VALIDATION OF INDIRECT MEASURES OF BODY-FAT IN LACTATING COWS

The objective of the study was to establish and compare equations that would estimate the body fat content of lactating dairy cows from diff erent indirect techniques. The techniques used were body condition sco ring, dilution of D2O in body water, and determination of mean fat cel l size diameter of the subcutaneous, abdominal, and perirenal depots. Each technique was validated against direct determination of body fat content of the same lactating cows. To apply equations to high produci ng, lactating dairy cows, cows were from a herd producing more than 95 00 kg of FCM/305 d, were lactating, and were in less than average body condition. Eight days prior to slaughter, a single injection of D2O w as injected into the jugular vein, and body dilution was followed for 4 d. Cows were scored for body condition on the day of injection and w eighed daily for the 4 d prior to slaughter. Samples of subcutaneous, perirenal, and omental adipose depots were taken, and adipocyte size a nd number were determined. Body fat was not predicted well by D2O spac e alone, but inclusion of BW did improve the prediction of body fat fr om this variable. The best equations were derived from use of BW with body condition score or subcutaneous fat cell diameter: observed body fat = -122.1 + .21 x BW + 36.0 x body condition score, and -195.6 + .2 90 x BW + .927 x subcutaneous fat cell diameter; standard errors of th e estimate were 4.6 and 5.5 kg, respectively. Equations using diameter s of abdominal and perirenal fat cells gave similar relationships. Equ ations using all four predictors (live BW, fat cell diameter, conditio n score, and D2O space) were only slightly improved over these equatio ns. Thus, use of body condition score, adipocyte diameter, and BW in l aboratory and field conditions may help in the study and management of the use of body fat in lactating dairy cows.