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Altering Dietary Cation-Anion Difference in Lactating Dairy Cows to Reduce Phosphorus Excretion to the Environment

¹ Department of Animal Science, McGill University, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9
² Département de Biomédicine Vétérinaire, Université de Montréal, St-Hyacinthe, Quebec, Canada, J2S 7C6

Corresponding author: L. E. Phillip; e-mail: Leroy.Phillip{at}McGill.ca.

Four early-lactating dairy cows were randomly allocated to 4 diets with dietary cation-anion difference [DCAD; (Na + K) – (Cl^– + S^2–) mEq/100 g dry matter)] values of +14, +18, +24, and +45. Diets were formulated to be isoenergetic and isonitrogenous, and supplied similar levels of P (0.46%) and Ca (0.77%). The salts, MgCl₂, MgSO₄, K₂CO₃, and NaHCO₃ were used to alter DCAD. The main objective of the study was to ascertain whether a decrease in DCAD would reduce fecal P excretion in lactating dairy cattle. The experiment was conducted as a 4 x 4 Latin square design with 21-d periods. During the last 5 d, diets were offered at a restricted level and samples of blood, milk, feces, and urine were collected. Measures of acid-base status of the cows were linearly related to DCAD, but the animals did not experience metabolic acid stress. Neither fecal P nor urinary P was affected by DCAD, and there was no change in overall P balance. Plasma P tended to increase and blood concentrations of ionized Ca were enhanced as DCAD decreased; P excretion in milk showed a quadratic response to DCAD. Milk yield and milk composition were unaffected by changes in DCAD. Although DCAD may have influenced P homeostasis in lactating cows, there was no evidence that, within the range of + 14 to + 45 mEq/100 g dry matter, DCAD could be used as a nutritional strategy to reduce manure P from dairy cattle.

Key Words: phosphorus excretion • cation-anion difference • dairy cattle • acid-base balance

Abbreviation key: DCAD = dietary cation-anion difference, MP = metabolizable protein