A: The optimum starch content of diets fed to milking cows is not well-defined in controlled research trials, but 24 percent to 28 percent starch diets (DM basis) have been typical in the field. With today’s high corn prices, there is much interest in feeding diets that are lower in starch content than the norm. If low-starch diets are going to be fed, then the starch present in the diet needs to be highly digestible. A research trial summary shows a milk-yield increase of about 5 pounds per cow per day as the digestibility of dietary starch increases from low (80 percent) to high (98 percent). To achieve high starch digestibility, high-moisture corn should be 27 percent to 30 percent moisture, with a mean particle size of 1,500 microns or less. Drier high-moisture corn will need to be ground finer. The mean particle size of dry corn should be less than 700 microns. Sieving tests to determine mean particle size are available through commercial feed-testing labs. Reduced dry matter intake, milk yield and milk protein content and increased milk-urea nitrogen content signal that diet starch content and (or) digestibility are too low.
— Randy Shaver, extension dairy nutritionist, University of Wisconsin-Madison