Phosphorus repletion of cattle previously exposed to dietary nitrogen and phosphorus deficiencies

This work reports the effects on growth and metabolic performance of steers fed 2 levels of phosphorus (P) during their repletion from a 15-week depletion period when they were fed diets containing various levels of nitrogen (N) and P. The repletion diets containing either 5.6 g P/day (marginal) or 8.3 g P/day (high) were fed to steers for a period of 12 weeks and were based on barley straw, molasses, sorghum grain, urea, and formaldehyde-treated gluten [mean dry matter (DM) digestibility of 0.62 and 14.6 g N/kg DM]. These repletion diets were formulated to simulate the dietary regimens of cattle grazing wet-season pastures in northern Australia and other tropical and subtropical regions. Food intake, liveweight gain, plasma metabolites, and P kinetics were measured. There was no significant difference between the repletion marginal- and high-P treatments in the mean liveweight (LW) gains and DM intakes (g/kg LW) of the steers. However, for both P treatments, the steers that had been fed a suboptimal level of N during depletion had marked increases (>60%) in feed intake throughout the repletion period. Within the marginal-P treatment, the growth rates of steers that had been fed a suboptimal level of N during depletion were higher than for those that had been fed higher N levels, and were similar to those of the high-P groups. Plasma inorganic P, bone thickness, and bone P concentration remained lower for the marginal-P treatment at the end of the repletion period. Furthermore, within the marginal-P treatment, steers that had been fed the high-N diet during depletion were unable to increase bone thickness and had lower growth rates than anticipated for the quality of the diet. Although N balance was positive and similar for both P treatments, the apparent P balance for the marginal-P treatment of 7.6 mg/kg LW was only about half that of the high-P treatment. It was concluded that the requirement for dietary P in steers that had been exposed to a dietary P deficiency, particularly when in association with high N, was higher than the 5.6 g/day provided.

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