With feedgrain prices higher than normal, grain sorghum will be planted on more acres than normal in Louisiana. I’ve been asked recently about the more important agronomic issues that need to be paid attention to.
Let’s start with planting date. This year, thus far, it appears as if we have planted earlier than is customary. With soil temperatures in the mid-60s and moisture being depleted from north winds, we had to get started at some point.
Several much older grain sorghum hybrids were brought back into the state in an attempt to alleviate some of the demand for seed. Seed supply for sorghum is now tighter than it was for corn. Corn seed can be found fairly readily throughout the state.
Once grain sorghum hybrids are selected, plant population became a popular subject. Last week, I had numerous calls from producers who had begun planting and were planting more than 130,000 seed to the acre.
These numbers are much higher than what we customarily recommend. I was prompted to do more research on the subject.
Currently, the LSU AgCenter is recommending a final plant population of around 70,000 plants to the acre. To achieve this plant population, you need to plant 80,000 to 90,000 plants to the acre based on 85 percent germination rates.
Please note that the worst thing that you can have in a grain sorghum field is a thin stand, because that brings up many more issues to deal with, especially late in the season.
I checked with my grain sorghum colleagues to the east and north and both agreed that around 50,000 to 70,000 plants to the acre is essentially the optimal plant population needed to maximize yields.
Rick Mascagni, an LSU AgCenter agronomist located at the Northeast Research Station in St. Joseph, La., has conducted some recent research on plant population, row configurations and nitrogen rates.
On a Gigger silt loam in Winnsboro, La., using plant populations of 78,500 plants to the acre, yields were maximized when 100 pounds of nitrogen were used in both the single- and twin-row configuration.
When plant population was increased to 104,500 plants to the acre, results were the same as the lower plant population. Yields were maximized when 100 pounds of nitrogen was applied.
The most interesting point to note is that there were no statistical differences between the treatments, but the highest yields were achieved with the twin-row system with a plant population of 78,500 plants to the acre.
I asked Roger Leonard, a research entomologist at the Macon Ridge Research Station, in Winnsboro, La., what other IPM problems could develop with excessive plant populations.
“There are at least three potential negative effects with dense grain sorghum plant populations on insect pest management,” he said. “The first is a direct increase in production costs associated with more insecticide-treated seed. The LSU AgCenter recommends all sorghum seed be planted with an insecticide seed treatment. As the seeding rate is increased, the cost of this treatment increases.
The second issue is the effect on plant development from excessive crowding. Any delay in individual plant development will increase the range of head emergence and duration of flowering across the field.
Increasing the flowering interval increases the time period of grain susceptibility to sorghum midge. This, in turn, will likely increase the need for additional insecticide applications to protect the crop from sorghum midge-damaged seed.
Finally, higher plant densities are more conducive to caterpillar pest (webworm, corn earworm, fall armyworm) movement between seed panicles. Rather than staying on individual panicles, these pests can migrate to adjacent panicles and cause seed injury because the seed heads in dense plantings often are in contact with each other.
Although I have not observed a pesticide application problem in dense populations of plants, there is a possibility of negatively impacting insecticide efficacy by increasing the surface area of the plant that needs to be treated to achieve satisfactory control.
Another important thing is to not plant grain sorghum too deep. Sorghum seed is much smaller than corn or beans and cannot not tolerate a very deep seeding depth. Usually, .75 to 1.5 inches deep is adequate to achieve a good stand.
Regarding planting date, we target April 1 to May 1 in south Louisiana. In north Louisiana, the range is April 15 to May 15.
Grain sorghum is going to germinate when soil temperatures are between 60 to 65 degrees F, but it is better to be on the higher end of that soil temperature range.
I know some producers are traditionally planting 120,000 seed to the acre and harvesting over 100 bushels to the acre. This is fine, but if you can harvest the same yields with fewer seed to the acre, why not save the money? It will pay dividends in a year like this.