Once upon a time, the species we know as sorghum (Sorghum bicolor) came in three distinct flavors: sorghum, which was primarily grown for its grain and was often known as “grain sorghum”; sudangrass, which was used exclusively for hay, silage and grazing; and sorghum-sudangrass, the hybrid between the two that was also used for hay, silage and grazing. Sorghum-sudangrass shows hybrid vigor – it’s a forage with wider leaves and higher tonnage than sudangrass. Basically, these genetic flavors represented three convenient categories, and that’s how we taught about them in agricultural courses. But no more. Plant breeders have changed things. Although you’ll still see the traditional three names in seed catalogs, the reality is more blurred. Plant breeders have developed some new traits that can now be found in all three categories, making this species a true continuum. And more useful than ever. I’ll cover three of these important new traits, and then you can decide how to mix and match them for your fields.
First, however, some background. Sorghum-sudangrass is an annual warm-season (C4) grass that grows quite tall; has wide leaves; provides high tonnage; and, in the vegetative stage, looks a lot like corn. Farmers plant it after the last frost, when the soil temperature has reached at least 55ºF. Sorghum-sudangrass requires less water than corn, and it is often used in water-deficient situations, especially because it can be planted later than corn. With good fertility, these plants explode in growth during the summer, and yields can exceed 15 tons per acre of green forage. Although sorghum-sudangrass has traditionally been used as a single-cut plant for hay or silage, we can also use it for multiple grazings during the heat of the summer. In Oregon, with livestock happily grazing fields of sorghum-sudangrass, I’ve personally seen daily gains of 0.5 pound for lambs and 2.5 pounds for yearling steers. It’s true that you can’t really see the lambs when they graze in it, but at least they have shade.
The other two categories of this species are grain sorghum and sudangrass. Not surprisingly, the varieties grown for grain have a thick stem designed to hold up a top-heavy load of grain (milo). Traditionally, these plants were never bred as a livestock forage. In contrast, the classic sudangrass varieties have thinner stems and lots of leaves. These plants can be grazed multiple times during the season, but their yields are generally lower than the hybrids.
All the sorghum types, however, do have some caveats – namely, potential problems with cyanide and nitrates. But these can be avoided fairly easily with good management. You can find lots of documents online with detailed descriptions of techniques for avoiding these problems.
The three traits
The first is the “BMR” gene. BMR? Simple: brown midrib. Look at the wide leaf of a sorghum plant and turn it over. The large vein (midrib) on the underside of the leaf is usually whitish green. In 1931, researchers discovered a gene in corn that caused this midrib to appear brownish. Wow, big deal – a color-coded plant. But seriously, the BMR gene is a big deal. The brownish color (sometimes reddish brown or even tan) is the secondary result of a primary change in the lignin biochemistry in the plant. This change is very important.
Lignin is a type of plant fiber that is critical to the structural integrity of plant cell walls. Think, for a moment, about how buildings are constructed, especially the angular internal bracing that builders attach to the main beams. Chemically, lignin resembles that cross-bracing. Although this geometry is important for plant structure, lignin is totally indigestible by livestock. As a plant matures, lignin molecules increase in number and combine with other types of fiber (primarily hemicellulose). This causes the fiber to become stronger but also less digestible. The bottom line is that high levels of lignin are associated with reduced fiber digestibility.
But the BMR gene changes things – it reduces lignin production, often by more than 20%. Translation: Plants with the BMR gene have a higher digestibility than plants without this gene. The practical result is an increase in the digestible energy level of the forage by as much as 5-7 percentage units of TDN. That is a big deal.
A note for the technically minded: Lignin is synthesized in a complex metabolic pathway that contains many steps, and each step is controlled by an enzyme. Any gene that alters or slows down one of these enzymes can slow down the entire pathway. Researchers have identified three genes that reduce lignin synthesis and cause the brown midrib appearance: BMR-6, BMR-12 and BMR-18. (There are other genes that can slow lignin production, but these three are the important ones for commercial forage breeding.) The BMR-6 gene reduces the activity of the enzyme cinnamyl alcohol dehydrogenase; the BMR-12 and BMR-18 genes both reduce the activity of the enzyme caffeic acid O-methyltransferase. In fact, these latter two are actually different versions (alleles) of the same gene.
Although the BMR mutation was first observed in corn, the mutation can occur in two other species: pearl millet (Pennisetum glaucum) and sorghum. Plant scientists bred for the BMR gene in sorghum in the 1990s (this is not a GMO trait), and now many commercial varieties have it. Seed companies seem to favor the BMR-6 gene because of its agronomic characteristics. However, all three BMR genes will increase forage digestibility. Interestingly, the BMR trait only occurs in C4 (warm-season) grasses. It has never been found in the cool-season grasses or in legumes.
Let’s switch gears and focus on a shorter topic, literally: the “brachytic dwarfs.” If you’ve ever grown (or seen) sudangrass or sorghum-sudangrass, you know that it grows tall, very tall – sometimes as high as 12 feet. That height can equate to a huge forage yield for hay or silage, but it also increases the risk of lodging (plants falling over). And grazing these tall plants can be a real challenge: The stalks are much less digestible than the leaves, and tall plants contain a high percentage of stalks. Also, have you ever tried finding your animals in that jungle?
A shorter plant would be nice, but shrinking a plant is not that simple. We can’t simply miniaturize the plants into bonsai sorghums because this would reduce yields and the plants would still contain lots of stems. In an ideal world, our shorter plant would retain a high yield of digestible leaves, but it would be shorter because it had a shorter stem. Well, the brachytic dwarf genes solve this problem nicely.
The brachytic dwarf trait is a special kind of selective dwarfism: The leaves remain the same, but the stems are shorter. Four genes are involved with this trait. These genes control the internode length – the distance between the leaves. Plants with these genes have a shorter distance between the nodes, but they retain the high number of leaves and the large leaf size. These plants also exhibit increased tillering, which fills in the open space between plants. All this means a higher leaf-to-stem ratio and a lot more digestible leaves because of the extra tillers.
Brachytic dwarfs grow no higher than 6 feet – which is short for this species. Most of the other characteristics are unchanged, like leaf size, number of leaves, maturity and yield. Essentially, brachytic dwarfs are shorter plants with higher leaf-to-stem ratios, better digestibility, more tillers and easier grazing management. Not bad.
The final trait
Our final genetic trait involves the plant’s ability to detect day length.
Like all grasses, the sorghums like to set seed. Actually, that’s the reason for the existence of the grain sorghums – to set seed. Otherwise, without seed, it would be tough for farmers to sell a crop of milo. Similarly, all the sudangrasses and the sorghum-sudangrasses will show inflorescence during the latter part of the growing season (inflorescence = seedhead). In any case, all these plants have built-in maturity dates when they convert from vegetative growth, with a high nutritional value, to reproductive growth, with severely reduced levels of protein and digestibility.
But the “photoperiod sensitive” (PS) trait in the sorghums changes this schedule. PS sorghums can detect changes in day length and adjust their seed set accordingly. Shorter days send a signal to these plants to set seed. The PS trait gives an extreme response. Plants with the PS trait may delay setting a seedhead until the first killing frost, effectively keeping the plant in vegetative growth until the frost kills it. The bottom line is that all through the summer and early autumn, these PS plants may stay leafy and vegetative. This trait, of course, is not particularly useful for farmers who want to harvest a grain crop, but for livestock folks who want high-value vegetative forage for grazing or hay or silage, this is wonderful.
So there we have it: three traits that have completely altered how we can use this plant. Look at some modern seed catalogs. Seed companies now offer varieties with a delicious combination of traits: BMR, brachytic dwarf and photoperiod sensitive. Almost like a restaurant menu. The old three categories of sorghums, sudangrasses and sorghum-sudan hybrids now mean less than they used to. All three sorghum types are now being sold with some or all of these new traits. For example, I’ve seen catalog offerings like BMR forage sorghums and dwarf brachytic sorghum-sudangrasses.
With the sorghum family of forages, we now have a cornucopia of choices, a real salad bar of traits. Summer pastures are looking better and better.