Male-Sterile Forage Sorghum Seriously Challenges Corn Silage
Male sterility means a plant will not translocate nutrients for grain or seed production. All of the nutrients remain in the leaves and vegetative plant tissue, producing very high-quality forage. With male-sterile sorghums no grain is produced, unless foreign pollen from a sorghum-type plant is made available.
We can thank Texas research agronomists for introducing male-sterile hot climate summer annuals (HCSAs) to crop growers. A male-sterile plant of Texas black-hull kefir (another HCSA) was discovered in a plot of the sorghum variety test at the Texas Research Substation in Chillicothe, Texas, in 1935. Those workers explained that with the second generation of hybrids with this plant, the progeny segregated into classes of approximately 75% normal plants and 25% male-sterile plants. To describe the advantages of male-sterile traits, let’s first describe the shortfalls of regular sorghum.
Normal sorghums have fertilized seeds at the top of the plant. The nutrients formed by photosynthesis after seed fertilization are preferentially moved to the seed site, much like a corn plant moves nutrients to the kernel on the ear. The difference is that the sorghum seeds rapidly become very hard and indigestible to the rumen. Their small size makes any processing difficult without destroying the forage effective fiber and turning the crop into mush.
Additional research found that breaking the seed does little to increase its digestibility and the broken seed’s nutrition is voided out as manure. Compounding this problem is the fact that having several pounds of seed at the top of an eight- to 12-foot stalk predisposes to serious lodging. Often crop researchers are disappointed as they watch their high-yielding HCSA crop completely lodge before reaching harvest maturity.
Ruminant nutritionists praise the benefits of male-sterile brown-midrib (BMR) sorghum with this comparison: “Much like a beef steer, the plant does not have a nutrient sink in the seed head. Thus, with delayed harvest, all the photosynthetic material is accumulated in the forage portion where it was formed. There is no fertilized seed to accept it. We clearly measured this phenomenon occurring in both 2020 and 2022 research trials.”
Steers devote most of their energy to production – and none of it to reproduction. (A natural genetic trait called BMR has been used in numerous varieties and hybrids of summer annual grasses for many years. This trait makes them more digestible and enables cattle to extract more energy from these forages.) Following are a few male-sterile BMR forage sorghum management pointers.
Historically, sorghum farmers would wait until just a week after heading and then chop forages. Often, they were upset about the wetness of the feed and the lack of energy compared to corn silage. The more current (and informed) line of thought regarding harvest delay for sorghum is to match what occurs in corn silage when it tassels and then is chopped eight weeks later. This allows both crops to be compared on an equal playing field. The result was a huge increase in enhanced nutrition as the sorghum’s digestible components accumulated in the forage cells.
Quoting ruminant nutritionists (many of them at Miner Institute in Chazy, NY), “When comparing the data for the two crops the end total energy result is nearly the same, but how they get there is very different. Corn silage has digestible fiber in the 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 with the eight-week enhanced nutrition has all the energy stored in digestible fiber and forage cell contents. A critical point for both nutritionist and farmer is 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 (1% NDF [neutral detergent fiber] by body weight).” I stress that this balancing process demands forage testing.
Here are a few more management pointers, purposed to help HCSAs deliver everything their growers hope for. These crops, drilled in seven- or eight-inch rows, suppress weeds quite well (without herbicides and/or row cultivation) – unless soil fertility is very poor. Additionally, sorghum, sudangrass, their hybrids and millets all have fibrous root systems, unlike corn and soybeans. This means that they stop degrading soil organic matter.
Also, when conditions turn droughty, HCSAs need about half as much moisture to produce a pound of forage dry matter as does corn silage (according to University of Texas research). Dairy folks whose farms are certified grass-fed organic are very fortunate to have these crops to choose from as their prime forage energy source. I caution folks that HCSAs are quite insistent on having their seeds placed in soils that are at least 65º F. In stating that temperature floor, I remind folks that both sorghum and sudangrass both originated in sub-Saharan Africa where moisture is very lacking in the early part of the growing season; average annual rainfall in that region is only nine inches. Corn’s productive temperature ceiling is 85º; HCSAs tolerate 105º.
This super-arid background of HCSAs introduces another important trait: all four of these plant classes are C-4s. This means that the species in question build their carbon structures in four-carbon modules. This means that C-4s’ ability to retain moisture (as regulated by minute openings called stomates) is much more efficient than is the case with non-C-4 plants.
In addition to the HCSAs just mentioned, sugarcane and corn also belong in their class, and so, unfortunately, does pigweed and its renegade child, Palmer’s amaranth.
Don’t forget: in order for these HCSAs to do their water retention thing, soil potassium must be adequate. A plus for sorghum’s negative trait – hydrogen cyanide or prussic acid – is that it remains in the plant’s roots to kill any corn rootworms, should corn follow sorghum the next year. Fortunately, prussic acid only appears in sorghum forage less than 20 inches tall.
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