Diet influence on nitrous oxide emissions in lactating dairy cows

Nitrous oxide emissions from soil are of concern since they represent a loss of N from agriculture and they contribute to global warming and the reduction of stratospheric ozone. The present work is part of an integrated research project aiming at reduction of N losses in dairy farm systems. N₂O were quantified over 65 days under two slurry sources. Urine and faeces were collected from two different N intake ratios of one cross-over experiment.

Slurry was applied to the soil at a standard rate of 120 kg N-NH4⁺ ha^-1. Treatments were as follows: Control (C), Slurry from lower N intake ratio (diet A) and Slurry from the higher N intake (diet B). The N intake is presented in Table 1.

Table 1. Diets fed to dairy cattle from which fresh manure was obtained. DM: Dry matter

N₂O emissions were measured using the closed chamber technique using a photoacoustic infrared gas analyzer (Merino et al., 2001)

Table 2. N excretion in urine and feces. Values are the mean for the five cows on each diet.

During the trial, urine and faeces N concentration in diet B was significantly higher than in diet A (Table 2). No stastitically different cummulative N₂O losses were measured during the 65 h period, with 241.2 and 208.5 g N-N₂O/ha for diets A and B respectively, having shown a similar emission pattern during the trial in both diets in function of the soil water content. Thus, under similar N application rates, soil water content has a greater influence on nitrous oxide emissions.