A novel test for measuring and managing potential phosphorus loss from dairy cattle feces

Pollution of waters resulting from phosphorus (P) runoff from agricultural land receiving long-term manure application is one of the most serious threats to water quality in many regions of the world. Of various approaches to alleviate the problem, reducing P surplus on animal farms through optimizing P intake and minimizing P excretion in manure offers a great opportunity. Here, we present a fecal P test method that has the potential to identify over-feeding of P in dairy cattle. Previous research has suggested that water-extractable P in dairy cow feces closely reflects dietary P changes and may indicate the animal's P status (adequate vs excessive). However, the notion was somewhat confounded when a subsequent study found other factors (pH and Ca content as well as sample handling method) also affecting P extractability in water. In the present work, we hypothesize that the impact of those factors on P extractability can be overcome by selecting dilute acid solutions to replace deionized water as the extractant. Using samples from 25 commercial dairy farms, we tested an array of acid solutions (including HCI, citric acid, and acetic acid) and found that 0.1% HCI is the most suitable extractant. Inorganic P (P(i)) released in 0.1% HCl closely reflected dietary P changes among the farms (R2 = 0.69) and was independent of pH, Ca, or sample handling method. Knowledge of P metabolism and partitioning in dairy cows and our experimental data suggest that excess P intake by the animal leads to greater amounts of bioavailable but unabsorbed P, which is excreted in feces. Its relative magnitude may be estimated by measuring P(i) extractable in 0.1% HCl. This novel and simple fecal P test could potentially be used as an indicator of the animal's P supply utilization status and thus serve as a screening tool for the presence of P over-feeding on dairy farms.