Effects of dietary cation-anion difference and potassium to sodium ratio on lactating dairy cows in hot weather

Forty-two lactating Holstein cows 188 ± 59 d in milk were used in an 8-wk randomized complete block trial with a 2 × 3 factorial arrangement of treatments. The objective was to determine the effects of high dietary cation-anion difference (DCAD) and K:Na ratio on milk yield and composition and blood acid-base chemistry. Treatments included DCAD concentrations of 45 or 60 mEq (Na + K −Cl)/100 g of feed dry matter and K:Na ratios of 2:1, 3:1, or 4:1. Mean DCAD values were later determined to be 41 and 58. Dry matter intake was similar across treatments. Yield of milk and energy corrected milk were lower for the 3:1 K:Na ratio compared with 2:1 and 4:1 ratios. Blood urea N was lower for the highest DCAD, suggesting that DCAD possibly reduced protein degradation or altered protein metabolism and retention. Mean temperature-humidity index was 75.6 for the duration of the trial, exceeding the critical value of 72 for all weeks during the treatment period. Cows maintained relatively normal body temperature with mean a.m. and p.m. body temperature of 38.5 and 38.7°C, respectively. These body temperatures suggest that cows were not subject to extreme heat stress due to good environmental control. Results of this trial indicate that the greatest effect on milk yield occurs when either Na or K is primarily used to increase DCAD, with the lowest yield of energy-corrected milk at a 3:1 K:Na ratio (27.1 kg/d) compared with ratios of 2:1 (29.3 kg/d) and 4:1 (28.7 kg/d). Results also suggest that greater DCAD improves ruminal N metabolism or N utilization may be more efficient with a high DCAD.