Concentration and ruminal digestibility of starch in rations of lactating cows has important effects on productivity. Starch is more digestible and less filling than forage fiber and provides more glucose precursors than fiber from any source. Ruminal fermentability of starch is affected by grain and endosperm type, processing and conservation method, and diet and animal factors, and affects production of fermentation acids and microbial protein in the rumen. Excessive ruminal fermentability can decrease fiber digestibility, efficiency of microbial protein production, and alter ruminal biohydrogenation, decreasing synthesis of milk fat and increasing energy partitioned to body condition at the expense of milk.

The concentration and ruminal fermentability of starch affects feed intake, and energy partitioning of cows differently as they progress through lactation. High-producing cows in early to mid-lactation thrive on high-starch rations with highly fermentable starch sources while starch concentration and fermentability should decrease as lactation progresses to maintain yield of milk fat and prevent excessive body condition. Highly fermentable starch sources should be limited in rations for the first two weeks following parturition to avoid further depression in feed intake, and decrease risk of ruminal acidosis and displaced abomasum. Grouping cows by physiological state (fresh, early to mid, maintenance) is required to formulate diets for starch to optimize health and production.