This past summer, I noticed that quite a few experienced graziers in western Oregon planted a forage that has not been popular here in many years: sorghum-sudangrass. Farmers were using these tall, coarse, broad-leaved plants that resemble corn for hay, and also, surprisingly, for grazing. But this modern sorghum-sudangrass isn’t the same plant that our grandparents grew. Genetics have changed it, plus there is one new twist. Now that summer is over, this may be a good time to talk about summer forages so we can begin thinking about next summer.
First some background: Sorghum-sudangrass is really one part of a genetic continuum of plants of the same species. This species has one scientific name: Sorghum bicolor. At one end of the spectrum is the tall, leafy forage plant sudangrass. At the other end of the spectrum is a plant bred specifically for seed production, with a thick, rigid stalk designed to support a top-heavy mass of seeds. This is the classic grain sorghum plant. The seed crop of this plant is called, not surprisingly, sorghum, but these seeds also have another name in the feed industry: milo. And in the middle of the spectrum is the hybrid plant called sorghum-sudangrass. Because it is a crossbred plant, it boasts hybrid vigor, similar to the hybrid vigor we get when we cross two different breeds of livestock. Ideally, this hybrid vigor shows up as increased vigor, wider leaves and higher yields when compared to traditional straightbred sudangrass. At least that was the original design. Modern genetic improvements may have blurred these categories, but you get the idea.
Sorghum-sudangrass is a C4 (warm-season) annual. It can easily grow 10 feet tall and yield more than 5 tons of dry matter per acre during the hot summer. Although it physically resembles corn, it doesn’t use as much water as corn, making it particularly tolerant of droughty conditions or fields with limited irrigation resources. Although sorghum-sudangrass has traditionally been cut for hay or silage, it can also be harvested multiple times during the season, which makes it suitable for grazing.
So what is its niche? In brief, this is a hot-weather annual plant with a potential for explosive summer growth. It’s suitable for hay or grazing, and it thrives in fields that lack enough water for corn.
Its resemblance to corn, however, can be a bit disconcerting to folks who aren’t used to it. This year a Douglas County, Oregon, farmer planted 10 acres of sorghum-sudangrass right next to a major country road. Later in the summer, so many people drove by it and thought it was corn that “didn’t look right” that our local newspaper ran an article about that field. Frankly, it was the first time I’d ever seen the journalistic profession interested in forage identification.
Sorghum-sudangrass is a warm-season grass. It grows best at 75º-85ºF, and it shouldn’t even be planted until the soil temperatures reach 55ºF. It also needs to be planted deep enough (0.75-1.5 inches) – otherwise the seeds become a real treat for the local bird population. Producers around western Oregon generally drill or disk it in late May or June, or even later. Jumping the gun is risky; unsprouted seed will rot in the wet, cooler soils. But once the young plants begin to grow, they really take off. Sorghum-sudangrass can be harvested in 30 days and then grazed every 20-30 days thereafter until frost. If you’ve ever seen a flock of lambs in a field of sorghum-sudangrass, it’s quite a sight – because you can’t see them. Oh yes, the lambs are in that mass of forage, somewhere, eating like pigs and probably gaining 0.4 pound per day or better, but you just may have to accept on faith that they’re still there.
Sorghum-sudangrass has a reputation for mediocre-quality hay – only 10%-11% crude protein – but that’s because those plants are typically harvested to maximize hay yields when they are huge and relatively mature. But when they are young and grazed at 30 inches or so, the plants can be quite good nutritionally. Well-fertilized young vegetation can boast protein levels of 15% or more and total digestible nutrient (TDN) values greater than 67%. Not bad for a midsummer forage. There is, of course, one well-known warning – never harvest plants that are less than 18 inches tall – but we’ll talk more about that later.
Here’s the new twist. Some varieties of sorghum-sudangrass carry the BMR gene, which does not mean “basal metabolic rate” or “bratwurst medium rare.” No, BMR stands for brown midrib. Which means what? Well, the underside of each wide leaf has a prominent center vein or “midrib” that’s usually green or whitish green. The BMR genetic mutation causes the midrib to appear brownish. By itself, this color mutation means little, but that BMR gene is actually a genetic marker for another gene that is exceptionally important. This important gene reduces the activities of some enzymes that synthesize lignin. Lignin is a type of plant fiber that is indigestible. Lignin also lowers the digestibility of other plant fibers, like cellulose and hemicellulose. Plants carrying the BMR gene, therefore, have fiber that is more digestible than plants without this gene. The bottom line is that the BMR gene increases the TDN value of the vegetation by 5-7 percentage points.
Scientists actually first observed the BMR trait in corn in 1931, but it wasn’t until the 1990s that plant breeders incorporated the BMR gene into commercial corn varieties. Unfortunately, the BMR gene also reduces grain yield and total plant yield by 15%-20%, as well as increasing the rate of lodging (plants falling over), so it wasn’t greatly popular with corn silage enthusiasts in spite of the improved nutrition. In the 1990s, the BMR gene was also incorporated into sorghum-sudangrass, and commercial BMR varieties are now easily available. To some extent, BMR sorghum-sudangrass suffers some of the same yield and lodging drawbacks as BMR corn, but the impact is lower, especially if the plants are used for grazing. Basically, plants harvested by grazing never get as high as plants harvested for silage, so lodging is less of an issue. In any case, BMR varieties give a distinct energy boost at a time during hot weather when extra nutrition is needed to finish cattle or lambs or maintain high milk production.
Now the warning. Sudangrass, sorghum-sudangrass and grain sorghum all come with a red flag: “Plants contain cyanide, beware!” Well, actually, these plants don’t contain pure cyanide, and you certainly don’t need to wear a gas mask around them, but they do contain a compound called dhurrin. One portion of this dhurrin molecule is the carbon-nitrogen combination – CN. When plant material containing dhurrin enters the mild pH of the rumen, this -CN portion splits off and becomes HCN (hydrogen cyanide, also called prussic acid) that is quickly absorbed into the blood. Although livestock can successfully detoxify low levels of cyanide, it doesn’t take much more to be impressively toxic.
The good thing is that the amount of dhurrin in sorghum-sudangrass and sudangrass is greatly reduced as the plants gain height. Hence the simple rule of thumb: Never graze or harvest these plants until they are at least 18 inches tall. Usually this is not a problem, but there are three additional caveats: (1) Don’t graze these plants immediately after a frost. Freezing temperatures disrupt the leaf cells, which release some of the dhurrin and make it more available. Just wait 10 days before using plants for grazing or greenchop. (2) Don’t allow animals to graze the young shoots that appear soon after a break in a drought or after an early frost. These young, deep-green shoots are high in dhurrin. Move the animals off that field until the plant shoots are 18 inches tall. (3) Don’t use grain sorghum plants for grazing. Grain sorghums were never bred for low dhurrin levels. Some varieties may contain high levels of dhurrin, even when the plants are tall.
Oh yes, there is one other practical caution about sorghum-sudangrass, although it is not a common problem. Under some conditions, these plants can accumulate high levels of nitrates. But these are the same conditions as for corn – fields highly fertilized with nitrogen that suffer from drought or long periods of overcast skies. There are lots of factsheets from universities and online on how to cope with this situation, so I won’t go into it here.
For some producers, in some fields, sorghum-sudangrass and sudangrass can be important tools. Grazing, BMR genes, improved varieties, low-moisture situations – combined with the potential for high summer yields and good nutrition – these plants can be impressive. Impressive enough to give them a try.
