Getting ‘the rest of the story’ on fecal starch
Posted on September 29, 2011 in Dairy Performance
By Jon Urness Legendary commentator Paul Harvey always ended his daily lunchtime radio broadcasts with the affirmation, “Now you know the rest of the story.” I’m not so sure we’ll ever know the full “rest of the story” when it comes to all the factors affecting total tract starch digestibility. A survey conducted by Vita Plus nutritionists continues to shed light on what goes into optimizing starch digestibility and reducing the amount of undigested starch that passes out the south end of a northbound dairy cow. With corn futures dancing around the $7 level or higher, efficiently feeding and utilizing starch has been a hot topic. Today, an extra 5 or 10 percent fecal starch passing through an animal can have a huge financial impact. According to Dr. Jim Ferguson, University of Pennsylvania, a 1 percent change in fecal starch content equates to 0.72 pounds of milk yield. We’d like to see less than 5 percent fecal starch in a sample and preferably less than 3 percent. But a significant number of test results are coming back well over that level. Dave Taysom from Dairyland Laboratories, Inc. in Arcadia, Wis. summarized 16 months of results from the lab that showed almost one-third of all samples were higher than 5 percent fecal starch. A Vita Plus survey showed similar results with 32 percent of all samples above that level. So what leads to the higher levels of fecal starch? Here are a few things we found with a Vita Plus field survey of 71 herds. Corn silage processing still a factor You may recall, corn silage processing scores (CSPS) below 50 are considered under-processed, scores between 50 and 70 are adequately processed, and those above 70 are optimally processed. When we look at the best and worst processing scores, we see a considerable difference in the resulting fecal starch. Corn silage moisture a huge factor It’s been repeated numerous times at dairy seminars that corn silage moisture is one of the most important factors in affecting starch digestibility. Our survey bears that out very clearly. Harvesting dry corn silage often results in excessive undigested starch. Is ration starch a factor? Our observations as well as Ferguson’s show very little correlation between the amount of starch in the TMR and fecal starch. This lack of a clear trend may seem counterintuitive. Ferguson explains it this way: “Our correlation with dietary starch and fecal starch was low. There was a small association…but it was weak so I tend to ignore it. I think the data suggests that cows use starch well if it is processed appropriately. There is a trend with higher milk production that all dietary inputs are digested slightly less, probably from higher DMI and faster passage. It makes one think that as production increases, we need more rapidly and available digestible NDF and starch sources.” Time in storage…the big one Our survey showed that fecal starch was almost twice as high for herds fed corn silage stored less than four months compared to corn silage fed more than four months. It was once thought that corn silage needed to remain in storage for an extended amount of time to allow for complete fermentation. Not so. A good fermentation is pretty well complete in a week or two. Instead, University of Wisconsin-Madison research suggests that the real culprit seems to be the protective layer of protein that encapsulates the starch layer in the kernel. Bacteria need to penetrate the coating that surrounds the starch granule to free the starch. Much of this occurs in the rumen, but the process starts in the bunker or silo as bacteria and acid “soften” the protein. Thus, in this case, the more time the better.
Feed quality and nutrition