Ontario Dairy Farmer October 2023 Issue, by Laura Martin, M.Sc

COWS EVOLVED ON sugar-rich pastures, and even the rumen microbes have a "sweet tooth" that isn't satisfied with the highly fermented, corn-based TMRs typically found on Ontario dairy farms. Supplementing the ration with some sugar can help improve cow performance, with research showing benefits, beyond just improving the palatability of the ration.

The sugar level of dry forages, like hay, can vary depending on forage variety and harvest timing. Plants make sugar all day and then consume it at night, so hay harvested in the afternoon should have more sugar than hay harvested in the morning. Fermentation of forages consumes a lot of the natural sugars during the process, so these feeds may contribute very little sugar to the ration.

Research has shown that the types of sugars that are left in fermented forages are harder for the rumen microbes to use. Mature grains also have very little sugar as, by harvest time, most of the sugar has been converted into starch.

A typical Ontario dairy ration, with fermented forages and mature grains, may contain low levels of sugar, about 2 - 4 % OM. However, a recent research is an ideal sugar level for milking cows, along with adequate fibre and moderate starch levels.

Supplementing diets with a sugar source (Table 1) can boost the total level of sugar into the ideal range. Adding sugar works especially well in high corn silage rations and rations that have low fibre digestibility. Bakery by-product is a source of sugar and is readily available in Ontario.

It can be used to replace some of the starchy corn in the ration, and is very palatable. However, the sugar level and type depend on the mixture of bakery products and can vary from load to load. Wet beet pulp, from sugar beets, is available seasonally, and can help boost sugar levels. It can work well as a partial replacement for corn silage, especially on low silage inventory years.

Feeding table sugar on farm can be an easy way to get sugar into cows. Commercial liquid sugar supplements often use both whey and molasses in combination. These two ingredients have different sugar profiles, with molasses having mostly sucrose and whey containing mostly lactose, which makes these work well together in the rumen. Liquid sugar supplements have the added benefit of reducing TMR sorting, as they can help the different components of the TMR stick together.

Fibre, the main energy source in most forages, passes slowly through the rumen and can take over a day to digest. Starch is digested faster than fibre but it still takes several hours to degrade, which helps provide a steady source of energy between feeding. Most sugars are digested much more quickly, they can ferment within an hour. Even though sugars ferment more rapidly than starches, they don't pose the same risk for rumen acidosis and reduced butterfat.When sugars are broken down in the rumen, they tend to increase the levels of butyrate, and this plays a big role in how sugars benefit the cow.

Butyrate produced in the rumen is used as a fuel for the lining of the rumen. Butyrate has been shown to be more effective at increasing the development of rumen papillae (the microscopic projections from the rumen lining) than the other volatile fatty acids created in the rumen. This implies that by feeding sugar, and increasing the levels of butyrate, the absorptive surface of the rumen can be increased. Cows with higher levels of butyrate may also be more resistant to subacute rumen acidosis. Multiple studies have supported this mode of action, of sugars through butyrate, by showing that cows fed supplemental sugar in rations have improved rumen pH, fibre digestion, milk fat and milk production.

Feed	% Suger, DM
Table Sugar	100
Molasses	70
Whey	70
Wet Beet Pulp	15
Bakery	10
Pasture	10
Dry Hay	7
Corn	5
Grass Haylage	4
Corn Silage	3
Alfalfa Haylage	2

Cows fed additional sugar will often respond with higher butterfat percentage and yield. Butyrate is used for milk fat synthesis, so the increase in butyrate can help explain this increase in butterfat. Also, the breakdown of sugars in the rumen, unlike starches, does not contribute to the production of trans fatty acids, which are linked to decreasing milk fat levels. Actually, sugars may help reduce the build up of these fatty acids. This, combined with the more stable rumen pH, contributes to the higher butterfat levels associated with feeding sugars. Some trials were using liquid sugar supplements, so the reduction in ration sorting may also explain some of the improved butterfat yields.

Feeding sugars can also increase feed intake in milk cows. One obvious reason is that it sweetens the feed and, as every producer knows, cows have a sweet tooth. Another reason that cows may eat more is that by increasing butyrate, and lowering propionate in the rumen, the cows don't feel full as fast. High levels of propionate are responsible for signalling to the cow that she is full, which is why diets with high starch levels can reduce intakes because the cows are receiving signals that they need to stop eating.

What sugars really do is feed the rumen microbes. Rumen microbes use the fast energy supplied by sugars to convert dietary fibre into energy for the cow. Adding sugar to the diet changes the population of microbes and can increase the production of microbial protein.

This microbial protein is then used by the cow as it is an excellent source of amino acids (protein) that can be used for milk production. When feeding supplemental sugars, it is important to ensure that the ration provides enough rumen degradable protein to support the growth of the microbes. If there isn't enough protein for the microbes to use, the extra energy won't have as big of an impact. The change in rumen environment, from the higher pH to the increase in absorptive surface, contributes to the increase in fibre digestion observed when additional sugar is fed.

This increase in fibre digestion, along with the increased intakes and improved gut health, helps explain why feeding sugars can increase both milk production and milk fat.

Hitting your ration's sweet spot can increase production in a healthy way. Replacing some of the fast-acting starch in the ration with sugar can increase feed intakes, milk production and components.

When adding sugars to the ration it is important to make sure that the right sources are used, and that the other feeds on farm are balanced to optimize the rumen microbes' response. Look at adding sugar to your ration to help improve your cows' performance.