Estimation of Forage Dry Matter Intake in Lactating Dairy Cows Using a Deuterium Oxide Dilution Technique

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Abstract
A method for estimating dry matter intake was investigated in dairy cows fed orchardgrass-bluegrass forage. Four Holstein cows in late lactation were confined to tie stalls and fed green chopped forage to verify accuracy of the technique. Intake of dry matter was estimated using equations that described the kinetics of total body water turnover. Total body water and water turnover rate was determined by a single intravenous injection of deuterium oxide (270 mg/kg body weight). Change in concentration of deuterium oxide in the blood was determined by infrared spectrophotometry for 8 d. Total body water was calculated by zero time extrapolation of deuterium oxide concentration time curve using single pool kinetics. Water turnover rate was defined as reciprocal of the slope of the time curve times total body water. Equation used for calculating forage dry matter intake was dry matter intake (kg/d) = percent forage dry matter .times. [water turnover rate (kg/d)-grain water (kg/d) - drinking water (kg/d) - fecal water (kg/d)]/percent forage water. Fecal water was estimated as 4.435 + .264 .times. water turnover rate. Intake of dry matter was underestimated by 4.6 to 7.2%. Actual differences between predicted and measured intake, however, was only 0.5 to .76 kg/d. Potential exists for this method to determine intake of dry matter of dairy cows grazing forages.