Eutrophication risk arising from intensive dairy cattle rearing systems and assessment of the potential effect of mitigation strategies

Nitrogen (N) and phosphorus (P) are fundamental nutrients in dairy cattle nutrition, but the majority of N and P fed to dairy cattle is excreted in urine and faeces, which may cause water eutrophication. The aim of the current study has been to assess the eutrophication risk of dairy production in order to evaluate potential mitigation strategies. A group of 16 dairy farms, which follow an intensive rearing system and are located in the northwest of Italy, was involved in the study. Data pertaining to the general characteristics of the farms, to the diet compositions, feed samples and the livestock productive and reproductive performances as well as information on the management practices were collected to evaluate the N and P feed contents of the diets, the N and P contents of the manure and the apparent N and P utilization efficiency at a herd level and for each category of reared animal (i.e., lactating cows, dry cows, heifers and calves). In order to accurately assess the environmental risk, the N release losses into the atmosphere, prevalently as ammonia, were considered, and the different forms of released N and P were converted into their phosphate equivalent (PO₄). On the basis of the characteristics of each farm, some common dietary manipulation mitigation strategies, such as the adoption of precision feeding and the reduction of the protein content of the diet, were proposed to evaluate their potential reduction of the excreted nutrients. The results of the feed analysis showed interesting and notable differences from previously reported N and P contents in the diets of dairy cows. The N balance conducted on these farms showed relatively good fitness of the diet for lactating cows, with only a slight deficit in crude protein supplementation (-2%), while the diets for dry cows were slightly more unbalanced for N (+17%). The P balance calculations revealed a large excess of P in the diets of the farms in this study, especially for heifers (+106%). The highest levels of N- and P-use efficiency were found for cows (20 and 28%, respectively), while the lowest ones were found for young heifers and calves (0–12 months; 7 and 11% N- and P-use efficiency, respectively). It is possible to reduce N and P losses by levelling out this unbalance and in addition, in particular for N, by generally reducing the diet crude protein content by at least 1%. The estimated reduction in PO₄ by dietary manipulation was assumed to decrease the environmental eutrophication risk by 17%. On the basis of these results, and considering the number of the dairy cattle reared in northern Italy, it is possible to estimate that the potential reduction in N and P excretion by dietary manipulation could be equivalent to 5693.5 metric T y⁻¹ or 1 metric T km⁻² of Utilized Agricultural Area per year of PO₄.