We can reliably estimate the digestibility of the combined starch sources in a diet with a single measurement that is both inexpensive and quickly analyzed in the lab. Unlike protein, fat and fiber, which require internal markers and multifaceted interpretation, starch digestibility in dairy cows can be predicted directly from the starch content of fecal samples. Most recently, researchers at the University of Wisconsin found a close linear relationship between fecal starch content and total tract starch digestibility (R2=0.94) across 30 diverse diets and 564 individual measurements (Fredin et al, JDS 2014).
To utilize fecal starch analysis on a dairy farm, producers can take a representative sample from the high- or mid-lactation group (30-150 days in milk). Typically this can be done by mixing 7-10 individual samples in a bucket and submitting a single sub-sample to the lab. Within the lab, samples are oven dried overnight, ground to a fine powder, and analyzed for total starch content. The end result back to the farm is a report that includes the fecal starch content, as well as a calculated estimate of total tract starch digestibility.
There is some debate as to whether optimal fecal starch content is less than 3% or less than 5%, but researchers at the University of Wisconsin recommend any herd with a fecal starch content greater than 5% should investigate their individual feeds for opportunities to improve starch digestibility. Working with a nutrition consultant, a farm may decide to use one of the many other tools available for individual feeds including the Corn Silage Processing Score, grain particle size, in vitro starch digestibility, UW Grain 2.0 or volatile fatty acid analysis.
Results from almost 3,000 fecal starch samples through Dairyland Laboratories suggest there is still a significant amount of unused starch making its way into manure pits. While there are other lab analyses to quantify factors that influence starch digestibility in individual feeds, fecal starch analysis provides a unique opportunity to check the end result of all of these factors, and of all the feeds within a diet, with a single analysis that is accurate, reliable and timely.
Kyle Taysom is Business Development Manager for Dairyland Laboratories, Inc. Contact him via e-mail: firstname.lastname@example.org