University of Nebraska Institute of Agriculture and Natural Resources Cooperative Extension

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July 13, 2001

In this week's Crop Watch

Uneven and slow-starting soybean patches or stands were not unusual in some areas. Limited soil moisture and developing dry conditions in some areas may have spurred the need for early soybean irrigation. (Photo courtesy IANR Photographer Brett Hampton)

Crop Production

How high temps affect corn pollination

Adjust alfalfa cutting for the heat

Insects

Scout for corn rootworm beetles

Resources

Crop Production Field day July 20

Wheat

Low test weights, price discounts typical of early harvested winter wheat

Climate

Rain welcome, but soil moisture still lacking

Research

Nelson coordinates NU variety testing

Other

Field updates

AgNews

Nebraska's July 1 wheat production forecast

Weeds could meet their match in a novel bioherbicide

Biodiesel jet fuels offer new market for soybeans

How high temps affect corn pollination

With a national record-breaking high in Nebraska last week and predictions for more temperatures in the 90s to 100s next week, talk in the coffee shop is turning to how this will affect silking and pollination in nearby corn fields. As of Sunday, July 8, Nebraska Agricultural Statistics Service reported that only 7% of the state's corn crop was in the silking stage. This was slightly ahead of the five-year average of 5% but behind last year at 12%.

Staging corn growth For a primer or review of the many growth stages of corn and management concerns at each stage, view "How a Corn Plant Develops", an Iowa State University Cooperative Extension publication and web site. Detailed descriptions of each stage along with photos to aid in differentiating the stages are included.

As temperatures soar and with tasseling in progress, we should be concerned about pollen viability and silk receptivity. Pollination is a critical period for corn development and yield. The following summarizes information from various sources on how this stress affects corn at this critical stage.

Pollen shed occurs over a two-week period. For kernels to develop, silks must emerge and be fertilized by viable pollen. Silks grow about 1 to 1.5 inches a day and will continue to elongate until fertilized. Temperatures greater than 95°F with low relative humidity will desiccate exposed silks but not impact silk elongation rates greatly. Pollen is killed by temperatures in the mid 90s or greater, especially with low relative humidity. Fortunately, pollen shed usually occurs from early to mid-morning when temperatures are lower.

Drought stress slows silk elongation but accelerates pollen shed. This can result in pollen shed occurring before silk emergence. Any stress such as inadequate water, low soil fertility, or too thick of a planting rate can delay silking two or more weeks and reduce seed set if pollen is not available. This is potentially a major problem although I have not heard of it happening often. The fact that pollen from one plant in ten is sufficient to pollinate a field provides a degree of compensation and improves the opportunity for fertilization in stressful environments.

The bottom line is that high temperatures will not severely stress corn if soil moisture is adequate. Reports of subsoil moisture across the state indicate that levels are varying widely, with some sites reporting their lowest of the growing season and others reporting two to four inches of moisture in the top two feet. Obviously we don't have to tell farmers to keep up with irrigation at this time of year. It is one of the best ways to reduce the impacts of high temperatures on corn pollination and fertilization.

Rain-fed fields are more of a concern. Drought stress with high temperatures at pollination and silking can have serious effects. If the current dry-hot conditions continue, I would expect to see major differences among fields based on management practices and hybrids. Practices that conserved soil moisture this spring or last year such as no-till or reduced till will improve a crop's performance during drought. Early-season hybrids probably will do better than other hybrids if pollination occurred before temperatures soared or moisture reserves were depleted. Full-season hybrids with good stress tolerance may do better than others with less stress tolerance.

Roger Elmore
Extension Crops Specialist

Rain quite welcome, but soil moisture still lacking

Substantial rainfall was observed across a broad area of western and north central Nebraska July 4-10. The heaviest rainfall was reported north of the Platte River, with precipitation generally ranging from 2 to 4 inches across the northern Panhandle through most of the Sandhills.

Rainfall did occur south of the Platte River, with the heaviest levels in a small area from Hastings though Grand Island and from Fairbury east through Nebraska City. Preliminary rainfall totals for the seven-day period ranged from 0.75 to 1.25 inches within these areas. Otherwise, most rainfall rates were between 0.25 to 0.75 inches.

Unfortunately, the driest sections of the state (southwest and south central Nebraska) failed to receive enough precipitation to reduce our concern for drought in that area. This week's national Drought Monitor will not show a change in the current drought area in Nebraska, but some of the area north of the Platte River shown as abnormally dry will be reduced. The recent storm activity is the result of a combination of monsoonal moisture and a slight retrogression of the massive upper air ridge that was situated over Texas and Oklahoma.

If the numerical models are correct, temperatures across Nebraska should begin to inch toward the century mark by early next week. The greatest likelihood of 100°F or greater readings should be over Nebraska. If this forecast proves true, atmospheric temperatures will probably be too warm for widespread rainfall. It's too early to tell whether Nebraska will see a repeat of the extreme heat recorded across southwestern Nebraska July 4-8. McCook had a high temperature of 111°F during this five-day period. If abundant rains fail to materialize through this weekend, crop stress will reappear rapidly next week.

Soil moisture monitoring sites south of the Platte River have seen a significant erosion of stored moisture during the last 30 days. Currently, all monitoring sites in this region are either at or within two inches of their lowest moisture levels of the year. If hot dry conditions return next week, most of the benefits of the fall and spring recharge season will have been eliminated. Models indicate that by July 18, virtually all of the state's corn crop should be in the silking stage. Above normal temperatures are likely during this period and sufficient moisture will be critical to limiting plant stress. If enough moisture falls prior to the return of the high temperatures, canopy moisture levels may be high enough to minimize excessive drying of silks.

Al Dutcher
State Climatologist

Adjust alfalfa cutting for the heat

With the recent extreme temperatures, alfalfa plants are suffering. Growth rates decrease and moisture stress is common, even in moist soil. Growers attempting to produce high-quality hay also will have a problem if temperatures don't cool down enough at night. High nighttime temperatures cause high metabolic rates in alfalfa, burning off valuable nutrients that the plants had accumulated during the day. This often produces alfalfa hay with fine stems that contain high protein, as well as high fiber and low relative feed value. It is nearly impossible to make dairy hay under these conditions.

Another problem is the rate at which alfalfa plants mature. When it's hot, alfalfa may begin to bloom in less than four weeks. Since many growers use blooming as a signal to harvest, this early blooming can be misleading. During hot weather alfalfa plants need more time not less time to rebuild nutrient reserves in their roots because they burn off nutrients instead of moving them to the roots when it is hot. Often five to six weeks of regrowth is needed.

Growers also may need to adjust the time of day when hay is cut. As long as hay stays above 50% moisture, plant cells will continue to respire, burning away valuable nutrients. Cutting hay late in the day forces plant cells to respire all night long, losing yield and quality. Instead, it may be wiser to cut in the morning on good drying days if plant cells can be stabilized before nightfall.

Bruce Anderson
Extension Forage Specialist

Low test weights, price discounts
typical of early harvested winter wheat

In southwest Nebraska the wheat harvested to date is showing below normal test weights. In southeast Nebraska where seasonal rains were timely, test weights were somewhat better.

Southwest Nebraska

Most test weights have been in the 52 to 58 pounds per bushel range. Only a few have been 60 or above, the weight needed to meet the minimum grade standard for U.S. No. 1.

This year's lower test weights are a result of unfavorable conditions during grain fill. High temperatures (above 85°F), especially when accompanied by winds, shortage of soil moisture and foliar disease such as stripe rust causes shriveled kernels which lowered test weights.

Grade requirements for winter wheat test weight Grade Test weight (lbs/bushel) U.S. No. 1 60 U.S. No. 2 58 U.S. No. 3 56 U.S. No. 4 54 U.S. No. 5 51 U.S. Sample Grade is less than 51 Discounts add up quickly Unfavorable conditions during grain fill have led to less than favorable test weights in some areas, leading to discounts at the elevator for test weights below 60 lbs per bushel. This schedule of discounts was reported by an elevator in west central Nebraska this week. Additional discounts would likely be taken for too much moisture, jointed goatgrass in the grain, and diseased kernels. Test weight (lbs/bushels) Discount (Price/bushel) 59-59.9 1 cent 58-58.9 2 cents 57-57.9 4 cents 56-56.9 7 cents 55-55.9 11 cents 54-54.9 17 cents 53-53.9 23 cents 52-52.9 29 cents 51-51.9 36 cents 50-50.9 43 cents 49-49.9 50 cents 48-48.9 57 cents 47-47.9 64 cents 46-46.9 71 cents 45-45.9 78 cents

In addition to the low test weights being graded lower and discounted at the elevator , they will likely affect seed quality. Large, dense kernels are considered better quality for seeding. Large seed tends to tiller more than small seed. Seed quality is also affected by the protein content of the seed.

The amount of protein in the seed is very important to early seedling vigor. This is not protein percentage and a large seed may indeed have a lower percentage than a small shriveled seed, but it will have more total protein in each seed.

Test weight is a bulk density or a weight per volume measurement and since small seed can pack well, it may have a high test weight. Therefore, test weight is not the best measure of seed quality.

A better measure of seed quality is thousand kernel weight or seeds per pound. For many varieties 15,200 seeds per pound is used as a minimum for good quality seed. If test weight is used as a measure for seed quality, use seed with test weights above 57 pounds per bushel.

Seed cleaning will usually add one to two pounds per bushel by removing straw, chaff, weed seeds, small kernels, etc. to the test weight. It is always advisable to plant the largest seed available for a particular variety.

Bob Klein
Extension Cropping Systems Specialist
West Central REC

Southeast Nebraska

Wheat harvest in Saline County was excellent. It was typical to hear about yields of 38 to 60 bushels per acre with test weights of 60 to 62 lbs per bushel with an average of 61. By July 10, the harvest was 85% complete. Residual weed control was important in many fields this year due to the early spring weather.

Randy Pryor
Extension Educator
Saline County

The wheat crop was thin because of winterkill, but yields were average or a little above. Most producers reported yields of 40-45 bushels per acre. Test weights were 59-61 lbs per bushel.

Producers harvesting wheat are having a few problems with weedy fields due to thin stands and now more chinch bugs than normal moving into adjacent milo and corn fields.

Paul Hay
Extension Educator
Gage County

Scout for corn rootworm beetles;
plan control for next year or initiate adult control

Western corn rootworm beetles began emerging in early July in southeastern and south central Nebraska. Beetle emergence will be somewhat later in northeastern and western Nebraska. Although rootworm populations may be down somewhat compared with last year due to winter weather, areas with economic populations of rootworm last year will likely have economic levels this year.

Western corn rootworm beetle

Beetles emerging before silk emergence may feed on corn leaves. They feed by scraping the surface tissue, leaving a white parchment-like appearance. Once silks emerge this is the favored food. The earliest silking fields in an area often are most heavily damaged because beetles will move around in search of green silks. There are no thresholds for silk-clipping damage based on beetle numbers because damage levels are not correlated well with beetle densities. Usually an average of at least 10 beetles per ear are required to seriously affect pollination. Severe silk feeding at 25-50% pollen shed may indicate the need to apply an insecticide, especially in seed production fields. Visit the NU Department of Entomology web site for a list of insecticides labeled for adult rootworm control.

During late July and August these beetles will lay eggs in corn fields. These eggs overwinter in the soil, hatch into rootworms in the spring, and feed on corn roots if continuous corn is grown. However, not all continuous corn fields have economic infestations of corn rootworms. Weekly scouting of adult rootworm beetles in July and August will provide the information needed to decide whether a rootworm insecticide is needed next year. People using adult beetle control programs should base the decision to treat and spray timing on information from field scouting.

Average number of western corn rootworm beetles present in cornfields that may produce an economically damaging rootworm population in corn the following year. Plants per acre Average number of rootworm beetles Continuous corn1 First year corn2,3 Per plant Per ear zone Per plant Per ear zone 14,000 1.28 0.64 0.96 0.48 16,000 1.12 0.60 0.84 0.42 18,000 1.00 0.50 0.75 0.37 20,000 0.90 0.45 0.68 0.34 22,000 0.81 0.40 0.61 0.30 24,000 0.75 0.37 0.56 0.28 26,000 0.69 0.34 0.52 0.26 28,000 0.64 0.32 0.48 0.24 30,000 0.60 0.30 0.45 0.23 32,000 0.56 0.28 0.42 0.21 1Based on a 50:50 ratio of females to males. 2Based on a 70:30 ratio of females to males. 3Use this threshold for continuous corn fields that did not have larval populations earlier in the season (adult beetles are immigrants, similar to first year corn).

Begin scouting for corn rootworm beetles soon after beetle emergence begins and continue scouting weekly until threshold levels are exceeded or beetle activity stops. Examine 50 plants per field, taking samples from each quarter of the field. Sampled plants should be several paces apart, so that examining one plant doesn't drive beetles off of the next plant to be sampled. The most reliable method is to examine the whole plant for beetles. Beetles may hide behind leaf sheaths or in the silks, so care is required to observe all beetles present. An alternative method is to check for beetles only in the ear zone (the area including the upper surface of the leaf below the primary ear and the under surface of the leaf above the primary ear).

In continuous corn if beetle counts exceed 0.75 beetle per plant, damaging populations of corn rootworms are possible in that field next year. In first year corn, there is a higher proportion of female beetles, so the threshold is lowered to 0.56 beetle per plant. These thresholds are based on a 24,000 plant population per acre. The number of beetles per plant to equal a threshold level should be adjusted for different plant populations (see table or NebGuide G86-774, Western Corn Rootworm Soil Insecticide Treatment Decisions Based on Beetle Numbers). People scouting using the ear zone method should divide the above thresholds in half, since on average only 50% of the beetles on a plant are counted using this method. In fields with insect levels over the threshold, consider rotating out of corn or plan to use an insecticide at planting or cultivation to prevent economic damage. Fields remaining below the threshold level do not need to be treated with a rootworm insecticide next year.

Individuals using adult beetle control programs should begin treatments when the beetle threshold is exceeded and 10% of the female beetles are gravid (abdomen visibly distended with eggs). This is an important point since the first beetles to emerge are mostly male, and females require at least 10-14 days of feeding before they can lay eggs. Treatments applied too early may be ineffective if large numbers of females emerge after the residual effectiveness of the treatment has dissipated. Continue to monitor fields weekly after treatment for rootworm beetles. If beetle numbers exceed 0.5 beetles per plant, retreatment is warranted. Late maturing fields are particularly susceptible to corn rootworms moving into them from nearby earlier maturing fields.

A complete discussion of adult corn rootworm management can be found in NU Miscellaneous Publication 63, Adult Corn Rootworm Management, by UNL Entomologist Lance Meinke. Be aware that reduced adult rootworm control with foliar insecticides due to insecticide resistance has been documented in parts of south central Nebraska (see NebFact 99-367, Adult Western Corn Rootworm Insecticide Resistance in Nebraska). If you experience poor control with repeated applications of foliar insecticides, and high numbers of beetles are still present, it may be better to consider rotating that field out of corn next year rather than continuing to treat for beetles.

Bob Wright
Extension Entomologist
South Central REC

Crop production field day July 20

Field demonstrations and presentations on farmer-identified topics will be the focus of this year's University of Nebraska Crop Production Field Day. The program will be 9 a.m. to 1:30 p.m. July 20 at the University of Nebraska Agricultural Research and Development Center near Mead.

"Quick-hitter" topics will include:

Anhydrous ammonia properties and timing with Neal Christensen, agronomist with Agriliance
No-till weed management and glyphosate brand comparisons with Alex Martin, Extension weed specialist
Impact of plant spacing uniformity on corn yield with Dale Flowerday of DalMar Consulting
Management strategies for cutworms, wireworms, and white grubs with Barbara Ogg, Extension educator and entomologist
Early season diseases in corn and soybeans with Jim Stack, Extension plant pathologist
Update on early planted soybean and corn project with Walker Luedtke, ARDC farm research project coordinator

The Crop Production Field Day provides an opportunity for producers to view various crop production systems. Speakers will address a variety of questions and issues that producers face during the growing season.

The program is free and includes doughnuts and lunch provided by agribusiness. No CEU credits are available for this program. For more information, contact the Agricultural Research and Development Center at (402) 624-8030.

The ARDC is on County Road 10 in Saunders County, 2 miles north of Nebraska Hwy 63 or 3.8 miles south of Nebraska Hwy. 92.

Nelson coordinates NU variety testing

This is the fifth in a series of stories on the research pursuits of our contributing authors. Most NU Extension specialists also have major appointments in the NU Institute of Agriculture and Natural Resources' Research Division.

View variety test results online Results of the last seven years of University of Nebraska variety trials are available online at the UNL Department of Agronomy Variety Test web site. As winter wheat harvest continues over the next several weeks, results will be posted to the web. Variety test results also are available for previous years for alfalfa, corn, grain sorghum, oat, proso millet, soybean, spring wheat, sunflower and winter wheat.

Lenis Nelson has coordinated NU's crop variety testing program since 1988. Prior to that, he was the proso millet breeder at the Panhandle Research and Extension Center for nearly 18 years. During that time, he was involved with the variety tests of winter wheat, corn, and grain sorghum as well many other crops.

Nelson is an Extension crop variety and seed production specialist and a professor in the NU Department of Agronomy. He received his B.S. in agricultural education from South Dakota State University, and his M.S. and Ph.D. degrees in agronomy from North Dakota State University.

Variety tests are conducted across the state with agronomists at the district Research and Extension Centers. Corn is tested at 30 locations with some irrigated, some dryland, and some no-till sites. Fewer than 10 of these are on University land, the rest are on farmers fields. Extension educators help identify local cooperators and fields that would be representative of most of the county.

Soybean variety tests are conducted primarily in eastern Nebraska. As the soybean acres increased in western Nebraska, locations were added in Custer, Perkins, and Brown counties. These tests have made a huge shift from conventional to Roundup Ready.

Winter wheat tests are mostly in western Nebraska. There are two tests in southeast Nebraska, two in south central Nebraska, five in west central Nebraska and 10 in the Panhandle and eastern Wyoming. Only two are irrigated; the others are planted into the predominant tillage system of a given area.

"The Extension goal of the variety testing program is to provide accurate and unbiased data which compares the yield and performance of seed from all sources, Nelson said. "We gather this data from a sample of soil and climatic conditions. Our research goals are to relate the plant and environmental characteristics to allow us to more quickly and accurately place the proper varieties where they are best adapted."

"Research on alternate crops has been a considerable challenge," he said. "In times of low prices, there is an interest in new and totally different crops which can return greater profit. A number of factors work against this process: