Predicting orthophosphate in feces and manure from dairy cattle

Abstract

Dairy cattle excreta are a valuable source of orthophosphate (Ortho-P), an inorganic form of phosphorus (P) that is readily available for microorganisms, plant growth, and development. There is, however, a growing environmental concern about the potential negative environmental impact of excessive amounts of Ortho-P excretion, which can lead to the eutrophication of water bodies. As a result, the development of mathematical equations to quantify and manage Ortho-P excretion on dairy farms could prove valuable for environmental sustainability. This study aimed to use literature data to develop empirical predictions for Ortho-P (g/kg dry matter [DM]) excretion using total P (TP [g/kg DM]) content of dairy cattle feces (Ortho-P_f) and manure (Ortho-P_m). Data sets from studies that evaluated and characterized the different forms of P in feces and manure from dairy cattle were compiled. After outlier exclusion, the final retained database for feces included 37 treatment means from 4 published papers while the manure comprised 23 treatment means from 7 published papers. A linear-mixed model was used to develop the predictive equations, incorporating the random effect of the study. A leave-one-out cross-validation procedure was used to evaluate the predictive ability of the developed models, whereby studies were regarded as folds. The fecal equation was determined as Ortho-P_f (g/kg DM) = -2.447 (0.572) + 0.966 (0.083) × TP (g/kg DM) (R² = 0.79) and resulted in a root mean square prediction error as a percentage of mean observed value (RMSPE, %) of 32.8% and error due to random sources of 97.6%. Additionally, the manure equation was determined as Ortho-P_m (g/kg) = -0.204 (0.446) + 0.590 (0.065) × TP (g/kg) (R² = 0.77) and had an RMSPE of 43.3% with a random error of 93.9%. Both models revealed minimal mean and slope biases on feces and manure data. Findings suggest that these sets of equations can be used to estimate excreted Ortho-P from total excreted P, helping nutritionists and farmers to understand the impact of dietary P changes on the environment. Further, these equations can be incorporated into extant models such as the Cornell Net Carbohydrate and Protein System (CNCPS) to aid in understanding and mitigating P and Ortho-P excretion from dairy cattle and to clarify the portion of P that migrates more rapidly into watersheds.

Summary: Orthophosphate (Ortho-P) is an essential nutrient for plant growth and development when applied in appropriate amounts; however, it can be detrimental to the environment by causing eutrophication of waterbodies when present in excess. Empirical mathematical equations can be used to manage and monitor its use and discharge to eliminate negative impact on the environment. To address this issue, predictive models of Ortho-P (g/kg dry matter) release using total phosphorus (TP; g/kg) content of dairy cattle feces (Ortho-Pf) and manure (Ortho-Pm) were constructed. Both proposed models showed minimal systematic bias. Therefore, these easily applicable models can be used without the need for costly, laborious laboratory analysis.

Figure 1

Plots of observed versus predicted fecal or manure orthophosphate (Ortho-P; g/kg; black diamonds), and residuals (red squares: observed − predicted values) versus predicted Ortho-P (g/kg; black diamond shapes represent the observed fecal or manure orthophosphate) generated from fecal (n = 37) and manure databases (n = 23). The solid blue lines indicate the relationship between predicted and observed fecal or manure Ortho-P and predicted values and the residuals. The solid black lines represent the line whereby predicted = observed values.