Cattle nutrition as a strategy to mitigate gaseous nitrogen losses from dairy farming

Resumen

Optimisation of dairy cow nutrition has been described as a key factor to reduce N pollution from dairy farms. Nitrogen pollution derived from dairy farming has been historically focused on nitrate losses. However, the study of gaseous N losses has become the main objective in the recent times. Therefore, this PhD-thesis was planned with two main objectives: a) the evaluation of the feasibility of dietary strategies to reduce N pollution in commercial dairy farms from the Basque Country; b) the study of the effect of dietary manipulation (correct protein and energy balance in the rumen or fitting N intake to animal requirements) on cow N balance, slurry characteristics and the subsequent NH3, N2O and NO volatilisation from dairy barn floors and/or slurry amended grasslands. The first trial conducted on 64 commercial farms from the Basque Country showed that protein overfeeding is a common practice for lactating herds. In fact, 69.7% of sampled rations exceeded in N intake, in which exceeding metabolizable protein was estimated by 7.4%. As N intake was the best predictor of N excretion (R2 = 0.7), fitting dietary crude protein (CP) content to cow requirements may be a feasible strategy to reduce N excretion in commercial farms. Other strategies such as the manipulation of the quality of protein fed, the periodical ration reformulation, the use of different feeding groups in lactating herds and the selection of a feeding system did not improve cow N use efficiency. Considering the variability of N excreted per milk kilogram in sampled herds, we estimated that N reduction might reach up to 35.5% in lactating herds when whole milk quota is produced. However, when cow N excretion was referred to farmland availability (intensification characteristics) results showed that the effect of dietary manipulation may be limited in highly intensified farms (dietary N manipulation explained 11.2% of variance on herd N excretion per hectare). In the second experiment, we studied the effect of energy supplementation to isonitrogenous rations on cow N use in order to reduce animal N excretion and the subsequent N accumulation in diet-derived slurries. Slurries were afterwards applied on grassland and gaseous N losses were measured (NH3, N2O and NO). Ration modification from low forage content diets (45:55) (high energy content diets usually used in specialised farms) to high forage content diets (75:25) (lower energy content diets which are usually considered feed and environmentally sustainable) showed that increasing forage content of diets might limit voluntary dry matter intake (especially for high NDF content forages). As a consequence, N intake will be reduced in lactating cows, minimizing therefore N excretion and slurry NH4+-N content. However, milk N use efficiency (NUE) or N excreted per milk kilogram may not be improved due to the lower response of milk yield. Reducing slurry NH4+-N content may involve management and environmental implications when slurry is applied on grassland whether slurry is applied on field to fit plant N requirements or is applied on fresh matter basis to empty slurry storages. The emission pattern of NH3, N2O and NO gases will be similar from high or low forage content diets derived slurries whether equal amount of NH4+-N is applied (120 kg NH4+-N in the current study). The emission factor of high forage treatment was 15.6% (17.8 kg N ha-1), while averaged 9.6% (11.5 kg N ha-1) in low forage diet. Ammonia volatilisation represented 60% of total N gas losses after 2 months of measurements. Ammonia, N2O and NO emission pattern and cumulative emission will vary depending on slurry management on grassland. In the third experiment, we studied the effect of varying dietary CP in isoenergetic diets on mid-late lactating cow N use, and fecal and urinary N excretion. In addition, NH3 and N2O accumulation was measured in barn floors (tie-stall). Results showed that reducing dietary CP (diets contained 14.0% CP, 16.0% CP and 17.0% CP) N excretion decreased. However, milk yield also decreased with lower N intake. Although milk N use efficiency was not improved significantly, N excreted per milk yield tended to decrease through lowering dietary CP content. Increasing CP intake enhanced NH3 concentration from barn floors, whose values ranged from 7.1 mg NH3 m-3 in low protein diets to 10.8 mg NH3 m-3 in high protein diets (atmospheric concentration, 0.4 mg NH3 m-3). This result meant that NH3 concentration is reduced 13% per each unit of dietary CP reduced. The lower NH3 concentration in low CP diets might be explained because urinary urea N also tended to decrease. In contrast to NH3 concentration, on-farm N2O concentrations did not respond to the different dietary CP levels. Treatments averaged 1.21 mg N2O m-3 in low protein diets while high protein rations showed concentrations around 1.10 mg N2O m-3. Nevertheless, these data support that N2O concentration in dairy barns may be significantly different from atmospheric concentration (0.55 mg NH3 m-3). Temperature monitored during the experiment ranged between 12.6ºC and 18.0ºC. The narrow range of temperatures together with the variability detected in N concentration of urinary and fecal samples contributed to mask the effect of temperature on NH3 and N2O accumulation. However, results recorded from fecal and urinary incubations (ratio 2:1) at 3 temperatures (4ºC, 19ºC and 29ºC) demonstrated that temperature affects NH3 concentration in different CP contets. In addition, NH3 concentration may differ between different CP contents at high temperatures. In contrast to NH3, increasing temperature did not enhance N2O concentration.