Improving prediction of phosphorus intake of cattle grazing tropical pastures

Summary

Nutritional deficiencies of phosphorus (P) commonly occur in northern Australian rangelands, but estimation in grazing cattle of the diet P concentration and the adequacy of P intake is difficult. Relationships to estimate diet P concentration from measurements of constituents of faeces (total P, acid insoluble P and near infrared spectroscopy of faeces (F.NIRS) to measure diet constituents) in cattle ingesting tropical forage diets were examined using multiple regression models.

A data set representing 167 forages and 601 diet-faecal pairs of samples (usually 3-4, range 1-12 animals per diet) from pen feeding experiments with young Bos indicus cross cattle was analysed. Separate regression models were necessary for grass and grass-legume versus legume diets. Diet P concentration was poorly predicted from faecal P concentration (R2=0.53). Inclusion of measurements of the concentrations of faecal total N (F-TN) and diet total N (D-TN) improved the prediction (R2=0.67). Diet P concentration predicted from faeces was also examined in cattle grazing tropical pastures where there was other information of P status.

Diet P in cattle grazing tropical forages could be estimated from faecal P concentration and F.NIRS with accuracy sufficient to improve field estimation of the diet P concentration.