting the results of Exp. 2, pigs fed BA alone had improved gain compared to pigs fed the low protein diet. Feeding benzoic acid, alone or in combination with the DFM, did not the apparent total tract digestibility (ATTD) of dry matter, gross energy, phosphorus, or nitrogen. However, the pigs fed BA with the DFM had decreased calcium ATTD compared to pigs fed BA alone. Pigs fed BA or the low protein diet excreted less nitrogen in urine compared to the control diet, which contributed to improved nitrogen retention in pigs fed BA compared to the control. Furthermore, decreased urine pH in pigs fed BA or the low protein diet resulted in reduced manure pH and ammonia emissions. Overall, research presented in this thesis indicated that supplementing 0.3% benzoic acid to grow-finish pigs improved nitrogen retention and reduced manure pH and ammonia emissions similarly to feeding a low crude protein diet but improved the average daily gain of pigs compared to feeding a low protein diet.