*Dr. David Weakley is director of dairy forage research for Calibrate Technologies at Forage Genetics International.
AS economic challenges continue to evolve in the dairy industry, improvements are required in how cows are fed in order to maintain profitability.
One strategy that can potentially improve feed intake, milk production or components or reduce feed costs is to formulate dairy rations using rumen digestible starch data. Relying merely on the crude starch content of ingredients to formulate rations is becoming a thing of the past.
Fermentation of starch by microorganisms in the rumen produces high amounts of propionic acid, in addition to other volatile fatty acids. Propionic acid is a major substrate for glucose synthesis in the liver, which, in turn, fuels lactose synthesis in the mammary gland.
However, while starch digestion in the rumen helps support milk volume, excessive amounts can reduce ruminal neutral detergent fiber (NDF) digestibility (Weakley and Reutzel, 2013). Furthermore, excessive starch digestion in the rumen can negatively affect energy metabolism and dry matter intake (DMI) in lactating dairy cows (Allen, 2000; Allen et al., 2009), resulting in reduced milk and milk fat production.
Unfortunately, most on-farm formulation systems attempt to control dietary starch variation by simply adjusting crude starch in the diet. However, the cow is reacting more directly to the digestibility of starch in the rumen rather than responding to crude starch inputs.
To better provide dietary solutions that are more optimized for cow performance, a method is needed for more rapid, accurate and frequent monitoring of ruminal starch digestibility and supply, particularly when high levels of a variable ingredient — like corn silage — are being fed.
Consequences of variation
Complicating matters further, ruminal starch digestibility can be influenced by many factors, including grain source (Nocek and Tamminga, 1991; Firkins et al., 2001), moisture content (Oba and Allen, 2003), grain processing (Owens 2009) and starch endosperm type (Allen et al., 2003; Phillippeau and Michalet-Doreau, 1998).
Furthermore, starch content and digestibility of some higher-moisture ingredients (particularly corn silage, high-moisture corn and earlage) change over time (Allen et al., 2003), as can be seen with corn silage in Figure 1.
All of these factors lead to a wide variation in the amount of starch digested in the rumen and can lessen the usefulness of static tabular values for both crude and rumen digestible starch in diet formulations, which emphasizes the need for more frequent sampling and testing to adjust diets in response to these changes over time.
Corn silage samples collected from across the U.S. from 2007 through 2012 contained anywhere from 1% to 68% starch, with in vitro ruminal starch digestion ranging from 50% to 96% of starch (Figure 2). Furthermore, the distribution of this variation can differ across years due to differences caused by hybrid effects, growing conditions and harvest management. This can present a major challenge to nutritionists when formulating diets.
One illustration of the effects of hidden variation in ruminal starch digestion can be seen in a study conducted by the author using four corn grain hybrids of different ruminal starch digestibility fed to cows in mid-lactation in a total collection unit (Weakley, 2001). The four grain hybrids were combined with two corn silage hybrids to create eight treatments, all having the same diet formula of 30.7% corn grain and 40.0% corn silage (dry matter basis).
As shown in Figure 3, there were great differences in milk production across the different treatments, even though all eight treatments contained the same formulated crude starch level (25% of dry matter) and had similar total tract starch digestion (92-98%). Formulating to a common crude starch level did little to control variation in milk production.
Another example of the effects of dietary starch levels and ruminal digestibility on performance can be seen in a study conducted with early-lactation cows (Weakley et al., 2011). In weeks 4-12 of lactation, 66 cows (22 per treatment) were fed diets differing in starch level and ruminal digestibility.
After three weeks on a common diet, cows were changed over to either a diet low (20%) in starch or one of two diets high (28%) in starch. The high-starch diets had either all supplemental starch in the form of fine-ground corn or a 50% replacement with fine-ground milo (which had a lower rate of ruminal starch fermentation).
When half of the corn in the all-corn treatment was replaced with milo, DMI was improved by 2.2 lb. per day (P < 0.01) and 3.5% fat-corrected milk production was significantly improved (P < 0.01) by 5.7 lb. per day (Figure 4).
Oba and Allen (2003) observed an 8% reduction in feed intake when providing corn in the diet in a more fermentable form as high-moisture versus dry grain, but only on a higher-starch diet.
These studies support the potential for higher DMI and milk production when replacing more ruminally fermentable starch sources with less fermentable sources, particularly in high-starch diets.
It becomes apparent that controlling the consequences of variation in ruminal starch digestion by using crude starch formulation standards is insufficient to reduce variation in milk production. Rather, a method for rapid and frequent testing of the ruminal starch digestibility of ingredients, combined with formulating to known dietary targets, should better optimize ingredient use and animal performance.
In particular, formulating diets with ingredients containing high levels of fermented starch (corn silage, high-moisture corn, earlage, etc.) should benefit from this approach because of their greater inherent variation in both starch content and ruminal digestibility.
Allen, M.S. 2000. Effects of diet on short-term regulation of feed intake by lactating dairy cattle. J. Dairy Sci. 83:1598-1624.
Allen, M.S., B.J. Bradford and M. Oba. 2009. The hepatic oxidation theory of the control of feed intake and its application to ruminants. J. Anim. Sci. 87:3317-3334.
Allen, M.S., R.J. Grant, G.W. Roth, W.P. Weiss and J.F. Beck. 2003. Effect of endosperm type of corn grain on starch degradability by ruminal microbes in vitro. J. Dairy Sci. 86(Suppl. 1):61.
Firkins, J.L., M.L. Eastridge, N.R. St.-Pierre and S.M. Noftsger. 2001. Effects of grain variability and processing on starch utilization by lactating dairy cattle. J. Anim. Sci. 79(E. Suppl.):E218-E238.
Nocek, J.E., and S. Tamminga. 1991. Site of digestion of starch in the gastrointestinal tract of dairy cows and its effect on milk and composition. J. Dairy Sci. 74:3598.
Oba, M., and M.S. Allen. 2003. Effects of corn grain conservation method on feeding behavior and productivity of lactating dairy cows at two dietary starch concentrations. J. Dairy Sci. 86:174-183.
Owens, F. 2009. Effects of chemical and physical characteristics of corn on starch digestion. Proceedings of Tri-State Dairy Nutrition Conference. p. 129-142.
Philippeau, C., and B. Michalet-Doreau. 1998. Influence of genotype and ensiling of corn grain on the in situ degradation of starch in the rumen. J. Dairy Sci. 81:2178-2184.
Weakley, D.C. 2001. Influence of feeding diets containing combinations of four corn grain varieties with two corn silage varieties on digestibility and nutrient balance when fed to dairy cows in mid-lactation. Calibrate Technologies research, Gray Summit, Mo.
Weakley, D.C. 2011. Increasing silage levels in dairy diets using starch and NDF digestibility data. Proceedings of. Mid-South Ruminant Nutrition Conference. Texas Animal Nutrition Council. p. 19-24.
Weakley, D.C., and L.F. Reutzel. 2013. Improving ruminal digestibility. Proceedings of Tri-State Dairy Nutrition Conference. The Ohio State University, Columbus, Ohio. p. 97-115.