Nebraska is known for an abundance of two things: irrigated corn and cattle. Put the two together, and you've got the perfect place for cornstalk grazing in winter.
In the late 1970s, University of Nebraska-Lincoln animal science professor John Ward stated there was enough corn and grain sorghum residue in the U.S. to feed 35 million to 40 million cows through the winter.
“That hasn't changed. If anything, that's grown,” said Galen Erickson, UNL animal science professor, at the recent Sorghum Symposium in Curtis. “It's probably more critical here in Nebraska than any other state, because we have plenty of residue and plenty of cows.”
When it comes to grazing crop residue, however, research has focused primarily on corn residue. About 140,000 acres of grain sorghum were planted in Nebraska in 2017. With more interest in the crop in some parts of the state, questions about the grazing value of sorghum have come up.
For the most part, sorghum residue is similar to corn residue, Erickson said. “It's very useful for grazing, and in fact, has some attributes that are better than corn for grazing.”
Similar to corn residue, sorghum residue has about 50% to 55% total digestible nutrients. The leaves are similar in TDN, but the stalk is actually more digestible than cornstalks.
While grain sorghum leaves plenty of residue behind, Erickson said cattle don't remove much of it.
“Cattle that are grazing sorghum residue are only going to remove 20% to 30% of the residue. So they're actually an excellent harvester, because they take out the best stuff and leave plenty for cover,” Erickson said. “The leaf is the primary residue that's being consumed. The stem is actually OK from a nutritional standpoint. But cattle must not like it, because they tend to graze it last. But if they did graze it, they would get some good from it — which is not the case in corn.”
Because of the way grain sorghum is harvested — the head is clipped off and the leaves and stems left standing — sorghum leaves more standing residue behind, and that taller residue is easier to graze in ice and snow over 1 foot deep.
This means there's less risk of down grain, but if there is grain on the ground, Erickson notes there's less risk of digestive issues due to overconsumption of sorghum, because sorghum berries are so much smaller in size compared to corn.
With too much grain available, however, cattle can still founder. Similar to corn, about 16 pounds of dry matter is produced per bushel of grain. However, only about 50% of that is consumed by the animals. So, about 8 pounds of dry matter residue is available for every bushel, or 56 pounds of grain produced per acre.
One key consideration in grazing sorghum residue, just like grazing corn residue, is supplementation. If grazing backgrounding, growing cattle, or lactating cows, supplementing energy and protein is necessary. Supplementation is necessary for calves to gain 1 pound per day. UNL research shows calves gain about 1.5 per day when supplemented 3 to 5 pounds of dried distillers grain per head per day.
On the other hand, older dry or gestating cows will maintain and even gain 0.5 to 1 pound per day on corn or grain sorghum residue without supplement, as long as some grain, leaves and husks are available.
“You certainly do have to supplement if you're grazing backgrounding or growing cattle, or lactating cows,” Erickson says. “If you're grazing older, dry, gestating cows, you probably don't need to supplement in most situations.”
What about BMR?
Some in Nebraska have planted brown midrib (BMR) sorghum and sorghum sudangrass for silage, and Erickson notes BMR type residues offer some advantages for grazing.
A UNL study published in 2008 compared BMR sorghum residue with conventional sorghum residue. Erickson notes the study found cattle grazing BMR sorghum residue with a small amount of protein supplement had a higher average daily gain compared to those grazing conventional sorghum residue — with an average of about a third of a pound ADG in both 2006 and 2008.
The reason for the improvement is the higher digestibility of the fiber in BMR sorghum, thanks to reduced lignin. In addition, BMR sorghum had about 10% higher NDF [neutral detergent fiber], or hemicellulose, cellulose and lignin.
“The brown midrib trait doesn't really affect the amount of fiber much, but it has a fairly dramatic improvement on how digestible the fiber is that's in that residue, which leads to increases in gains when you're grazing growing cattle,” Erickson says. “If you're grazing [dry] cows, I don't know if there's anything wrong with conventional sorghum residue. Because cows are just going to be at maintenance, I don't know if you need the extra gain.”