Author: Mary Beth de Ondarza
As seen in the Hoards Dairyman, December 2020
TYPICAL U.S. lactating dairy rations containing no added sugars run at sugar levels between 1.5% to 3%. However, we are learning that 6% to 8% dietary sugar may actually be optimal. As we feed more silages and processed feeds, many sugars that would otherwise naturally be in the dairy cow diet have been removed by the shift in ration ingredients. Common sources of supplemental sugar include (with percentage sugar on a dry matter basis); sugar (100%), molasses (70%), whey (70%), candy (40%), chocolate (35%), citrus pulp (25%), cookie (22%), and beet pulp (15%).
It starts in the rumen
Carbohydrates and degradable protein are used by the rumen microbes to make microbial protein. As more microbial protein is made in the rumen, there is less need for expensive dietary rumen undegradable protein (RUP). Rumen microbial protein is high-quality. It contains a blend of amino acids that are easily converted into milk protein. Thus, improving rumen microbial protein synthesis raises milk production.
For efficient growth of the rumen microbes to occur, the availability of carbohydrate and protein to the microbes should be synchronized. If too much protein is supplied without an available source of carbohydrate, the microbes will use the protein as a source of energy and waste the nitrogen found in the protein.
On the flip side, sugars are digested and used by the rumen microbes very rapidly. Studies show that adding sugars to diets almost always reduces the amount of ammonia in the rumen. This suggests that sugars help the rumen microbes capture and use more of the nitrogen in the diet, especially nitrogen coming from rapidly digestible sources such as the soluble protein in silages.
Rumen pH impacted
When the rumen microbes ferment feed, they produce acids. If this acid builds up, rumen pH drops, reducing microbial growth. Subclinical rumen acidosis occurs when the pH of the cow’s rumen drops below 5.8. As a result, feed intake declines and becomes variable, fiber digestibility is reduced, and rumen microbial protein production is limited by rumen acidosis. Milk production and milkfat production are also often reduced.
Dietary sugars can help to control rumen acidity (increase pH) via a few different mechanisms. First, if more sugars end up as part of the rumen microbes, a lower percentage of the total rumen degraded carbohydrate is converted into fermentation acids. Second, because sugars are rapidly available, they are more apt to be converted into the storage polysaccharide, glycogen, by rumen bacteria and protozoa….slowing fermentation to control rumen acidity.
Finally, dietary sugar is more likely to be converted to the volatile fatty acid, butyrate. The production of butyrate generates only one hydrogen ion, while the production of propionate from starch generates two hydrogen ions. So, if sugar is substituted for dietary starch, its fermentation to butyrate would help to reduce rumen acidity. Butyrate also stimulates the rumen epithelial cells, improving absorption of volatile fatty acids from the rumen to reduce rumen acidity.
Canadian researchers replaced corn grain with sugar to produce diets containing either 3% or 6% sugar. The high sugar diets resulted in a higher daily minimum rumen pH (5.61 versus 5.42) as well as a higher mean rumen pH (6.30 versus 6.17).
A common effect of sugar supplementation is an improvement in milkfat percentage and/or milkfat yield. This can be explained by a number of mechanisms. First, sugars typically increase the proportion of rumen butyrate, which is used for milkfat synthesis. Second, if substituting sugars for starch helps to control rumen pH, you would expect milkfat percentage to go up.
The rumen microbes convert dietary unsaturated fatty acids (like those in vegetable oil) to saturated fatty acids (like in butterfat) by a process called biohydrogenation. Researchers have found this process occurs in two general ways; one is normal and one is abnormal.
Very small amounts of the products of abnormal biohydrogenation result in milkfat depression. Added dietary sugars have been shown to promote normal fatty acid biohydrogenation and reduce abnormal fatty acid biohydrogenation. When Wisconsin researchers replaced starch with 2.5%, 5%, and 7.5% sugar, milkfat yield climbed from 3.23 to 3.37, 3.63, and 3.56 pounds per cow per day, respectively. Milkfat percentage changed from 3.81% to 3.80%, 4.08%, and 4.16%.
A dataset with 85 observations from published research was used to determine the effect of adding dietary sugar at 1.5% to 3%, 3% to 5%, and 5% to 7% of dietary dry matter. Sugar sources included molasses, whey, and dry sugar.
Milk yield was 70.2 pounds per cow per day with no added sugar and rose to 73.3 and 72.6 pounds per cow per day with 3% to 5% and 5% to 7% added dietary sugar. Likewise, 3.5% fat-corrected milk (FCM) climbed from 71.2 to 74.4 pounds per cow per day with 5% to 7% added dietary sugar. Milk true protein yield grew from 2.16 pounds per cow per day without supplemental sugar to 2.31 pounds per cow per day with 5% to 7% added dietary sugar.
This net improvement in milk true protein yield suggests an increase in rumen microbial protein synthesis with dietary sugar addition. Milk urea nitrogen (MUN) was numerically lower with additional supplemental sugar, but this change was not statistically significant.
Higher-producing cows had greater responses to added dietary sugar. Cows producing more than 73 pounds per day of milk produced 4.7 pounds per day more 3.5% fat-corrected milk with 5% to 7% added dietary sugar. However, cows producing less than 73 pounds per day only responded with 1.7 pounds per day more 3.5% fat-corrected milk.
Consider supplementing sugar in locating dairy diets to achieve 6% to 8% total diet sugar for optimum rumen function and performance. Generally, 1.5 to 2 pounds per cow per day of supplemental sugar would be needed to achieve 6% to 8% total sugar in typical U.S. diets. Higher-producing cows would be expected to have more positive responses to added dietary sugar. Liquid sugar sources have the added benefit of potentially reducing total mixed ration (TMR) sorting.
Research and field experience suggest the following optimal nutrient ranges based on percent of dry matter (%DM);
- Starch at 22% to 27%
- Soluble fiber at 6% to 8%
- Rumen degradable protein (RDP) at 10% to 11%
Further, consider the impact of starch and protein degradation rates on responses to supplemental sugars. Sugars would be expected to have a more positive effect within a diet containing a lower percentage of rapidly digestible starch. Consider increasing soluble protein and using milk urea nitrogen (MUN) levels as a guide.
Hopefully, future research will help us to characterize and understand the effects of dietary sugars by type (glucose, sucrose, fructose, lactose, and so forth) as well as to define multiple starch pools based on digestion rate. This will help us understand their impact on dietary sugar recommendations.