Our grain sorghum crop growth stages range from just a few leaves to reaching the stage of growth at which time many things begin to happen with the plant. For the older sorghum plants, the total number of leaves has been determined and the grain head size is being determined at this time. Now is also the time that rapid nutrient uptake is occurring and yes we are seeing some nutrient deficiencies. These nutrient deficiencies tend to be more obvious in situations with adequate soil moisture, when the plant is growing rapidly, and in many cases has a small root system, so it cannot capture all of the nutrients that it needs. Until grain sorghum develops an extensive root system, young plants may not be able to obtain enough ferrous iron to maintain normal growth on some soils. Although iron is not a part of the chlorophyll molecule, it is required to supply enough chlorophyll to support the growth of new leaf tissue.
High-pH, calcareous soils not only reduce the availability of soluble iron in the soil; they also change the soil’s cation-exchange capacity so that less iron is present and available for exchange overall. The result is a more slowly growing plant and subsequent uneven flowering dates. This not only delays ripening of grain and harvest, but the uneven pollination undermines an effective insecticide spray program for controlling sorghum midge.
Mild chlorosis ranges from a lighter green leaf color that progresses to increasing interveinal striping, too almost no visual symptoms. Moderate chlorosis is seen where sorghum plants are yellow or yellow-green in strips, or irregularly shaped areas of the field. Fields may have intermittent blotches of chlorotic plants scattered in a salt-and-pepper (random) arrangement. Iron chlorosis is often sporadic throughout the field, or it may be associated with some land-moving event.
Visual symptoms are often the best indicator of whether or not to treat chlorotic fields or parts of fields for iron chlorosis. Applying iron as a foliar spray is effective in restoring the green color and they are most effective when repeated at 10-day to two-week intervals. Several products are available for field use to correct iron chlorosis. Iron sprays require a spreader-sticker or detergent in order to be effective. If a commercial spreadersticker is not available, ordinary household detergent may be used at rates of 1/4 to ½ pint per 50 gallons of solution. Thorough coverage and wetting of the entire leaf surface is necessary for good results. Avoid too much detergent to minimize the chances of leaf burn.
As one evaluates the overall condition of our grain sorghum crop, many fields are at that critical stage in which moisture demand is increasing rapidly and the plant is going into the reproductive stage of development. The average daily water use by grain sorghum is usually less than 0.10 inch until approximately the seven-leaf stage, depending upon climatic conditions. At this stage, water use increases rapidly and is likely to be 0.30 inch per day during the boot, bloom and early grain development stages. We sure need a good rain to move this crop along.
More information and on sorghum production can be found at http://www.sorghumcheckoff.com/sorghum-production-handbooks
