Hay to reduce dietary cation-anion difference for dry dairy cows

Timothy grass has a lower dietary cation-anion difference DCAD = (Na + K) - (Cl + S)] than other cool-season grass species. Growing timothy on low-K soils and fertilizing it with CaCl₂ could further decrease its DCAD. The objective of this study was to evaluate the effects of feeding low-DCAD timothy hay on dry dairy cows. Six nonpregnant and nonlactating cows were used in a replicated 3 × 3 Latin square. Treatments were as follows: 1) control diet (control; DCAD = 296 mEq/kg of dry matter); 2) low-DCAD diet based on low-DCAD timothy hay (L-HAY; DCAD = - 24 mEq/kg of dry matter); and 3) low-DCAD diet using HCl (L-HCl; DCAD = - 19 mEq/kg of dry matter). Decreasing DCAD with L-HAY had no effect on dry matter intake (11.8 kg/d) or dry matter digestibility (71.5%). Urine pH decreased from 8.21 to 5.89 when L-HAY was fed instead of the control. Blood parameters that decreased with L-HAY were base excess (− 0.4 vs. 3.8 mM) and HCO₃⁻ (23 vs. 27 mM), and blood parameters that increased were Ca²⁺ (5.3 vs. 5.1 mg/dL), Cl⁻ (30.5 vs. 29.5 mg/dL), and Na⁺ (60.8 vs. 60.1 mg/dL). Compared with the control, L-HAY resulted in more Ca in urine (13.4 vs. 1.2 g/d). Comparing L-HAY with L-HCl, cow dry matter intake tended to be higher (11.5 vs. 9.8 kg/d), and blood pH was higher (7.37 vs. 7.31). Urine pH; total dry matter; Ca, K, P, and Mg apparent absorption; and Ca, K, Na, Cl, S, P, and Mg apparent retention were similar. Absorption as a percentage of intake of Na and Cl was lower for L-HAY as compared with L-HCl. In an EDTA-challenge test, cows fed L-HAY regained their initial level of blood Ca²⁺ twice as quickly as the control treatment (339 vs. 708 min); there were no differences between L-HAY and L-HCl. This experiment confirms that feeding low-DCAD hay is an effective means of decreasing the DCAD of rations and obtaining a metabolic response in dry dairy cows.