Number 214 October 16, 2009

Transcription

1 Number 214 October 16, Possible causes of blank heads in grain sorghum 1 2. Fall freeze damage to soybeans 4 3. Prussic acid and nitrates in sorghum regrowth after harvest 5 4. Considerations when pasturing wheat 6 1. Possible causes of blank heads in grain sorghum There are several fields of grain sorghum in different areas of Kansas this year in which the sorghum heads are partially blank, or with very tiny kernels. Often the affected area is near the top of the heads, but other areas of the head could be affected. Field in Reno County on Oct. 14, The upper third of the heads is blank and darkened, throughout the field. Photos by Jim Shroyer, K-State Research and Extension. 1

2 There are several possible causes for this: Closeup of affected head. * Unusually cool weather at or near the time of pollination. Studies have demonstrated that cool temperatures during formation of the pollen can induce male sterility. Pollen formation typically begins when the flag leaf is emerging and extends until flag leaf collar development (near the boot stage) well before heading and pollen shed. Night temperatures at or below 55º F during critical stages of pollen formation can induce pollen sterility and drastically reduce seed set. Poor seed set may be related to cool night temperatures when pollen was forming at about the time of flag leaf emergence. After the plants begin pollination, temperatures near 50º F will slow pollen germination and growth. Once formed, however, pollen can survive and remain viable at temperatures below 40º F -- and successfully germinate and grow as temperatures warm up later in the day. Cool temperatures can still reduce seed set during pollination by slowing growth of the pollen tube leading to the ovule. Flowering/pollen shed is shed beginning at the tops of the heads and working its way down over a period of several days. Occasionally, a band of poor seed set can be observed in sorghum heads if pollen shed or pollen growth is inhibited for only one or two days. With new pollen being shed every day, normal fertilization usually occurs unless the cool temperatures persist for a week or more. * Application of a growth regulator herbicide (e.g. dicamba) beyond the labeled height restriction. These herbicides can cause partial or total head sterility if applied after the labeled height restriction. The heads will show some twisting or distortion if this is the problem. * Bird damage. This is a common problem, usually occurring most severely near tree lines or fences. Bird damage is usually fairly obvious, with hollowed out areas where the kernel had been present, and droppings on the leaves or soil. 2

3 * Insect damage. Sorghum midges damage sorghum by laying eggs at flowering. The larvae then feed on the developing kernel, effectively destroying that seed. Each larva will generally destroy one kernel. The adult midge is a tiny gnat-like insect that is attracted only to those heads or part of heads that are flowering. No kernel develops inside the glumes where larval feeding has occurred, thus it gives the appearance of a blasted area. As a result, the name for midgeinfested and destroyed seeds is that the head has a blasted appearance. After feeding, the larva crawls up to the top of the spikelet where it pupates. Then when the adult emerges it leaves behind a telltale pupal case, which is relatively weather resistant. So if the glumes are empty due to midge infestations, there will probably be a few pupal skins still hanging out. Infested heads will have normal kernels scattered among the spikelets that do not contain kernels, as the midges don't normally uniformly infest every kernel. The adult only lives about 1 day and lays about 45 eggs. Sorghum midges are usually found in the southeast quarter of Kansas, but have been reported as far west as Dodge City, and south of I-70. Diseases are probably not the cause of this head damage. The blank areas of the heads are typically darkened, and may appear moldy or sooty. This may lead producers to believe that sooty mold or some other disease may be causing the problem. But those fungi are merely saprophytes, meaning they infected the affected areas of the heads after the problem had occurred instead of causing the problem. Likewise, freeze injury is not the cause of the problem. If there are no kernels at all, the problem had to have originated at the time of pollination and fertilization. If there are very tiny kernels, the problem occurred early in the filling period. In either case, the cause of the problem occurred well before the recent hard freezes. So what is the most likely cause of the problems this year? Unusually cool conditions during pollination, at least in those cases where the problem has occurred uniformly throughout the field. The low temperatures at the South Central Experiment Field near Hutchinson on August 30 and 31 were 49.5 and 46.4 degrees F. The low on August 22 was 51.2 degrees. Sorghum starts flowering and pollinating at the top of the heads, and progresses downward for a period of several days. If the unusually cool period of weather lasted for only one or two nights, then just part of the head (or one side of the head) would be affected, and that s what we re seeing this year in many cases. -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu -- Vara Prasad, Plant Physiologist vara@ksu.edu -- Chris Little, Plant Pathologist crlittle@ksu.edu -- Jeff Whitworth, Extension Entomologist jwhitwor@ksu.edu 3

4 2. Fall freeze damage to soybeans It is typical for some late-maturing soybean fields to suffer from freeze damage in the fall. This year, more acres than normal of soybeans in western Kansas were affected, with the recent low temperatures on October 10 at of 24 degrees F at Garden City, 20 degrees F at Tribune, and 20 degrees F at Colby. The earliest killing freeze occurred on October 3 for much of this area. This early freeze caused many late-maturing plants to prematurely die before the seeds were fully developed. Soybean tops are damaged at temperatures in the range of 30 to 32 degrees F. Wisconsin data reported that 80% of soybean leaves were damaged at 26 degrees F for five minutes. These numbers are generalizations as freeze tolerance may be slightly higher in thick stands and narrower rows. Wisconsin research showed that when frost occurred at or before R6 (seed fills the pod cavity at one of the four uppermost nodes), yields were reduced. The most sensitive growth stage for economical yield loss was found to occur at the R5 (seed 0.3 cm long at one of four uppermost nodes) growth stage. Yield reductions are a result of a reduction in number of beans per plant and reduced bean size. The maturity of freeze-damaged soybeans is never delayed, but can be accelerated. Soybean seeds that are soft and green at the time of the freeze will shrivel, but those in pods that have turned yellow may still mature normally. Clearly green soybeans are undesirable because of low seed quality and yield, but also have poor end-use qualities. Green soybeans result in offcolor meal and oil. They have to be refined out of the oil because of high levels of free fatty acids, which can cause the oil to become rancid. So what are the options for green soybeans? If they are not moldy, they can be fed to beef cattle if introduced slowly. They should not be fed to young calves that do not have a functional rumen, but can make up to 7% of a growing calf s diet. In a roughage diet, the fat content should be limited to less than 4%, which is key in feeding green soybeans to cattle. Another good option is to graze freeze damaged soybeans. The best growth stage for grazing is when the plants are in the R4 to R5 (full pod to 0.3 cm long seed at one of the four uppermost nodes) growth stage. The cattle need to have another forage source available, which could include grass pasture, hay, or silage. Bloat is a concern at times, but is much less a concern than with alfalfa or clover. Freeze-damaged soybeans can also be a great source of hay or silage for livestock. The best time to harvest soybeans for silage is near maturity, but before leaf loss occurs. When ensiling, another source of fermentable carbohydrates needs to be added, such as ground corn grain or molasses (minimum of 10% on a dry matter basis) because soybeans are low in soluble carbohydrates near maturity. Without the addition of other materials, the silage will have an unpleasant smell and will be low quality. Soybeans may be cut for hay at any time, as both forage and pods provide digestible protein. Some portions of the hay may not be eaten by the cattle due to refusal, which can be substantial in some cases. If soybeans are not sold or fed to livestock, they must be stored. Special considerations must be taken when storing green soybeans. First note that the green color will not be lost from the soybeans, it will simply be camouflaged by the mixing with other beans. Aeration does not affect the internal green color of the soybeans, but it is very important to aerate the soybeans quickly. 4

5 This is important because the spoilage risk from mold or insects in storage is greater with green beans. During aeration, the grain temperature should be reduced to below 40 F as soon as possible. With these smaller, immature beans, note that it will take longer to aerate them, and make adjustments accordingly. Storage risks are the same for green as for yellow soybeans as long as they are stored at the recommended 18% moisture. -- Kent Martin, Southwest Area Crops and Soils Specialist kentlm@ksu.edu -- Brian Olson, Northwest Area Crops and Soils Specialist bolson@ksu.edu -- Stu Duncan, Northeast Area Crops and Soils Specialist sduncan@ksu.edu -- Doug Shoup, Southeast Area Crops and Soils Specialist dshoup@ksu.edu 3. Prussic acid and nitrates in sorghum regrowth after harvest Grain sorghum regrowth after harvest can occur anytime until a hard, killing freeze (25-28 degrees F) has occurred. Parts of Kansas have already had a hard, killing freeze (25-28 degrees F) by now, but much of central and eastern Kansas has not. High levels of nitrate or prussic acid can occur in sorghum regrowth, or in the stems of stubble where there is no regrowth. Under normal conditions, grain sorghum contains little free prussic acid, or cyanide. However, when freezing, drought stress, wilting, or mechanical injury damages plant tissue, free cyanide is produced. Grazing can continue on grain sorghum stubble until frost, or even after frost if the plants are allowed to turn brown before they are grazed. Do not graze frost-damaged or stunted grain sorghum until they have been killed (turn brown) by the frost. If plants begin to grow again after being frost damaged, they should not be grazed until the regrowth is 18 inches tall or the entire plant is killed by frost and turns brown. There are some important precautions producers should be aware of before grazing or utilizing grain sorghum stubble fields. * Don t graze on nights when a frost is likely. High levels of the toxic compounds are produced within hours after a frost occurs. When only plant tops have been frosted, new shoots may regrow at the base of the plants. These can be very dangerous because of high prussic acid content, and because cattle will selectively graze them. Do not graze frosted summer annuals until regrowth of shoots is 15 to 18 inches tall, or until several days after the entire plant and shoots are killed by subsequent frost. * Don t graze or green chop grain sorghum for at least 4-6 days after a killing frost, or until the plant is dry. Plants that are frozen may release high concentrations of prussic acid for several 5

6 days. After wilting, prussic acid release from plant tissues will decline. Dead plants have less free prussic acid. Prussic acid concentrations are higher in fresh forage than in silage or hay because HCN is volatile and dissipates as the forage dries. However, if the forage had an extremely high cyanide content before cutting, or if the hay was not properly cured, hazardous concentrations of prussic acid could remain. * Before a frost has occurred, grain sorghum stubble is usually safe to graze, but small green shoots of sorghum or shattercane may develop in the stubble. Abundant regrowth can be dangerous. Young plants or regrowth after grazing contain higher concentrations of prussic acid and should not be grazed until plants have reached a height of 18 inches. Never turn hungry animals into questionable forage. Prussic acid does dissipate from plant tissue; therefore, hay that has been properly cured is safe to feed. Properly ensiled forage is also safe to feed. When forage is being utilized as green chop, it is important to feed the green chop in a timely manner. If the green chop is allowed to heat, cyanide is released and the forage becomes toxic. If questionable forage must be grazed or utilized as green chop, feed dry hay along with the fresh plant material. Grain sorghum, forage sorghum, sudangrass, sudan-sorghum hybrids and pearl millet are notorious nitrate accumulators. Nitrate content generally is highest in young plant growth and decreases with maturity. Sorghums and sudangrass are exceptions because concentrations usually remain high in mature plants. If plants are stressed at any stage of growth, they can accumulate nitrate. Nitrates normally accumulate in stems and conductive tissues. Highest nitrate levels occur in the lower one-third of the plant stalk. Concentrations tend to be low in leaves because nitrate reductase enzyme levels are high there. A light frost can damage plant leaf area and reduce photosynthetic activity. With less available energy, nitrates accumulate in the plant. To be safe, sorghum producers should have sorghum tested for nitrates before feeding to livestock. The same precautions for prussic acid poisoning will help prevent nitrate poisoning. -- Based on Nitrate and Prussic Acid Toxicity in Forage K-State publication MF Considerations when pasturing wheat Producers who want wheat for pasture this fall have some decisions to make that will affect both the forage production and ultimate grain yield potential of the wheat. Cattle should not be put onto wheat pasture until there is crown root development to anchor the plants. Don t just look at topgrowth and assume if the wheat is tillered then crown roots have developed; sometimes that s not the case. Check some plants to make sure there is good root development. Cattle should not be able to pull the plants up out of the ground as they graze. In general, there should be about 6-12 inches of topgrowth before pasturing wheat, but the true test of when the wheat is ready is to see if crown roots have developed enough that the wheat is hard to pull out of the ground. 6

7 Producers will have to plan to use extra nitrogen on wheat that is pastured. Cattle remove N in the wheat forage. It s not uncommon to see N deficiencies in wheat after cattle have been removed. For every 100 lbs per acre of animal gain, producers should apply another 40 lbs per acre of N in order to maintain grain yields. Producers should use split applications of N, with part of the N going on in early fall, and part of it applied as soon as the cattle are pulled off in the late winter or early spring. Producers should be sure to have a dry area available to move the cattle onto when the fields get wet. This will help limit soil compaction problems. -- Jim Shroyer, Extension Agronomy State Leader jshroyer@ksu.edu These e-updates are a regular weekly item from K-State Extension Agronomy and Steve Watson, Agronomy e- Update Editor. All of the Research and Extension faculty in Agronomy will be involved as sources from time to time. If you have any questions or suggestions for topics you'd like to have us address in this weekly update, contact Steve Watson, swatson@ksu.edu, or Jim Shroyer, Research and Extension Crop Production Specialist and State Extension Agronomy Leader jshroyer@ksu.edu 7