Impact Of Ration Starch And Crude Protein For Lactating Dairy Cows

 

Formulating optimal starch and protein levels in lactating dairy cow rations represents a major challenge because of their huge influence on nutrition, production, and health, all of which directly impact dairy farm profitability. Starch is a major energy source for lactating dairy cows when digested in the rumen and/or absorbed in the intestine as glucose. Increasing ruminal starch digestion improves microbial protein synthesis, which is the main amino acid source for absorption in the small intestine. Feeding a low starch ration may affect the nutritional and productive performance of the lactating dairy cow. Reduced production of glucose precursors (propionate) and amino acids needed for the production of milk and milk protein may negatively impact milk production and profitability. Conversely, feeding too much starch may negatively affect cow health due to ruminal acidosis.

 

Feed protein represents another source of amino acids for the production of milk protein by the dairy cow. Rations low in crude protein concentrations can limit ruminal microbial protein synthesis, which reduces the availability of amino acids for milk production. In contrast, feeding cows rations too high in crude protein can affect health and fertility (reproductive efficiency). Therefore, the inclusion of the proper amounts of starch and crude protein in the dairy cow ration may improve dairy farm profitability.

 

The SDSU Dairy Science Department recently conducted a literature review to analyze the relationship between the concentrations of starch and crude protein in the ration on the performance of lactating dairy cows. Five studies were selected for this summary that used early to mid-lactation dairy cows and were published in the Journal of Dairy and/or Journal of Animal Science between 1989 and 2014. These studies used ground shelled and steam-flaked corn, steam rolled barley, dextrose, and molasses as starch sources, along with wheat, soybean meal, fish meal, animal fish protein blend, urea, and forages as protein sources. The concentrations of starch in the rations varied from 15 to 45% and crude protein varied from 11 to 20%.

 

In summary, dry matter intake was not influenced by ration starch or crude protein concentrations. However, milk production and milk protein yield increased by 0.98 lb/d and 0.03 lb/d, respectively, per percentage unit increase in crude protein concentration in the ration. Both, milk production and milk protein yield tended to decrease with crude protein concentration higher than 18%. Increasing crude protein concentration can improve performance, but there comes a point, where further increases (more than 18%) are detrimental. The summary indicated that the flow of essential and non-essential amino acids to the small intestine increased as ration starch and crude protein concentrations increased, which would explain the increased production of milk and milk protein yield.

 

As expected, milk fat percentage decreased 0.03%, while milk fat yield tended to decrease 0.008 lb/d, in response to each percentage unit increase in ration starch concentration. The depression in milk fat percentage becomes more pronounced when ration starch concentrations exceeded 30%. The milk fat percentage depression would be due to high starch rations reducing ruminal fiber digestion, an increase in ruminal bio-hydrogenation of fatty acids, or both, although rumen pH was not affected. Surprisingly, as ration crude protein concentration increased one percentage unit, feed efficiency increased 0.02 units, which was due to the lactating dairy cows having increased milk yield, while maintaining dry matter intake. Thus, increasing crude protein up to 18 % can improve ration digestibility and result in more milk.