INTERACTIONS OF TALLOW AND HAY PARTICLE-SIZE ON YIELD AND COMPOSITIONOF MILK FROM LACTATING HOLSTEIN COWS

An 18-wk lactation study was conducted to determine whether the effect s of tallow on the lactation performance of dairy cows were influenced by particle size of hay in the ration. A total mixed ration containin g 50% concentrate, 25% corn silage, and 25% alfalfa hay (dry matter ba sis) was fed to Holstein cows. Four total mixed rations were developed based on differences in the percentage of tallow in the concentrate a nd particle size of alfalfa hay: 1) 0% tallow, long-cut hay; 2) 0% tal low, short-cut hay; 3) 5% tallow, long-cut hay; and 4) 5% tallow, shor t-cut hay. Ration had no effect on dry matter intake, body weight gain or change in body condition score. Tallow increased milk and milk pro tein yields but reduced milk protein concentration. However, the effec ts of tallow on milk and milk protein yields were the same, regardless of hay length in the ration. A tendency for an interaction of tallow and hay particle size was detected for fat-corrected milk (FCM) becaus e tallow increased FCM more when hay was short. Ration had no effect o n volatile fatty acids in ruminal samples collected via a stomach tube . In this study, the effects of tallow on milk yield and composition f rom Holstein cows were the same, regardless of hay particle size in th e ration. The tendency for tallow to increase FCM more when hay was sh ort suggests at least a limited role of forage particle size in the de termination of how fat supplements in dairy rations affect lactation p erformance.