Phosphorus mass cycling and balance in dairy farms: Case studies in Puerto Rico

Intensively managed dairy farms in Puerto Rico are suspected of being important contributors of phosphorus (P) in runoff that can reach surface-waters. A whole-farm P balance is a practical tool that can be used to estimate the net amounts of P retained within farms. In addition, knowledge of the internal mass-P cycling within farms (cows, pasture-lagoon, soils and forage) can be used to assess the relative risk of the on-farm agricultural practices to the environment and to identify management practices that reduce P accumulation in soils. Two dairy farms representative of feed and management practices in the Caribbean were assessed to describe internal mass-P cycling and to produce a whole-farm P balance. Farm A with an area of 58 ha had 120 lactating cows that rotationally grazed on improved tropical-grass pastures, and Farm B with an area of 310 ha had 482 lactating cows that were managed in semi-confinement, were fed silage and concentrate during the day, and were grazed only at night. Over 60% (DM basis) of all feeds consumed by dairy cows were imported to the farms, accounting for 71% and 81% of the P consumed in Farms A and B, respectively. The majority of the total P consumed by lactating cows (65% in Farm A and 75% in Farm B) was from commercial dairy grain concentrate, and the remaining P inputs were from grazed and cut pasture. Phosphorus in milk and animals sold (outputs) was 21% of total P inputs in Farm A, and 17.5% in Farm B. Dietary P concentrations, in both farms, exceeded by more than 60% nutritional recommendations of P. On an annual basis, there was an annual surplus (i.e., positive P balance) of 3,132 kg P farm−1 (54 kg P ha−1) and 18,290 kg P farm−1 (59 kg P ha−1) in Farms A and B, respectively. At current milk production levels the practices with the greatest impact potential for reducing soil P enrichment in dairy farms are reducing P inputs in feeds, improving P recycling by maximizing harvested pasture, and reducing or discontinuing fertilizing P additions to grazed pastures.