Mid-season nitrogen and iron deficiency in sorghum
The rains this week don’t yet make our crop for the summer for dryland unless you received many inches; they just put us back in the ballgame. The NEXT rains will determine where we go. With that in mind as Pete Dotray, Wayne Keeling, and others will note, keep an eye on your weed issues. Some areas of the South Plains finally have enough moisture to trigger potentially major weed activity.
Is it truly too late to take a chance on grain sorghum?
Yes. Of course we can find those in the past that have had a successful crop planting this late, but it may be early next week before some fields dry out enough to plant. One Lubbock County grower asked on July 17 about still planting early maturity grain sorghum and “see what happens.” But it will be several days before he can plant so it is truly too late. For a Lubbock Co. example, if we could get a true early maturity hybrid in the ground on July 21 (allowing for drying), then at 55 days to half bloom (half of the plants in the field initiating bloom), that takes us to Sept. 14, then allow another five weeks to reach reasonable physiological maturity in the seed—that’s Oct. 19. Growth by then has for all practical purposes has ceased by about a week earlier. Remember that 1 day of heat accumulation in these days of July is worth 2 to 3 days in early October.
A more suitable alternative is sorghum/sudan or haygrazer. You do not need a physiological seed maturity and, with favorable conditions the rest of the summer, yield of 1.5 tons/A is certainly feasible. For dryland for seeds of ~15,000 seeds/lb. then 15 lbs./A is a good target, but even 10 lbs./A will give adequate results. Some haygrazers like the “three-way cross,” or the sorgo-sorghum/sudans, may have seed size of near 20,000 seeds/lb. If that is the case you can reduce your per-acre seeding rate.
Mid-Season Nitrogen on Grain Sorghum
With rainfall support some producers may now actually consider fertilizing grain sorghum that was planted late. This will depend on the amount of N you might have had on cotton ground that failed. Grain sorghum’s requirement is about 2 lbs. of N per acre per 100 lbs. of yield goal. I like for this N to be applied within 30 to 35 days of planting to ensure the N is “in the system” at the time when the all-important growing point differentiation occurs.
If you are irrigated, the bulk of this N should still be applied by 30-35 days after planting, but if you trickle out smaller amounts of N, then you might hold back 20% and fertigate, but for a medium-early maturity hybrid I recommend you still complete N applications with 50 days of planting.
If you have soil test information that indicates significant soil nitrate N then it should be credited toward your crop requirement. Be sure in your applications, however, to minimize root pruning with knife rigs or rolling coulters.
Iron (Fe) Deficiency in Grain Sorghum
Iron deficiency is related to caliche soils and outcroppings in West Texas (usually pH ≥ 7.9) or frequent alkali spots in fields (Coastal Bend, South Texas). This is a particular concern for sorghum. Chalky soils that appear whitish across the field should probably never have grain sorghum, and it is prohibitively expensive to correct it. Many fields, however, simply experience some degree of iron deficiency, the classical condition of interveinal chlorosis where the veins of the younger leaves remain green and the leaves are yellow between the veins. In the worst of cases, the leaves are almost completely bleached out and the plants do not grow.
Iron deficiency can also be induced temporarily due to water-logged conditions and possibly fast growth in young sorghum (root system not well established yet) or a possible response to herbicide (replant situation where residual is present or a subsequent post-emerge application). In modest cases where iron deficiency occurs, as the root volume expands iron deficiency diminishes. On the other hand, strong cases of iron deficiency in grain sorghum may merit treatment at as early as 10 to14 days after emergence.
Iron deficiency compared to N deficiency. Iron deficiency is normally expressed mostly on newest leaves, and iron is immobile within the plant. When iron becomes available again, newly emerging leaves will again be dark green. Older chlorotic leaves will not green up unless they receive a direct foliar feed. In contrast, N is mobile in the plant, and will move to the youngest leaves from older plant tissues (which may express N deficiency) and shows no striping symptoms.
Most soil tests will flag Fe < 4 ppm as deficient. Currently, there are no economical sources of soil-applied Fe available. Therefore, the only options for correcting Fe deficiencies are to apply foliar Fe sprays in-season or to apply manure for long-term correction. If iron chlorosis has been observed during previous years in a field, iron fertilizer materials may be applied preemptively to the foliage through multiple sprayings early in the season.