According to plant geneticists, male sterility means that crops in question will not translocate nutrients for grain or seed production. Almost all of the nutrients remain in the leaves and vegetative plant tissue, producing 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 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 early workers explained that with the second generation of hybrids with this plant, the progeny segregated into classes of approximately 3:1 ratio of normal plants to male-sterile plants. To better understand advantages of male-sterile traits, let’s 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 become very hard, indigestible to the rumen. Their small size makes any processing difficult without destroying forage’s effective fiber and turning the crop into mush.
Additional research found that breaking the seed does little to increase 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 sadly 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: they liken it to a beef steer, since “the plant does not have a nutrient sink in the seed head. Thus, with delayed harvest, all the photosynthetic material is accumulated and stored in the forage portion. 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 to reproduction. A natural genetic trait, 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 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 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 almost identical, 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, compared to corn silage, per University of Texas research. Dairy folks whose farms are certified grass-fed organic are 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. Corn’s productive temperature ceiling is 85º; HCSAs tolerate up to 105º.
The super-arid sub-Saharan Africa background of HCSAs introduces another important trait: all four of these plant classes are C-4s. This means these species build their carbon structures in four-carbon modules. Thus 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. Unfortunately, so do pigweed and its renegade child, Palmer’s amaranth.
Don’t forget: in order for these HCSAs to master water retention, soil potassium must be adequate. Potassium provides electrolyte benefits for plants. A plus for sorghum’s negative trait – hydrogen cyanide (prussic acid) – is that it remains in the plant’s roots to kill any corn rootworms, should corn follow sorghum next year. Fortunately for grazing ruminants, prussic acid only appears in sorghum forage less than 20 inches tall.
According to Dr. Michelle Wander, professor of soil science, University of Illinois, “Each 1% increase in soil organic matter helps soil hold 20,000 gallons more water per acre.” So the HCSAs – which need much less water than corn and soy – do a much better job building soil to store that water than these last two crops – a real head-scratcher.
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