Heavy rains in south Texas this year highlighted sorghum production’s weedy weak link.
“All our pre-emergence weed control got thrown out the window this year with the rain,” said Robstown, Texas grower Jim Massey. “I would say almost no farmers in this area are happy with weed control this year. Everyone has a field or two that got away from them.”
Post-emergence weed control for grasses is not an option for sorghum growers, because the grain is closely related to the grass weeds that often infest it, and no herbicide-tolerant varieties are yet on the market.
“All we can do is get out there after harvest and try to clean it up for next year,” Massey said.
Improving weed control is a top priority for the sorghum industry if it is to compete for acres with other grains, such as corn and soybeans, said Sorghum Checkoff Crop Improvement Director Justin Weinheimer.
“The No. 1 request from farmers right now, from an agronomic standpoint, is herbicide tolerance,” he said. So far, two agricultural companies — DuPont Pioneer and Advanta US — have ALS-herbicide resistant sorghum varieties in the pipeline, but commercialization remains several years away, company representatives said.
The industry hopes to avoid the errors that lead to herbicide-resistant weeds in other crops, especially since sorghum is vulnerable to gene movement into closely-related weed species.
A LONG-AWAITED TRAIT
A long-awaited sorghum management tool took its first step on a chilly December day in 2007: DuPont signed a license agreement with Kansas State University to commercialize the ALS-resistant sorghum lines the university’s researchers discovered using conventional breeding techniques.
The industry greeted the announcement with enthusiasm, but eight years later, growers are still empty-handed. “We’ve been expecting this for several years,” noted Massey. “It will be a big deal for a lot of growers to be able to spray an ALS herbicide over the top of sorghum. I certainly would have used it this year.”
The initial parent lines of the trait, which will be called Inzen, still need work to get up to the company’s standards for yield and performance, said Wayne Schumacher, the commercialization manager for DuPont’s Crop Protection division. Both university and company scientists are also working to breed out the sorghum’s tendency to turn an alarming shade of yellow when first sprayed with ALS herbicides.
“We are planning to come to market with the Inzen trait somewhere within the next five years,” said Doug Pilkington, DuPont Pioneer’s senior marketing manager. “We’ll target the High Plains geography initially, and then expand to other regions over time.”
In the license agreement, DuPont agreed to make the trait available to other companies. Only DuPont Pioneer and Advanta US have plans to release Inzen varieties, but other companies have expressed interest, Schumacher said.
The first herbicide to be registered for use with Inzen — an ALS herbicide from DuPont called Zest — is waiting on EPA approval, which the company expects by the end of 2015, Schumacher said. Getting the trait into high-yielding varieties without genetic engineering has contributed to Inzen’s long commercial timeline, but it will allow companies to skip the cumbersome domestic and international regulatory hurdles when the trait is ready to bring to market.
GENE FLOW FEARS
Sorghum’s kinship with the grass weeds that clog its fields poses a special danger when it comes to developing herbicide-tolerant varieties like Inzen. In fact, the gene that makes Inzen tolerant to ALS herbicides was transferred from existing populations of ALS-resistant shattercane, a weed and close relative of grain sorghum, Kansas State University weed scientist Curtis Thompson noted.
“Gene flow is always a possibility in sorghum because of its wild weed relatives,” he said. “There are some stewardship things we will need to try to abide by to minimize gene flow.”
For example, Inzen grain sorghum will be best used to control annual grass weeds like foxtails, barnyardgrass, witchgrass, volunteer wheat and crabgrass, Thompson said. “But if someone has a solid mat of grassy sandbur or shattercane in their field, I would not recommend using Inzen sorghum to try to clean that up. Some of it will just be using common sense.”
DuPont is prepared to apply lessons learned from the spread of glyphosate-resistant weeds to its rollout of Inzen sorghum, added Schumacher. “One of the requirements we’ll make in order to maintain long-term viability of the trait and chemistry is not planting sorghum in continuous years,” he said. “That’s going to be the biggest component.”
MAKING DO TIL MARKET DEBUT
Until Inzen makes its market debut, sorghum growers will continue to use the more onerous pre-Roundup Ready weed management practices.
That means being vigilant about burning down weeds in the fall, applying and rotating pre-emergence herbicides in the spring, and even cultivating between rows, Massey said.
Growers must also deal with the consequences of growing sorghum in a Roundup-Ready landscape. This spring, glyphosate drift wiped out 450 of Wayne Miller’s most promising grain sorghum acres just as the plants were poking out of the ground where he farms near Corpus Christi, Texas. “We still don’t know where it came from,” he said. Two miles downwind, a neighbor lost an additional 500 acres of sorghum from the same drift accident, he added.
In the Midsouth, the high concentration of Roundup Ready acreage can make this threat especially discouraging to potential growers, added University of Arkansas agronomist Jason Kelley.
“If you look at our soybean, corn, and cotton acreage, those are 95% or more Roundup Ready,” he explained. “So we’re growing sorghum in close proximity to all these and we’ve had some issues with glyphosate drift. And I know some growers simply say [they’re] not going to deal with it.”
Sorghum growers also face the growing threat of herbicide-resistant weeds in their fields, Thompson said. Palmer amaranth pigweed, waterhemp, and kochia with resistance to various herbicides are creeping into sorghum fields in the Great Plains and making weed control more difficult all year round.
Unfortunately, the ALS-inhibitor class of herbicides — which can be used with Inzen sorghum — generally have poor post-emergence activity on these problem broadleaf weeds, Thompson noted.
“I don’t see resistance going away,” he said. “So it’s something else growers will have to learn to manage.”
