There has been increased interest over the last few years in maximizing total-tract digestibility of starch from corn fed to dairy cows. This interest was primarily driven by the high price of corn over the last few years. Even with recent sharp decreases in corn prices, dairy producers and nutritionists still see value in improving total-tract starch digestibility as a tool to improve feed efficiency and potentially increase milk yield. There are several ways to improve total-tract starch digestibility. These include: planting and feeding corn hybrids that contain floury (opposed to vitreous) kernel genetics, feeding ensiled high-moisture or steam-flaked corn, decreasing corn grain particle size, allowing for complete fermentation (at least 5 months) of silage or high-moisture corn, or enzyme application.
For dairy producers struggling to obtain a total-tract starch digestibility above 95%, consider the following two strategies. First, decrease the particle size of dry ground corn or high-moisture corn. Recent research from the University of Wisconsin-Madison reported that total tract starch digestibility improved from 93 to 98% as particle size of dry ground corn grain decreased from 1270 to 552 µm (Fredin et al., 2014). Secondly, replace dry ground corn grain with high-moisture corn grain or steam-flaked corn. In a meta-analysis describing the effects of cereal grain type and processing methods on nutrient digestion by dairy cows, total-tract starch digestibility was increased for ensiled (94.2%) and steam-flaked corn (93.9%) compared with dry ground or rolled corn (92%).
It has been suggested that total-tract starch digestibility can be increased by reducing dietary starch content. Strategies to reduce dietary starch often include partially replacing dry ground corn grain with forages such as corn or hay crop silage, with non-forage fiber sources, or with molasses or glycerol. A reduction in dietary starch content often coincides with a greater proportion of starch coming from corn silage and less starch coming from corn grain. Since starch in well fermented corn silage is more digestible than starch in corn grain, total tract starch digestibility is expected to increase. However, a review of the literature suggests that total tract starch digestibility does not always increase when reduced-starch diets are fed. The figure above shows the effect of feeding reduced-starch diets on total-tract starch digestibility in lactating dairy cows. The dataset was compiled from research published in the Journal of Dairy Science from 1993 to 2014 where lactating dairy cows were fed high- or reduced-starch diets and represents 52 total treatment means from 26 peer-reviewed studies. The change in dietary starch content is presented as the x-axis. A value of 15 indicates starch concentration was reduced 15 percentage units in the reduced-starch diet.The change in total-tract starch digestibility is presented as the y-axis and indicates feeding reduced-starch diets has no effect on total-tract starch digestibility. Ferraretto et al. (2012) also reported that total-tract starch digestibility was unaffected by dietary starch content. Many but not all of the studies replaced corn grain with forage, non-forage fiber sources, or sugar. The unexpected response of reduced-starch diets on total-tract starch digestibility may have be due to increased DMI and a subsequent increase in rate of passage of starch for cows fed reduced-starch diets.
For dairy producers and nutritionists looking to improve total tract starch digestibility, decreasing corn grain particle size or feeding high-moisture or steam-flaked corn are likely the safest bets. When corn prices are high, reducing dietary starch content will likely decrease feed cost but should not be expected to improve total-tract starch digestion.