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Sorghum vs. silage? Some points to rememberSorghum vs. silage? Some points to remember

There are potential advantages to using a male sterile BMR sorghum instead of corn silage, but you can easily mess it up.

Tom Kilcer

March 21, 2023

7 Min Read
male sterile BMR sorghum plot
SORGHUM CONSIDERATIONS: A delayed harvest of male sterile BMR sorghum can provide many of the benefits of corn silage at a potentially lower price, but there are important points to remember. Courtesy of Tom Kilcer

A hypothesis that we tested in 2020 that supported the use of male sterile BMR sorghum with a later harvest has been replicated by research from the sorghum checkoff.

Normal sorghum plants have fertilized seeds at the top, and the nutrients formed by photosynthesis after seed fertilization are moved to the seed sink, much like a corn plant moves nutrients to the kernel on the ear. The difference is that sorghum seeds quickly get very hard and are not digested in the rumen.

Their small size makes any processing difficult without destroying the forage-effective fiber and turning the crop into soup. Additional research has found that breaking the seed does little to increase the digestibility.

Adding insult to injury, having several pounds of seed at the top of an 8- to 12-foot stalk is a setup for lodging.

Delayed harvest

The hypothesis of our research was to use a male sterile BMR forage sorghum. It was our theory that with delayed harvest, all the photosynthetic material would be accumulated in the forage, where it was formed. There is no fertilized seed to accept it. We clearly measured this in both 2020 and 2022 research trials.

In each trial, male sterile BMR forage sorghum was grown in narrow rows. Starting at heading, six replicate samples were taken each week. Each sample consisted of a single full plant that was chopped — to avoid subsample bias — inoculated with SiloSolve MC, vacuum sealed, and fermented for 21 days before being sent for analysis.

Sampling was done for six weeks in 2020 and eight weeks in 2022. Farmers traditionally waited a week after sorghum heading and then chopped, but they would often complain about the wetness of the forage and lack of energy compared to corn silage.

This harvest delay was to match what occurs in corn silage, where it tassels and is then chopped eight weeks later. It also allows both crops to be compared on an equal playing field.

The result was a huge increase in enhanced nutrition as the digestible components accumulated in the forage cells.

Over the weekly harvest time frame, the cellular sugar and starch level, the non-fiber carbohydrates (NFC), and nonstructural carbohydrates (NSC) increase significantly after heading. The question was how long after heading?

Most think that sorghum needs to be harvested at boot stage or immediately after heading to optimize nutrient content. Our research has found that this is false, at least for quality forage. Our multiyear replicated research found that eight weeks after heading is needed to have the digestible components similar to that of corn silage.

Silage vs. sorghum

When comparing the data for corn silage and sorghum, the end total energy is nearly the same, but how they get there is very different.

Corn silage has digestible fiber in stover, and almost half or more of the dry matter energy comes from the ear and the large amount of starch it contains. BMR male sterile forage sorghum, at the eight-week level, has all the energy stored in digestible fiber and forage cells. Yes, this increased the starch in the cells by 45% to nearly 10%, but this is much lower than the 40% corn silage contains. Processing is not needed to release this starch energy.

The other energy content of the sorghum that is moved to the grain in fertilized genetics is many times higher than that of corn silage in the forage.

But one critical point for both nutritionists and farmers is to realize that sorghum is not corn silage. When replacing one with the other, the ration needs to be rebalanced and a high-forage diet is strongly suggested.

The sorghum harvest delay really changes its energy level when compared to corn silage. Analytical results bear this out.

During this time frame, the NDF dropped 20% as more of the dry matter was digestible cell contents, and the NDF digestibility increased 13% to equal, or even exceed, corn silage.

The NFC increased 84% in eight weeks. This is equal to corn silage or slightly lower, but it doesn’t tell the whole story. The NSC increased over 200% from what it was at initial heading. By going eight weeks instead of six weeks, we nearly doubled its level in the forage.

Better type of sugar

Photosynthesis increased sugar levels over 550% with the mean level at eighth week of harvest at 26% of dry matter. This was not a fluke. In the 2020 research, which went only six weeks after heading, this increased 400% to 18.85% of dry matter.

Nutritionists have known that sugar plays a big role in rumen digestion, and the support of microbes and fungi is critical to breaking forage down. Unfortunately, if too much sugar is just dumped into the ration as molasses or other sugar type, there is a very good chance of tipping the rumen into acidosis. This has multiple negative effects on animal health and milk components.

The sugars in this sorghum are in a very different package, as they are contained in each individual plant cell. As the cells are individually ruptured by fungi, which need sugar to do their job, the sugars are slowly and steadily released in the rumen.

This slow release, coupled with our findings that a 1-inch length of cut with no processing is best for this crop, means you have the effective fiber — peNDF — needed for optimum rumen function without the explosion of sugar tipping the rumen into acidosis.

The longer pieces also ensure full extent of digestion, and this has been proven to have a major impact on reducing or eliminating leachate from the silo.

As a crop that is all forage and no grain, the potential that we had in 2020 and in earlier male sterile work was that crude protein should be about 11% at harvest.

Both sites in 2022 were over 11% crude protein at heading. As the plant laid down more highly digestible dry matter, the protein levels decreased because of insufficient nitrogen in the plant at the end of the season.

We have received a New York Farm Vitality Institute grant to study nitrogen and sulfur rates on male sterile BMR sorghum, and to look at the yield curve response and the crude protein economic curve response. Our previous work with winter forage found that these two curves are very different. It also appears that a heavy manure application with immediate incorporation may supply that late-season nitrogen from its organic matter breakdown.

Harvest delay increases dry matter

We found that the crop is very wet shortly after heading. By allowing the crop to continue photosynthesizing, the dry matter increased from 55% to 65% over the six- to eight-week period, with final dry matter over 30%. Thus, like corn silage, the crop looked the same but kept increasing yield as the dry matter increased. Much of the increase was in stalk size, which assisted standability if the crop was planted at correct seeding rate.

Our research found that utilizing the proper homolactic bacteria inoculant allowed complete fermentation with no butyric and Clostridia formation.

By delaying harvest eight weeks, you avoid harvesting, hauling and storing a very large volume of liquid for the small amount of dry matter nutrients that earlier harvest caused.

The bottom line

This is not a slam-dunk crop. You can screw it up. Having your crop next to, or downwind, of headed sorghum, sorghum-sudangrass or sudangrass will fertilize the male sterile crop and void the nutrient enhancement for which we are working.

Winter forage allelopathy can hurt the stand unless light tillage is used to break it up. Planting too early in cold soils will wipe it out or delay heading. This was a problem at our trial location of Willsboro, N.Y. Fortunately, we had someone come through and harvest a field for us in Pennsylvania.

Drilling more than 8 pounds of seed per acre will ensure that it will be all lodged flat when you harvest. Season length is determined backward. So, when you harvest you need to back up eight weeks as this is when it should be heading. Select a variety and plant date to achieve that point.

South of the Mason-Dixon line, we have the additional issue of needing a variety that heads out late enough in the fall. If it heads too early, it will increase nutrition for five weeks and then start sending up heads from the lateral shoots. This completely ruins potential nutrient enhancement as the energy is used to produce heads and lignin.

About the Author(s)

Tom Kilcer

Tom Kilcer is a certified crop adviser in Kinderhook, N.Y.

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