Low milk fat syndrome and magnesium oxide supplementation.

The low milk fat syndrome most commonly occurs in dairy cows fed diets with a high ratio of carbohydrate to roughage. It is characterized by markedly depressed yields of milk fat. Rumen buffers, such as sodium bicarbonate (NaHCO3) and magnesium oxide (MgO), are known to correct the milk fat depression observed on restricted roughage rations, although by apparently different mechanisms. The mode of action of NaHCO3 can be explained by control of the proportion of ruminal propionate via control of rumen pH. Cows fed diets supplement with NaHCO3 showed increased feed intake and improved digestibility of dry matter or fibre. The mechanism by which Mg increases milk fat concentration may operate at both rumen and tissue level. Most studies with MgO have suggested that it acts as a buffer or neutralizer in either the rumen or the intestine. During digestion, the addition of MgO increases digestibility of dry matter, as does NaHCO3. Among many different tissue effects, MgO supplementation increases triglyceride uptake by the mammary gland. Feeding MgO tends to increase lipoprotein lipase activity in the mammary gland and to decrease the linoleic acid content of milk fat. The percentage uptake of triglycerides from lipoproteins and milk fat by the mammary gland was greater in cows fed MgO-supplemented rations than in cows fed restricted roughage rations. Triglyceride-rich lipoprotein concentrations are uniquely low in cow plasma compared to other species, but still account for a major portion of the arteriovenous difference in plasma lipids across the mammary gland.(ABSTRACT TRUNCATED AT 250 WORDS)