by Dr. John Kruse, Cotton and Feedgrains Specialist and Dr. Josh Lofton, Macon Ridge Research Station Agronomist
The Louisiana corn crop developed more rapidly than normal this year, and more than a few producers will be ready to harvest this crop in the month of July. With the harvest comes a well-deserved feeling of completion and a sense that after months of intense labor and difficult management issues, it is time to rest – or at least move on to the next crop! However, before you leave the corn crop behind, there are a few issues to attend to that will put the next crop in a better position to succeed.
Final watering: A critical decision that must be made is how much more to water and when to discontinue irrigation. Corn continues to need water until it reaches “black layer. If irrigation is terminated prematurely without adequate rainfall you could see yield losses up to 25%. To estimate how many more irrigation events are needed you need to have knowledge of your soil as well as your crop’s maturity. If you break a cob that has begun to dent and look at the top side you should see the milk line. This line is the dividing line between a very bright yellow starch layer toward the top and a more opaque yellow toward the bottom. It takes 20 days for that milk layer to progress from the top to the bottom of the kernel. Therefore, you can estimate the number of days you have left until black layer. With the recent trends of hot temperatures, high winds, and lowered humidity with very little rainfall, it is going to be vital to continue irrigation until we reach the critical black layer growth stage.
Residue management: Corn produces a tremendous amount of biomass during the growing season, and a grower only harvests the grain, leaving a substantial amount of residue that needs to be managed properly. As a rule of thumb, the amount of residue produced by a corn crop is roughly equal to the amount of grain harvested, by weight. Therefore, a 200 bushel corn crop would produce about 11,200 pounds of residue per acre. LSU AgCenter staff received several calls this year about residue that spring rains washed into lower areas of fields, literally burying young corn plants and reducing plant stands. Instead of cutting corn stalks off at ground level after harvest, consider leaving knee-high stalks in place for the fall. The roots will do a better job holding soil in place over the winter, and by spring the stalks will be rotten enough that planting equipment will push the soft stalks over.
Fall fertilization: If you have not soil tested in three years, you really need a soil test. Modern crop cultivars with higher yield potential are pulling more nutrients – particularly phosphorus and potassium – out of the soil than in years past. We can no longer count on smokestack output to supply sulfur, and more and more fields are turning up sulfur deficient. Additionally, corn requires zinc in order to optimize yields, and if your soils are high in pH or naturally zinc deficient (less than 1.5 ppm soil test Zn), your yields suffered this year. Finally, most of our modern nitrogen fertilizers (UAN, urea, ammonium sulfate, etc.) create soil acidity as they transform from ammonium to nitrate in the soil. A low soil pH can prevent a crop from utilizing nutrients to their full extent. Lime is an investment in your future, and should be applied in the fall so that it can be incorporated and then dissolve with the winter rains. The only way to know if you need lime, and how much, is to take a soil test. If you have a soil best described as a loam (not too sandy and not too clayey), and the pH is between 6.0 and 6.9, then a fall application of phosphorus and potassium may make sense from a management perspective. However, if the soil is too sandy or has a low cation exchange capacity (CEC), some applied potassium could be lost over the winter. A clayey soil tends to tie up phosphorus, as does a soil with a too-low or too-high pH. In these cases, a spring application of P and K is better utilized by the next crop. Also, fall applied fertilizer is there to feed winter
weeds. Collecting a good representative sample is vital. In a corn production system for most nutrients, soil sampling can be carried out any time following harvest, but make sure there is adequate time for processing and analysis prior to desired application period. This is true for P, K, and micronutrient analysis. However, for N and S analysis, soil samples taken in the fall will not give a clear indication of N and S levels in early spring at planting. These samples should be collected shortly before desired applications. All soil samples should not be taken soon after nutrient or lime applications. Soil samples should be taken across each field in at least 20 randomly collected locations. These should be taken at least to the depth of the tillage layer (and potentially lower for deep rooted crops such as cotton) for cultivated fields and at least 6 inches for no-till fields. After the collection, the samples should be allowed to air dry for all nutrients except N and S which need to be placed in a drying oven or refrigerator soon after collection. A routine soil analysis at the LSU AgCenter soil testing and plant analysis laboratory ($10 for each sample under 10 or $8 for each sample over 10) should be adequate in most instances; however, if you would like additional analysis contact your extension agent or specialist to discuss. Once a soil test report has been received, pay close attention to those nutrients in the low and very low range. This indicates that supplemental fertilizer will be needed to sustain the corn crop for the coming production cycle. Those values that are indicated as optimum, suggest that nutrient levels in the soil are within the accepted range for optimum production. Soil test values that are high to very high, indicate nutrient concentrations are higher than the crop will take up. While these high to very high levels are typically not harmful for crop production, caution should be taken with continued applications due to: 1) limited economic return for additional fertilizers and 2) dangers of toxicity, especially for micronutrients.
Compaction: Today’s modern equipment exerts a heavy footprint on agricultural soils. Compaction layers led to several calls this year in which corn appeared to be potash deficient. It turned out that the rows on either side of the tractor tires were so compacted, the crop developed a shallow root system and when that shallow layer was fully exploited by the crop (by V5), there was no more potash available for it. Many producers are reporting that running a deep vertical shank down the row in the fall is breaking the plow pan and allowing winter rains to penetrate the soil more deeply and build greater moisture reserves for the following crop season. Roots are able to grow deeper into the soil profile and extract that moisture as well as nutrients.
Fall weed control: A late July or early August corn harvest leaves 6 or 7 months of fallow ground on which weeds can grow and mature. Weed scientists in the LSU AgCenter have been warning that winter weed management is becoming more problematic as Italian ryegrass and other weeds are displaying herbicide resistance. Henbit appears to be growing longer into the late spring and becoming a more robust competitor to spring-planted crops. Identify your fall and winter weeds and come up with a proactive program to control them that includes at least one fall application in addition to a spring burndown. For details on your specific situation, contact your County Agent or Dr. Daniel Stephenson.
Cover crops: While more Louisiana-based research needs to be conducted to quantify the benefits, much is already known about the advantages of winter crops. The plants hold soil all winter (drastically reducing erosion), allow for better winter-rain infiltration, suppress winter weeds, and build soil organic matter. Soils with higher organic matter have a much greater capacity to hold soil moisture during the summer, and have higher nutrient holding capacity.
The corn harvest for this year will be upon us, wrapping up a successful year. But it is also the beginning of a successful crop next year, if a producer is pro-active and starts his plan right away.