Dietary modification in dairy cattle: Field measurements to assess the effect on ammonia emissions in the Basque Country

Ammonia derived from cattle, especially dairy cattle, is one of the main types of atmospheric pollution. The amounts of N in excreta and subsequent emissions of ammonia can be reduced by manipulating cattle diets. Ten lactating cows were fed diets supplying different N intake and forage:concentrate ratio and the resulting slurries obtained were applied to grassland to study the emissions of ammonia to the atmosphere in the Basque Country. The forage:concentrate ratio was 2.5 in the lower N intake or diet A (405 g N d−1) and 1.0 in the higher N intake or diet B (498 g N d−1), respectively. Decreasing the N intake led to 11% decrease of N output (faeces and urine). This affected the N composition of the slurry, and larger amounts of slurry from diet A were applied to the soil to provide the same amount of N than in diet B. Slurries were applied to soil to provide 120 kg NH4+–N ha−1. When equal amounts of N (120 kg N ha−1) were applied to soil via the slurry, there was no significant effect on ammonia emissions with no effect of the diet supplied, possibly because of the high variability in emissions and the effect of edaphoclimatic conditions. Cumulative NH3 emissions over the 65 h following application of the slurry to soil resulted in 5.7 kg NH3–N ha−1 emitted from soil treated with diet B-derived slurry, while for diet A, 12.6 kg NH3–N ha−1 were emitted. In terms of ammonia volatilization per amount of slurry applied, no statistical differences between treatments were found, with 142.2 and 100.6 mg NH3 kg−1 of slurry applied for treatments A and B, respectively. Thus, dietary N control does not guarantee that ammonia emissions would be reduced under the same N application doses, nevertheless, the amount of slurry applied to reach the same N doses decreases in the lower N intake diet.