C.O.R.N. Newsletter 2006-20

Soybean Rust in Louisiana

Authors: Anne Dorrance

We stick by our press release that soybean rust will not be an issue for 2006 for our soybean producers - based on the very low level of inoculum and poor environmental conditions for rust in the southern US. After we made that press release, the folks in Alabama found a total of 5 lesions in one soybean sentinel plot-- we corrected and have reposted the press release since just 5 minutes before the 4th of July weekend started (Friday afternoon) Louisiana has now reported a find-- on kudzu -- not on soybeans.

The story still stands! There is not enough inoculum close enough to Ohio for concern this season. Fungicides are not needed nor economical for the Ohio crop.

The Louisiana find is interesting, because the inoculum for this probably came from Mexico during that last tropical storm. Looks like the guys in Louisiana will be scouting fields this week.

We always hope that our corn is "knee-high" by the fourth of July-- I expect in the future for Ohio for rust to be an issue for our soybean crop there will have to be a lot of reports of soybean rust present in Kentucky, Southern Indiana by the fourth of July.

Have recent environmental stresses affected corn ear formation?

Authors: Peter Thomison

In recent weeks, depending on its location, the corn crop has been subject to some form of environmental stress, heavy rains resulting in saturated soils and flooding, hail and wind injury, swings in temperature, and localized dry weather. Corn development across Ohio varies considerably - ranging from about V6,the six leaf collar stage (for corn planted in late May) to V14 or beyond (for corn planted in mid to late April). Ear formation is well underway in most fields. As early as the V4/V5 stage, ear shoot initiation is completed and the tassel is initiated on the top of the growing point. During the rapid phase of corn vegetative growth which generally starts by V7, ear yield components are being determined. Kernel row numbers per ear (ear “girth”) are generally established by about V12.

Have the recent environmental stresses impact ear formation and yield potential? It takes fairly severe stress conditions during the early vegetative growth stages to impact kernel row numbers per ear. Kernel row numbers are usually less affected by environmental conditions than by genetic background. Therefore, in most cornfields, it’s unlikely that kernel row numbers have been impacted significantly by recent weather related stresses. However, unlike kernel rows per ear, kernels per row can be strongly influenced by environmental conditions. Determination of kernels per row (ear length) is usually complete about one week before silking (R1) or about the V17 stage. Stress conditions such as drought or severe defoliation caused by hail during the two weeks prior to pollination can reduce kernels per row and lead to a reduction in grain yield.

Hail Damage to Corn Will Vary Depending on Growth Stage

Authors: Peter Thomison

Some of the severe thunderstorms during the past week were accompanied by hail, which may have caused damage to crops. In corn, the impact of hail damage is largely dependent on the crop's stage of development. Hail affects yield primarily by reducing stands and defoliating plants. Most of the hail damage results from defoliation. Generally, the corn plant is little affected by hail prior to the 6 to 7 leaf stage because the growing point is at or below the soil surface and in the leaf whorl. However, once the growing point is elevated above the soil surface due to internode elongation, the plant grows rapidly and becomes increasingly vulnerable to hail damage with the tassel stage/pollen shedding stage (VT) being the most critical period.

Severe hail damage prior to the 6 to 7-leaf stage can also result in “twisted” or “tied” leaf whorls as injured plants recover and new leaves try to unroll; however, most plants will grow out of this problem and tied whorls seldom cause major yield loss.

Leaf damage by hail usually looks much worse than it really is, especially during the early stages of vegetative growth. Shredded leaves and plants with broken midribs still have some capacity to contribute to plant growth. Plants not killed outright by hail usually show new growth within 3 to 5 days after injury occurs (i.e. if damage occurs prior to tasseling). For this reason, estimates of hail damage should be delayed several days to allow for this period of re-growth.

The hail insurance adjustor's growth staging system counts leaves beyond the last visible collar to the uppermost leaf that is 40-50% exposed whose tip points downward - usually this results in a leaf stage that is numerically 2 leaves greater than the "leaf collar method" (e.g. a V11 plant according to the leaf collar method would probably correspond to a 13-leaf plant according to the hail adjustor's method).

How do we estimate the potential yield loss from recent hail storms? Corn growth stages in Ohio vary considerably depending on location, planting date, etc. Within some corn fields, it’s not unusual to see corn differ by three or more growth stages because of differences in soil color and drainage. Most corn in Ohio has not progressed beyond the V14 stage (as of 7-3-06). Based on estimates of the National Crop Insurance Association (see table below), at the 13-leaf stage (or about V11) if 50% of the leaf tissue is destroyed by hail, a corn plant loses 10% of its grain yield potential; if 100% defoliation occurs, a corn plant loses 34% of its yield potential. Table 4-5 in the OSU Agronomy Guide (on-line at http://ohioline.osu.edu/b472/0005.html) also provides information on effects on yield loss caused by different levels of leaf destruction.

Percent Yield Loss in Corn based on Growth Stage and Defoliation (adapted from NCIA Corn Loss Instructions, rev. 1984)

-----------------------Percent Leaf Defoliation

Growth Stage*	25%	50%	75%	100%
9-leaf (V7)**	1	4	7	13
11-leaf (V9)	1	7	12	22
13-leaf (V11)	2	10	19	34
15-leaf (V13)	3	15	30	51

*as determined using the hail adjustor’s leaf staging method;
** approximate leaf collar stage within parentheses.

For more detailed information on evaluating hail injury in corn, consult "Assessing Hail Damage to Corn" National Corn Handbook Chapter 1 (NCH-1). Available on-line at http://www.agcom.purdue.edu/AgCom/Pubs/NCH/NCH-1.html(verified 7/3/06).

 

Soybean Aphid Update

Authors: Bruce Eisley, Ron Hammond

We continue to visit fields to determine the status of the soybean aphid in Ohio. Last week, we revisited northwest OH and sampled numerous fields to see if things had changed since our last visit two weeks ago. We continue to find few if any aphids. The single field that had aphids that we mentioned a few weeks ago in this CORN newsletter had even fewer aphids this week. We only found one additional field that had aphids, and even then, found a total of 2 aphids when examining over 50 plants. The aphid population continues to be extremely low in Ohio, which is similar to nearby states. We feel a combination of low aphid overwintering plus numerous beneficial insects are combining to keep the aphids at these low numbers. As always, we will keep an eye on them throughout July to see what happens in the coming weeks because things could change. Keep checking this CORN newsletter for further updates on the aphid.

A concern we do have at this time is the inclination for some growers who might want to “help” the situation by applying a “preventive” insecticide treatment in their late herbicide applications. This question always comes up in our talks, so it should be addressed. We highly recommend NOT doing this! There are only two things that can or might happen. The first is you will waste money! The aphid densities are too low or non-existent in most fields. There is absolutely no reason to be treating them, preventively or not. Second, because beneficial insects might be helping keeping aphid populations low, an insecticide treatment would only serve to also kill off those beneficials, perhaps allowing the aphids to start multiplying easier in the coming weeks. We urge you to allow the beneficial insects to do their job by helping to keep soybean aphid below economic levels.

Rootworm Larvae Still Active

Authors: Ron Hammond, Bruce Eisley

We checked some corn roots (corn following corn) last week and found rootworm larval damage and rootworm larvae in the soil. The larvae were still actively feeding and most were only about one-fourth inch in length so they will be feeding for a while. So, if you want to see if your insecticide (granular, liquid or seed treatment) controlled rootworm larvae or if your transgenic rootworm hybrid controlled rootworm larvae or if you are in the area where the western corn rootworm biotype might be found, now is a good time to get the shovel out and start digging corn roots. Check the newsletter from last week (http://corn.osu.edu/#G) for instructions on how to sample.

Foliar Fertilizers – How Much is Actually Taken Up and What is the Agronomic Value?

Authors: Peter Thomison, Robert Mullen

Every year there are more and more foliar fertilizer materials hitting the ag market, and the question always comes up – how efficiently does a plant take up foliar nutrients and should I be relying more on foliar fertilizers? (Especially when dry, soil applied fertilizers are increasing in price).

We will begin at a fundamental agronomic principle – if the soil is supplying an adequate amount of a given nutrient then application of that nutrient in a foliar form is unlikely to result in increased yield. This is true for any nutrient. Another agronomic principle that has to be considered – application of a foliar nutrient that results in an increase of that nutrient within the plant’s tissue will not necessarily lead to a yield benefit. Often times the application of a foliar nutrient will alter plant nutrient concentration, but at harvest no appreciable yield benefit is evident. These two agronomic principles being considered, how much of a foliar fertilizer is taken up through the leaf?

The amount of a nutrient taken up by plant leaves is highly variable and strongly dependent upon environmental conditions. Some researchers report nutrient uptake as high as 90% for foliar applied nutrients while others report values as low as 30%. Leaf uptake of a foliar applied nutrient is not trivial when you consider the physiologic structure of a plant leaf. Leaves have a waxy cuticle designed to act as a moisture retaining and consequently moisture retarding layer. Thus absorption of nutrients can only occur through small cuticular cracks, stomata, or aqueous pores.

The agronomic value of foliar fertilizers is primarily related to the nutrient being supplied and whether or not the soil is providing an adequate amount. Foliar application of nitrogen, phosphorus, and potassium is unlikely to overcome severe nutrient deficiencies because you can not apply enough to provide the crop adequate nutrition throughout the growing season (unless multiple passes are made). Much work has been conducted throughout Iowa and Minnesota with the results being less than promising (http://www.ipm.iastate.edu/ipm/icm/2005/6-20/folfert.html; http://www.extension.umn.edu/cropenews/2003/03MNCN21.htm). Maintaining a good soil fertility regimen should decrease the need for foliar fertilizers in a crop production system. Ohio State University (and other Land Grant Universities) continues to evaluate new fertilizer materials to determine their efficacy for crop production.

The application of micronutrients can be beneficial to crop production because these nutrients are needed in much lower concentrations, but care must be taken to ensure that phytotoxicity does not occur from the application. Application of foliar zinc, manganese, boron, iron, etc. can benefit grain yield if they are found to be deficient in the soil. Soil deficiency is the key to determining whether or not additional nutrient is required. Soil testing can be useful for some micronutrients, but most deficiencies are due to other soil factors affecting nutrient availability primarily soil pH. When a crop is deficient the foliar application of these fertilizer materials can benefit the bottom line, but make sure the deficiency exists before considering their application. Visual assessment may not be adequate for diagnosing a micronutrient deficiency because many plant stresses result in similar symptoms, therefore plant/soil sampling should be used to confirm the problem.

Presidedress Soil Nitrate Test (PSNT) – Understanding its Limitations and Knowing Where to Use It

Authors: Robert Mullen, Edwin Lentz

The presidedress soil nitrate test (PSNT) or late spring nitrate test (LSNT) is often promoted as a soil testing tool that can be used to aid in nitrogen management decisions. Soil samples are generally collected between mid-May to mid-June whenever sidedress applications of nitrogen are being made. The question is when should I use the PSNT and what does the PSNT tell me as a crop manager?

The goal is to provide better nitrogen recommendations for crop producers. Nitrogen transformations that occur in the soil are dynamic and strongly influenced by environmental conditions because it is the result of soil biological activity. The transformation we are primarily concerned with in agriculture is the mineralization of organic nitrogen to inorganic forms (initially ammonium and eventually nitrate). If mineralization rates for soils could be predicted for a given growing season nitrogen recommendation strategies could be improved. Because accurate models do not exist to quantify this phenomenon we must relay on a snapshot in time of what has occurred in the soil (i.e. the PSNT/LSNT) or some other alternative method (reference/nitrogen rich strips). The PSNT/LSNT conducted on soils that do not have a manure history or previous forage legume crop rarely return levels high enough to prompt a decision. The PSNT/LSNT is almost exclusively promoted for fields that have received recent manure applications or where corn is following a forage legume crop.

Depending upon the PSNT/LSNT level we get an estimate of the likelihood of seeing a response to additional nitrogen fertilizer, but we do not get an actual nitrogen recommendation. PSNT/LSNT values near 25-30 ppm are unlikely to benefit from additional nitrogen fertilizer, and the higher the value the less likely the need for supplemental nitrogen. The problem arises when PSNT/LSNT values are less than 25 ppm. PSNT/LSNT values below this level may or may not respond to additional nitrogen fertilizer, but the stock recommendation would be that they do require more nitrogen. There can be sites that have a low PSNT/LSNT value but show no response to nitrogen fertilization. This is a major limitation of the PSNT/LSNT.

Put a Notepad and Pencil in the Combine

The lower half of the state has begun wheat harvest. While you are sitting in the combine take the time to identify the weeds in your wheat fields and record the information for future reference. Use your time wisely during harvest and honestly evaluate the successes and failures of weed control in winter wheat. During our travels around Ohio this spring and early summer and based upon phone calls we received, there are more weeds than normal in Ohio wheat fields, especially winter annual grasses such as downy brome, cheat, annual bluegrass and others.

Evaluating your weed control in the current winter wheat crop will allow you to start planning ahead for this fall’s winter wheat crop. This will allow you to improve weed control in next year’s winter wheat crop. One of the best ways to reduce winter annual weeds and dandelion in no-tillage wheat is to apply glyphosate as a burndown prior to wheat emergence. This will drastically reduce most winter annual species and sometimes control all of the winter annual species and is the best method for controlling dandelion.

If you are considering to plant double-crop soybeans in wheat fields, we strongly encourage you to plant Roundup Ready soybeans. Roundup Ready soybeans will allow you to obtain the best weed control and the least chance for injury of double-crop soybeans. Go ahead and get the soybeans planted, then about two to three weeks after planting apply glyphosate at 1.1 to 1.5 lb ae/A to control any weeds that will have recovered during harvesting of the wheat. Waiting this long will allow for good regrowth and yet not let the weeds get much larger which should maximize weed control. Remember that some of the weeds present, especially marestail and dandelion, may be up to ten months old which will cause them to be very difficult to control. Scout the fields for later emergence of weeds and spray if there has been a significant flush of new weeds. 90-99% of the summer annual weeds for the growing season have already germinated, so a second postemergence application may not be warranted, but be prepared.

We will discuss in future articles how to control weeds in wheat stubble that are not planted to double-crop soybeans. This is another reason for evaluating weed control now, so you can more accurately plan ahead for reducing future weed problems rather than adding to future weed problems.