Relationship between intensive milk production and carbohydrate metabolism in high producing dairy cows

A high-yielding dairy cow with a milk production of 50 kg/day excretes 2,5 kg lactose daily. The same amount of glucose is necessary for the formation of lactose and, in addition, glucose is needed for glycerine synthesis and for pentose phosphate pathway which generates NADPH to satisfy the requirements of fatty acid synthesis. To sum up, the mammary gland of the cow mentioned above requires 3,5 kg glucose/day. (It should be considered that several organs, e.g. brain. and the fetus need also glucose.) Since starch and sugar ingested by the ruminant are fermented to volatile fatty acids by ruminal microbes, the amount of glucose that is absorbed by the gut is very small. Consequently, the dairy cow is constantly dependent upon gluconeogenesis, i.e. glucose synthesis from non-carbohydrate compounds. The main precursors for gluconeogenesis are propionate (50%), glucogenic amino acids (25%), lactate and glycerin (25%), respectively. This enormous amount of carbohydrate has to be produced by the liver and, to a smaller extent, by the kidneys. The process of gluconeogenesis and the carbohydrate metabolism in general of the dairy cow is regulated by different hormones. Insulin has an inhibitory effect on gluconeogenesis. The insulin level of lactating cows is relaively low. Glucagon stimulates gluconeogenesis. Pancreatic glucagon secretion increases after parturition. Cortisol also increases the rate of gluconeogenesis, but the circulating hormone level is relatively low during lactation. Growth hormone has a hypoglycemic effect, because it supports lactose production. Plasma growth hormone levels are positively correlated with milk yield. Since the fermentation of starch in the rumen is accompanied by inevitable losses in heat and gas, recently deliberate attempts have been made to increase post-ruminal digestion. If starch escaping the rumen (bypass starch) can be digested to an extent exceeding 70% of its digestibility in the rumen, such escape should improve the energetic efficiency of production by the dairy cow. It is known that starch digestibility is affected by the plant species, by the physical form of the feed (e.g. granulation) and by the grain processing methods. Among cereals, corn and sorghum have low starch digestibility. With corn, up to 30% or more can escape ruminal fermentation. According to several authors, the capacity for digestion of starch in the small intestine can be as high as 1,0 to 1,5 kg/day in lactating cows. Nevertheless, it should be taken into consideration that the relatively low activity of pancreatic amylase and the rudimentary transport mechanism for glucose in ruminants can reduce the rate of intestinal starch digestion.