Effects of 2-Hydroxy-4-(Methylthio) Butanoic Acid (HMB) and Its Isopropyl Ester on Milk Production and Composition by Holstein Cows

Abstract

The esterification of 2-hydroxy-4-(methylthio)-buta- noic acid (HMB) to isopropanol (HMBi) decreases the rate and extent of its ruminal breakdown. The modes of action of HMB and HMBi appear to be different. The quantification of the production response to HMBi has not been done. The objectives of this study were (1) to determine the lactation response to HMB, (2) to deter- mine the lactation response to HMBi, and (3) to evalu- ate whether the response to HMBi is affected by HMB in the diet. Sixty-one Holstein cows (24 primiparous, 37 multiparous) were assigned to 1 of 4 dietary treat- ments 21 to 28 d after calving. The base diet consisted of (on a dry matter (DM) basis) 32.5% corn silage, 17.5% alfalfahay,10%wholecottonseed,and40%ofapelleted concentrate made primarily of ground corn, soybean meal, and blood meal, and was fed for 16 wk as a control diet. To prepare the dietary treatments, the base diet was supplemented with 0.1% of diet DM with HMB (treatment 2), with 0.15% HMBi (treatment 3), or with 0.045% HMB and 0.15% HMBi (treatment 4). Results showed a significant increase in milk yield (2.9 kg/d), protein content (0.15%), protein yield (115 g/d), fat yield (165g/d),andlactoseyield(182g/d)fromHMBi.Supple- mentation of HMB had small and nonsignificant effects on milk yield and composition. There were no signifi- cantinteractioneffectsofHMBwithHMBionanyofthe production traits measured in this experiment. Plasma free Met as a proportion of essential amino acids was increased by HMBi, but not by HMB. Dietary supple- mentation of HMBi increased gross N efficiency ex- pressedastheproportionofingestedNsecretedinmilk. Consequently, HMBi significantly improved N effi- ciency.