Rumen starch digestibility: What drying technique agrees with on-farm cow performance?
By: John Goeser, PhD, PAS & Dipl. ACAN;
Rock River Laboratory Animal Nutrition, Research, and Innovation Director
Commercial laboratories like our own aim for logical laboratory science, accuracy, and ultimately to be in agreement with the cows that will be performing based on results we provide. As a nutritionist, I’ve found that sometimes commercial laboratories lose sight of this aim for cow agreement when developing new tools.
One such tool is starch digestion. The impact of drying techniques on starch has been discussed, but without regard for cow performance agreement. There has been considerable conversation recently centering on laboratory drying techniques. And while nearly 100 years of research papers investigating various drying techniques’ impact on dry matter and nutritional measures exist, we as an industry still leave much yet to learn. One paper discussing microwave drying impact (Sadeghi and Shawrang (2006)) demonstrated that microwave irradiation (drying) has variable effects upon rumen starch digestion, as assessed with in situ rumen incubation techniques. The authors observed digestion measures were related to microwave drying intensity (time dried) and found this drying method both increased and decreased effective rumen degradability. For microwave drying, Rock River Laboratory utilizes a low to medium drying intensity, depending on the initial sample dry matter content. This method is defined by drying a sample in the microwave for anywhere between 1 to 5 minutes over several drying intervals, depending on what it takes to get the sample to 90% dry. Drying using this means is kept to a low heat setting and drying times are sensitive to the sample type as not to burn the sample. In some cases, this could also mean several stages of drying over the course of 5 minutes to get the sample appropriately dry. This drying procedure is followed by a 6mm grind for samples with greater than 20 percent Neutral Detergent Fiber (NDF). Grain samples are not ground.
Is oven drying the answer?
Heuer (2014) compared rumen in vitro (a completely simulated digestion with samples ground at 4 mm) and rumen in situ (actual rumen incubation in live dairy cows with samples ground at 6 mm) starch digestion techniques. The author assessed 24 different oven dried high moisture corn, dry corn grain and corn silage samples in the study. Heuer noted that in vitro and in situ techniques were significantly different, and in vitro methodology over-estimated in situ rumen incubation results. This author further suggested that the in situ rumen incubation method results appeared to be similar to those published by other researchers, and more closely resembled in vivo rumen starch digestion (in live animals).
Since 2014, Rock River Laboratory has employed in situ rumen digestion techniques for starch digestion analysis, using both oven and microwave drying techniques. Outside of wet chemistry analysis, Rock River Laboratory 0, 3, 7 and 16 hour in situ starch digestion Near Infrared (NIR) calibrations are built on both oven and microwave dried samples.
With what does cow performance agree?
Powell-Smith et al. (2015) demonstrated that commercial laboratory in vitro rumen starch digestion was not correlated to in vivo starch digestion for commercial dairies. Later, Schuling et al. (2016) evaluated both in vitro and in situ rumen starch digestion by commercial laboratories. The authors found similar results as Powell-Smith et al. (2015) in that in vitro starch digestion was not related to commercial dairy cattle starch digestion, however, Schuling et al. (2016) found commercial laboratory in situ starch digestion results, measured by Rock River Laboratory, were significantly related to in vivo commercial dairy starch digestion. The authors also found that starch digestion rates (kd), calculated from 7 hour in situ measures, improved the CNCPS v6.5 model milk prediction (R2, or the coefficient of determination which is a common statistical measure of how close the data are to the fitted regression line, improved from 0.69 to 0.76 when using feed library versus in situ measured starch kd, respectively).
So to what does this all boil down? Starch digestion measures will continue to evolve and improve. However, Rock River Laboratory in situ rumen starch digestion is related to actual on-farm commercial dairy cattle starch digestion and the results from this analysis can improve diet formulation accuracy.
Heuer, C.R. 2014. Ensiling and processing of corn silage and high moisture corns and laboratory method comparison of starch digestion in ruminants. M.S. Thesis. University of Wisconsin – Madison, Madison, USA.
Powel-Smith, B., L.J. Nuzback, W.C. Mahanna and F.N. Owens 2015. Starch and NDF digestibility by high-producing lactating cows: A field study. J Dairy Sci. 98:E-suppl. 2. pg. 467.
Sadeghi, A.A., and P. Shawrang. 2006. Effects of microwave irradiation on ruminal protein and starch degradation of corn grain. Anim Feed Sci and Tech. 127:113-123. Schuling, S.E., D. Schimeck, and B. Vander Wal. 2016. Evaluation of in vitro and in situ starch digestibility assays. J Dairy Sci. 99:E-suppl. 1. pg. 777.