Dr. Kirby Krogstad, Assistant Professor, Department of Animal Sciences, The Ohio State University
Introduction
Cereal grains are an affordable and readily available energy source for dairy cattle because they contain greater concentrations of starch than forages which results in more digestible energy for the cow.
Its important to understand one key concept – not all starch is the same. The starch within cereal grains behaves differently depending on the grain type. This article will highlight the different things to keep in mind when feeding or selecting grains to feed in your dairy cattle rations.
1. Corn is less fermentable than barley, wheat, or oats.
Corn is the most prominent grain fed in dairy cattle rations; this is especially so in diets that include corn silage as the main, or sole, forage source. About 75% of the starch from corn is degraded in the rumen of cattle while starch from barley, wheat, or oats ranges from 80 to 93% rumen degradable.
Figure 1. The rumen degradability of different grain sources. Adapted from Huntington, 1997.
Why is starch degradability important to consider when formulating rations? The greater the rumen degradability of the starch, the greater risk of reduced rumen pH and ruminal acidosis which can depress fiber digestibility. Thus, a more rumen-fermentable starch source like wheat or oats will likely have a greater impact on the rumen environment than a less fermentable starch source like corn. These subtle differences must be factored in when balancing a ration.
2. Vitreousness of the grain effects fermentability.
What the heck is vitreousness? Vitreousness is the level of protein encapsulation of the starch within the grain. Floury grain has a white, opaque appearance, while vitreous grain (meaning “glass like”) has a yellow and glassy appearance. As the vitreousness (or protein encapsulation) of grain increases, the rumen digestibility of the grain steadily decreases which has been observed consistently in research.
How can you measure vitreousness? The kernel must be manually dissected to determine the percentage of vitreousness. Another measure that correlates with vitreousness is kernel density. As kernel density increases, vitreousness increases too. A denser grain will likely have reduced ruminal starch degradability. If you’re feeding a highly vitreous grain, you may be able to feed additional grain without risking ruminal acidosis because its rumen fermentability will be lower.
3. More fermentable grain → reduce total dietary starch to reduce ruminal acidosis risk
If you’re considering switching from less fermentable to more fermentable sources of grain (i.e. from corn to wheat), then you may need to adjust your overall ration. All other things being equal, you may want to reduce total dietary starch in this situation because the more fermentable starch source will more severely reduce rumen pH which increases the risk of ruminal acidosis and reduced fiber digestibility.
When you’re switching grain sources, keep an eye on manure scores, milk fat percentage, milk urea nitrogen, and feed intakes to make sure you’re not running into rumen problems. This is another way to enhance the use of technology like rumination collars or rumen boluses which can monitor rumen pH and rumination times. If either are reduced when changing grains in the diet, you may want to adjust the ration accordingly because ruminal acidosis may be occurring.
Conclusions
Providing fermentable starch from grain is a great strategy to deliver digestible energy to dairy cows in an affordable manner, but don’t forget that each grain source is unique. Corn is a moderately fermentable source of starch; if you switch from corn to oats, wheat, or barley, you may want to consider reducing the dietary starch concentration. Also, the vitreousness and maturity of the grain also play a role in starch fermentability. Pay close attention to the grain you’re feeding and make sure the cows and the rumens can handle it.