The Environmental Impact of Lowering Dietary Crude Protein in Finishing Pig Diets—The Effect on Ammonia, Odour and Slurry Production

Excess nitrogen excretion, ammonia and odour are environmental pollutants associated with pig production. Reducing dietary crude protein (CP) will lower the amount of nitrogen excreted, reducing the potential for ammonia emissions, if diets are adequately formulated to supply amino acids and production performance is maintained. Crude protein content in diets for finishing pigs has been lowered recently, but the quantitative effect of this reduction on ammonia, odour, and slurry output is not well-established. The relationship between ammonia and odour is equivocal, and the effect on slurry production is unclear. The objective of this study was to investigate the effect of lowering dietary CP on ammonia emission, odour emission and slurry output of finishing pigs. Thirty entire boar pigs were individually housed in groups of six, from 10 weeks of age (30 ± 3.0 kg) and offered standard diets. At 75 kg (±1.5 kg) pigs were assigned to one of three treatment diets; (1) 180 g/kg CP, 11.0 g/kg total lysine (High CP), (2) 150 g/kg CP, 11.1 g/kg total lysine (Medium CP), and (3) 130 g/kg CP, 9.0 g/kg total lysine (Low CP). After three weeks on the experimental diets pigs were moved, six each week, to individual calorimetry chambers to measure ammonia and odour emissions. Pigs were offered treatment diets ad libitum. At the end of the recording period, the pigs were removed from the chamber, weighed and feed disappearance recorded to calculate intake. The slurry in each chamber was collected and analysed. The reduction in CP in the diet from 180 g/kg to 150 g/kg resulted in a 22% reduction in ammonia emissions, and from 180 g/kg to 130 g/kg resulted in a 47% reduction ( p < 0.001). Slurry output from pigs offered the 130 g/kg CP diet was reduced by 39% ( p < 0.001) and dry matter increased by 35% compared to slurry from pigs offered the 180 g/kg CP diet ( p < 0.05). Water usage ( p = 0.017), slurry output and nitrogen and phosphate in the slurry ( p < 0.05, respectively) were found to decrease linearly with decreasing dietary CP content. There was no significant effect of reducing CP on performance or odour emission but hydrogen sulphide emissions decreased linearly ( p < 0.010) with decreasing dietary CP. There was a weak positive relationship between odour emission and ammonia (linear: R 2 = 0.25, p = 0.005) with odour emission reduced as ammonia emission reduced. Reducing dietary CP in finishing pig diets could reduce ammonia emissions, water usage and slurry and nutrient output from pig production.