Evaluating estimates of phosphorus maintenance requirement of lactating Holstein cows with different dry matter intakes.

The objective was to evaluate estimates of the inevitable fecal loss component of the P maintenance requirement of lactating Holstein cows consuming differing amounts of a low-P diet. The maintenance requirement for P is the sum of inevitable (e.g., unavoidable) endogenous fecal P plus endogenous urinary P when an animal is fed near its true P requirement (i.e., zero P balance). Urinary excretion of P is normally very low in healthy cattle. Inevitable fecal P is the main part of the total P maintenance requirement; it can be expressed as grams of fecal P/kilogram of dry matter intake (DMI). Twenty-one multiparous lactating Holstein cows (55 to 253 +/- 6 d in milk, range +/- SD; 0 to 171 +/- 64 d pregnant) with a wide range of pretrial milk yields (25.3 to 47.3 +/- 1.23 kg/cow per day) were selected to achieve a range in DMI and assigned to treatment groups of low, medium, and high DMI. To obtain an even greater range in DMI, rations fed to cows in the low and medium treatment groups were restricted to 75 and 50% of their pretrial ad libitum intakes, respectively. Dry matter intakes during the experiment averaged 11.3 (low), 15.3 (medium), and 25.1 (high) kg/cow per d, respectively. All cows were fed the same low-P diet (0.26% P, dry basis) throughout the experiment. Phosphorus balances of cows in all treatments were not different from zero and unaffected by DMI. Average daily total inevitable fecal P excretion was 15.3, 18.2, and 26.3 g/cow for low, medium, and high DMI, respectively. Inevitable fecal P excretion was 1.36, 1.19, and 1.04 g/kg of DMI for low, medium, and high and decreased linearly with increasing DMI. The regression equation to estimate inevitable fecal P excretion across the range of DMI was: (g/d) = [0.85 +/- 0.070 (g/d)] x DMI (kg/d) + [5.30 +/- 1.224 (g/d)]; (R(2) = 0.90). This equation can be used to estimate the inevitable fecal P component of the total P maintenance requirement of lactating Holstein cows.