Reduction of odorous compounds in fresh pig slurry by dietary control of crude protein

Two diets containing reduced levels of crude protein for both growing (35-65 kg) and finishing (65-95 kg) pigs were tested against standard commercial diets to determine changes in the odorants produced in fresh slurry. The experimental diets were formulated on a least cost basis using either a full commercial database or with restriction mainly to home-grown (UK) cereals and pulses. Synthetic essential amino acids were included in both formulations to obtain essential amino acids in an ideal protein ratio, with the crude protein being as low as possible. Nitrogen excretion by both growing and finishing pigs offered the low crude protein diets was significantly lower (P < 0.001) than that of pigs offered the commercial diets. Major odorants responsible for pig odour from slurries were identified and their concentrations determined using gas chromatography-mass spectrometry. Five out of 10 odorants from slurries produced by growing pigs were significantly lower (P < 0.05) in concentration for each of the low crude protein diets compared with the commercial diet. For the finishing pigs, nine out of ten odorants from slurries for the least cost and four odorants for the home grown diets were significantly lower (P < 0.05) than slurry produced from the commercial diet. Reducing dietary nitrogen and providing essential amino acids in an ideal protein ratio not only decreases the nitrogen excreted by the pig, but also reduces odorants produced in the resultant slurries.