Our grain sorghum is past flowering and moving through the soft dough stage. Grain fill is rapid with about half the total dry weight of the grain accumulating during this time. We may see nutrient stress and leaf loss since mobile nutrients are transolocated from the lower to the upper leaves. Between soft dough and hard dough stage, irrigation increases yield by improving grain fill test weight. We don’t see much if any response from water once grain reaches hard dough stage.
Once we get to hard dough stage, color, we no longer have to worry about sugarcane aphids. Also, keep in mind should try to avoid using pyrethroids for other insects and aphids. We need to check fields 2 – 3 weeks before harvest. We had issues with honeydew from SCA messing up the combines. A treatment may be needed if large numbers are in the head to prevent damage to combines. Hybrids with taller stalks and more space between the grain and upper leaves may make harvest easier by reducing the amount of leaf material going through the combine. Large infestation producing large amounts of honeydew and sooty mold may interfere with harvest desiccants.
The best way to tell hard dough is by pinching the kernel. The hard dough color is still similar to soft dough. MSU Grain Scientist Dr. Erick Larson says, “Grain Sorghum kernels change color and accumulate hard starch much the same as corn kernels mature. Kernels will mature first at the top of the head, so focus scouting on the kernels at the base of the heads. If you can quickly see a considerable amount of green kernels, rather than the burnt orange / brown color of mature kernels, you need to give the crop some more time to fully mature.”
Like corn, grain sorghum also forms ‘black layer’ from crown of the kernel to the base. Here is a photo by Dr. Erick Larson showing the progression from hard dough to ‘black layer.’
Sorghum Kernel Maturity from hard dough (left) to physiological maturity or ‘black layer’ (right) – Dr. Erick Larson
Here is information from UGA Extension Grain Entomologist Dr. David Buntin on pre-harvest intervals, rates, and efficacies:
- Sivanto Prime (Bayer Crop Protection). Sivanto prime has a full section 3 label and a supplemental 2ee label for lower rates on sorghum and other grain crops. The rates are 4 – 7 fl. oz per acre. Sivanto was very effective in my trials at rates of 4 to 7 fl. oz. per acre with Control usually lasting 21 days or more. At the 4 oz rate it can be applied up to 7 times during the season but has a 21 day PHI.
- Transform WG (Dow AgroSciences). Transform WG federal label was vacated last winter and a new federal label has not been approved yet. But Transform WG has an approved Section 18 emergency exception for use on sorghum in Georgia in 2016 through April 8, 2017. The big label change for 2016 is Transform cannot be used during bloom to protect pollinators. The label allows for 2 applications per season and not more than 3 oz per acre per crop and has a 14 day PHI. In my insecticide trails last season, rates of 1.0 and 1.5 oz per acre were effective. Use the 1.5 oz rate if aphid populations are increasing rapidly.
- Chlopyrifos (Lorsban Advanced, Nufos, other). Lorsban is labeled at 1 to 2 pints per acre. The 2 pint rate has a 60 day harvest interval. The 1 pint has a 30 day harvest interval, but is usually not effective. The 2 pint rate was 60-90% control for up to 2 weeks. At the 2 pint rate it cannot be used after the boot stage due the 60 day PHI. DO NOT USE CHLORPYRIFOS ON SWEET SORGHUM.
- Dimethoate ( Dimethoate, Cygon). Not recommended. In my trials dimethoate is variable in control and control if it occurs is only for a week or so.
The susceptible damage period for midge is bloom. Treatment after bloom is too late. The adults are hard to find. We either put a plastic Ziploc bag over the head. The midge will fly to the top of the bag. You can also use a white paper plate, but the midge can fly off. If it’s wings are wet in the morning, it may not fly as well. In either case, we only treat when an average of 1 adult per head is observed after 25 – 30% of heads are blooming. We can treat again 5 – 10 days later if midge is present. The eggs and larvae cannot be killed inside the glumes with insecticide.
NOTE: For midge, try not to use a prethroid because it may flare sugarcane aphids. If SCA are present and pyrethroid is used, we would have to tank mix with Transform or Sivanto.
Sorghum Midge – Photo by Ben Thrash