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USCP Agronomy Check
BRENT BEAN
LUBBOCK, TEXAS
By now, much of the grain sorghum in the Delta and Mid-South has headed, well on its way to harvest. Grain sorghum yield is determined by three components:
• number of heads per acre
• number of seeds per head
• individual seed weight
Once the sorghum heads out, the first two components have largely been determined. The number of heads is directly related to seeding rate and tillering, while the potential number of seeds per head is set between the fifth leaf stage to just before boot.
Adding weight
Once the grain fill period begins, sorghum management should focus on adding weight to the individual kernels and preventing the loss of kernels from insect feeding. Sorghum flowers from the top of the panicle and moves down over a three- to seven-day period. As soon as flowering is complete, the grain fill period begins and typically last 30-45 days until the starch content of the kernel reaches its maximum.
It is important that drought stress be minimized during early grain fill. If yield potential is high, consider applying 25-35 pounds of N during this time, especially if good rainfall amounts have been received that may have leached N out of the root zone.
Insect control
Monitoring fields for insects that feed directly on the grain is critical during early and mid-grain fill. Head worms and stink bugs are typically the insects that cause the most yield reduction. Small, recently hatched worms can be hard to see when they are in the sorghum head. A common scouting method is to bend the sorghum head into a clean white bucket and shake the worms off where they can be counted. Thresholds on when to treat are typically around one worm per sorghum head depending on the value of the crop, size of the worm and cost of the insecticide.
Since webworms are much smaller than corn earworms or fall armyworms, the threshold for their control is usually much higher. Insecticide application can be delayed until five or more webworms per head are present. Stink bug threshold is typically 4-5 per head.
Sugarcane aphids have been an issue up until the time of harvest in previous years. Successful control of sugarcane aphids is dependent on timely application as soon as the threshold level is reached. For the Delta and Mid-South, treatment should take place when populations reach 25 to 30 percent infested plants with localized areas of honeydew and high populations. Use either Sivanto™ Prime or Transform™ insecticide for sugarcane aphid control. If other pests are also present, consult state and local extension specialists for recommendations on tank mixes. Avoid use of pyrethroids when sugarcane aphids are present. ∆
BRENT BEAN: Agronomist, United Sorghum Checkoff Program
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