Milk Fat Synthesis on Restricted-Roughage Rations Containing Whey, Sodium Bicarbonate, and Magnesium Oxide1,2

Abstract

A study was conducted to elucidate the mechanism through which partially delactosed whey and minerals (NaHCO3 plus MgO) increase milk fat synthesis on restricted-roughage rations and to determine the minimum effective level of the whey. Thirty-six Holstein cows were fed concentrate ad libitum and 2.3kg hay/day for eight weeks. Concentrates contained varying levels of whey (0, 3, 7, and 14%), or minerals (2.5% NaHCO3 and 1% MgO), or both whey (14%) and minerals. Fat depression was least on the mineral rations, but concentrate intake and milk yields were also lowest. As whey in concentrate increased, milk fat also increased, with 14% whey almost as effective as minerals. Compared to controls, rumen pH was increased by minerals but not by whey. Both minerals and whey decreased rumen propionate and increased acetate; whereas, butyrate was increased only at 14% whey. Increased mammary uptake of fatty acids from heparin-precipitable lipoprotein was directly proportional to arterial concentrations and accounted for almost all of the increased milk fat synthesis on the mineral rations, but for about one-half the increase on whey. A larger uptake of β-hydroxybutyric acid was shown for the 14% whey than for other rations and was related to higher concentrations of the shorter chain fatty acids (C6 to C14) in milk. Concentrations of stearate and oleate in milk were increased by minerals and by whey; whereas, linoleate was decreased by additives. Stimulation of milk fat synthesis by whey and minerals mediated through increased mammary uptake of fatty acids from serum lipoprotein was not a metabolic reversion to conditions found on a normal ration sufficient in forage.