Answer provided by Mike Hutjens, extension dairy specialist at the University of Illinois.
Q: Do dairy cows have a starch requirement? I read in a magazine that you recommended 18 to 26 percent starch?
A: Lactating dairy cows have a fermentable carbohydrate (FC) requirement to optimize rumen microbial yield of energy (as VFA or volatile fatty acids) and amino acids (as microbial protein). FC sources include starch (such as corn or barley), sugar (such as molasses or candy waste), and soluble fiber (such as pectins in beet pulp or alfalfa). Each source of FC has different rates of fermentation, fermentation end-products (acetate or propionate), and potential impacts on the rumen environment (such as pH). I prefer to use FC sources rather than non-fiber carbohydrate.
Nutritionists and dairy managers can mix and match these FC sources based on price, fiber levels in the ration, and desired results (milk yield and components). A lactating cow does not have a starch requirement, but I target 18 to 26 percent starch (depending on processing and source), 4 to 6 percent sugar, and 10 to 12 percent soluble fiber in the total ration dry matter to attempt to balance FC levels and sources.
Ration-balancing programs with a computer model can provide interesting results and comparisons when changing the levels and relationships of various FC feed sources.
Bottom-line: Do not "cheat" the rumen microbes — they are critical when optimizing performance with current high feed prices (corn futures exceeding $7 per bushel) and unfavorable milk-to-grain ratios (less than 2-to-1).