Forage Sorghum as an Alternative Silage Crop

Right off the bat, one of the benefits to using sorghum over corn is the seed cost—sorghum seed is much less expensive than corn seed, resulting in a direct savings before the crop is even in the ground. Sorghum also thrives in hot, dry weather. While corn silage stops growing at temperatures over 85°F, sorghum can continue to grow up to 105°F. Sorghum is also very drought tolerant and water efficient; under dry conditions, sorghum species will produce twice as much dry matter per inch of water than corn silage. Other potential benefits to using sorghum include elimination of issues with corn root worm (for the sorghum itself and also for a subsequent corn crop), reduced susceptibility to pathogens and disease, greater flexibility for planting, and more rapid ground cover.

Selecting the right type and variety can help ensure good nutritive value at the time of harvest. As with other crops, sorghum hybrids with the brown midrib (BMR) trait are a good example of how nutritive value can be improved through genetic advancement. Sorghum plants with the BMR trait have less lignin and greater fiber digestibility compared to non-BMR plants, leading to improvements in intake, milk production, and milkfat concentration compared to conventional sorghum.

One of the most common issues with sorghum is that the plants will lodge as the seed head matures and the grain fills, causing complications at harvest time. Research out of New York has found that the male-sterile BMR sorghum, which does not produce viable pollen and therefore does not have any seed set, is capable of producing maximum yield while also having the best standability as it has no maturing seed weight to weigh the plant down. With this type of sorghum, energy is stored in the plant cells and as digestible fiber rather than in the seeds (which for sorghum are hard and indigestible). As a result, photosynthetic energy continues to build within the plant cells over time rather than going towards grain fill, increasing the quality and milk-producing ability of the forage. This results in greater digestibility for male-sterile BMR forage sorghum compared to its seeded counterpart and makes it a great match for sorghum silage systems.

Patience is a virtue, and this also holds true when planting sorghum—don’t rush the planting date. The most critical step in establishing sorghum is to wait until the soil is warm enough (60-65°F) and temperatures are increasing before you think about planting. If not, germination and seedling vigor will be compromised and you will be dealing with a poorer stand and a lot more weed issues.

Optimum planting for sorghum is drilling in narrow (6-inch) rows, followed by planting in 15-inch rows. In replicated trials, drilling into 6-inch rows increased yield by 18% compared to planting in traditional 30-inch rows. The narrow row also means the canopy closes quickly, reducing erosion, shading out weeds, and maximizing sunlight interception in a short amount of time. Sorghum should be planted at a 1-inch depth at a rate of 10 lbs/acre if drilled in 6-inch rows and 8 lbs/acre if planted in 15-inch rows. Avoiding high seeding rates will result in plants with thicker stems and will reduce issues with lodging (the male sterile genetics will also help with this). Weed control can be accomplished by using seed with a safener along with a registered grass control herbicide applied immediately after planting. The current recommendation is to fertilize sorghum silage similar to corn silage, making sure adequate sulfur is present to maximize protein formation and mitigate potential issues with nitrate toxicity.

As with all forage crops, defining the target maturity for forage harvest requires finding the optimum combination between yield and forage nutritive value. For sorghum species, a single-cut management system has been found to double yields with little reduction in quality while drastically reducing harvest costs compared to a multi-cut management system. That said, the ideal time for sorghum harvest is very different than corn silage, largely due to differences in energy partitioning in the plant.

Corn silage has some energy in the plant fiber and within plant cells but most of the energy comes from the grain starch. Male sterile BMR sorghum doesn’t produce grain, so instead of energy from grain starch most of the energy is stored in the plant fibers and within the plant cells. Replicated research out of New York has evaluated various harvest stages for the male sterile BMR type sorghum and has tested forage quality (measured post-fermentation) up to seven weeks post-heading. Overall, they found that as they delayed harvest the non-fiber carbohydrates increased by 71%, the non-structural carbohydrates increased by 185%, and the water-soluble carbohydrates (sugar) increased by 500%. Starch also increased, but because more of the energy is contained within the plant cells rather than being converted to grain starch, the increase was not as great. Due to this increase in highly digestible components, neutral detergent fiber (NDF) was diluted and actually decreased by 15% (Kilcer, 2022; preliminary data).

As a result, for male sterile BMR sorghum the recommendation is to wait to harvest until 6 to 8 weeks after heading to increase both yield and forage quality. The increase in digestible components in the forage each week after heading means more digestible carbohydrates will be available as a slow, steady nutrient release for the animal. In addition, harvesting later results in a greater dry matter at harvest, which means you are harvesting and hauling less water, will achieve more efficient fermentation, and will have decreased potential for leachate.

When chopping sorghum, the harvesting head should be low to get as much of the crop as possible as it is all highly digestible forage. The knives should be sharp to avoid knocking the plant over and contributing to plugging. Sorghum is easy to chop, so watch your forward speed not only to avoid plugging issues but also to avoid overtaking the cutting speed of the head. By driving too fast, you will start leaving longer stubble in the field, which basically amounts to lost yield.

For sorghum species, the length of cut the chopper is set at is critical. For poor quality forages, a shorter length of cut results in greater milk production. The reverse is true for highly digestible forages like the BMR sorghum species. Larger particles will stay in the rumen longer, allowing the microbes in the rumen to extract the maximum amount of nutrients before the forage moves out of the rumen. Larger particles will also decrease the number of plant cells cut open, decreasing the amount of leachate loss, which is essentially the loss of highly digestible material. A cutting length of 1 to 1¼ inches has been shown to work best and will produce forage with a desired consistency while minimizing leachate losses.

Similarly, processing sorghum is not recommended. Processing has not resulted in improved feed value and will also increase leachate losses while simultaneously increasing fuel use and reducing field speed. Avoiding processing will reduce plant cell rupture, preserving more nutrients within intact cells and increasing the amount of sugar conserved through fermentation (which is then made available to the animal during digestion). Even if using a seeded-type sorghum, research has shown that processing does not help and the seeds are still poorly (if at all) digested.
The high sugar content in sorghum at harvest means that there is a high substrate present to support complete fermentation. However, the use of an inoculant is critical for rapid silage pH drop to effectively preserve this highly digestible material. If the crop is below 30% dry matter, it is suggested to use a homolactic inoculant rather than a buchneri-type inoculant. This has been shown to promote proper fermentation, effectively decrease the pH, and avoid production of butyric acid with sorghum harvested from 17 up to 27% dry matter.

The most critical factor when formulating rations using high quality BMR sorghum is to remember that it is not the same as corn silage. With proper balancing, BMR sorghum can produce the same amount of milk as corn silage (or for other species, similar rates of gain), but it is not as simple as replacing corn silage in the ration with sorghum silage. Instead, the differences between the two crops must be taken into consideration.

Sorghum silage is slightly lower in energy than corn silage but is higher in protein. As a result, typically a small amount of cornmeal must be added to increase energy, but this can be offset by a reduction in soybean meal or protein supplementation. This change is often economically advantageous, resulting in an overall cost savings on the ration. Because sorghum is more digestible than corn silage, the ration must also be rebalanced to a higher fiber feeding level (i.e. feeding a greater amount of NDF overall). Additionally, research has found that sorghum-based rumen pH is higher, which enables higher components and has less potential for subclinical acidosis.

Overall, there is a lot of potential for sorghum to be used as a silage crop for producing low cost, high quality dairy forage. Like with any forage system, good management counts for a lot so be sure to adjust accordingly if switching from corn silage to sorghum silage.