By Chad Hale, Forage Grass Specialist
“All things in moderation” is a mantra that applies to many things in life. Most of us like sugar but we probably shouldn’t let it be the majority of our diet. Ruminants are no different. They like sugar too and dairy nutritionists have a rule of thumb that sugar should not exceed 5% of the total ration. But should it be included at all? Can’t we get the same energy out of starch? Research shows that sugar has some benefits in ruminant diets. Sugar is more rapidly fermentable in the rumen than starch, which is a good thing if there is ammonia in the rumen from the breakdown of rumen degradable protein. Without sugar, this valuable nitrogen would be lost and excreted.
Everyone knows that sugar tastes good, including ruminants! There is a popular video from Idaho showing cows fighting over hay that was cut in the afternoon versus
hay cut in the morning (more on that later). What they were actually fighting over was the higher sugar in the forage from the afternoon cutting. Grazing livestock almost always prefer grasses to legumes if given the choice. When it comes to making silage, bacteria that ferment the forage also need sugar. High sugar forages ferment faster and more completely than low sugar forages.
So there are compelling reasons to have high sugar forages, but how do you get them? It’s important to realize that different forage species have different sugar levels. Also sugar levels vary in the plants throughout the day in a predictable, diurnal pattern. Addressing the species issue first, Grasses and members of the sorghum family are among the forages highest in sugar content. Legumes are generally the lowest in sugar. An easy way to incorporate more sugar into your livestock’s diet is grow grass with alfalfa or put in some forage sorghum in place of corn for silage harvest.
Management is ultimately the key to harvesting high sugar forages. Many of you have probably heard of the AM/PM hay research done in Idaho. We get many calls each year in the Midwest from producers who tried to cut hay in the afternoon but when the feed test came back it was no better than morning cut hay. Producers out west believe that afternoon cut hay is definitely better. Who is right? They both are! The key is understanding how sugar levels fluctuate in plants and in understanding plant respiration (or breathing).
Photosynthesis uses sunlight to build sugars in the plant all day. So sugar levels are highest in the afternoon when the sun has been out for a long time. Conversely, sugar is lowest in the early morning before the sun is up because the plants have been burning sugar all night long without being able to make any. Respiration is the process of burning sugar. If you cut any forage plants, they will continue to respire until they dry down to about 40% moisture. Respiration rate is faster at high temperatures and lower in colder temperatures.
So applying this to our hay scenario: out west, hay is cut in the afternoon when the level is highest. It still may be above 40% moisture when night comes but the temperatures drop drastically at night, which slows respiration. Humidity is low, so hay may actually dry slightly at night. In the morning, the hay cut the previous afternoon is still higher in sugar than the forage that is still standing uncut. That is the only reason afternoon cut hay is higher in sugar out west. In the humid east, hay cut in the afternoon is almost always still above 40% moisture by nightfall but the higher temperatures and humidity keep respiration cranking all night. Forage harvested in the afternoon is no higher in sugar than standing forage by morning. Soon after the sun comes up, the standing forage may be higher than the afternoon cut hay. In addition, dry matter losses overnight can exceed 10% and the worst part is almost all of that dry matter loss is sugar that the plants burned during the night. Figure 1 shows the relationship between temperature and dry matter loss overnight.
This chart shows the dry matter loss through respiration (C6H12O6 (Sugar)+ 6O2 ——> 6CO2 + 6H2O + Energy) throughout the nighttime . Greater temperatures lead to greater DM losses. Chart courtesy of Tom Kilcer at Advantage Ag Systems, LLC.