You are here
C.O.R.N. Newsletter: 10-2021
Weather and herbicides – what to do (or not) this weekAuthor(s): Mark Loux
Current forecast is for fairly warm temperatures through late evening Tuesday evening, followed by a substantial drop in temperatures and chance of snow, followed by cold/cool temperatures through the weekend. Primary question concerning this scenario seems to be whether it is okay to apply wheat or burndown herbicides prior to this cold snap. Some things we know about herbicides and cold weather:
- Herbicides applied to an emerged crop just prior to or during cold weather may be more injurious compared with favorable weather conditions. During cold weather when plants are not actively growing or growing slowly, the rate of translocation and metabolism of herbicide by the plant slows down, which can mean an accumulation of herbicide that is not being metabolized. This can increase the risk of crop injury since metabolism of herbicide by the crop, or conversion to an inactive form, is what allows that herbicide to be safely used on the crop in the first place. For some herbicides, there is such a large margin of safety with regard to crop safety that this is all inconsequential. For others the margin is narrower and issues such as cold weather and sprayer overlaps are more important. The inclusion of safeners in herbicide formulations reduces the risk of injury, usually be increasing the rate of metabolism, but may not completely solve issues that arise because of adverse weather or too high a dose. So with regard to this week and risk of injury to wheat, we would recommend avoid applying herbicide once the cold weather starts (from Wednesday on), until warm weather resumes.
- There is less certainty in making a recommendation about whether to treat wheat on Tuesday prior to the cold weather. We have seen instances in corn where application just prior to cold weather has resulted in greater injury. Wheat is actively growing now under favorable weather, and should readily translocate and metabolize herbicides. Much of this process occurs within the first few hours of application. Temperatures do not really start to plunge until early Wednesday morning per the forecast. While it’s somewhat of a guess, it seems that application during the first part of Tuesday would be possibly safer to the crop than later in the day. Past experience has shown us that some wheat herbicides are just generally safer than others, so one option would be to omit the ones that have stricter growth stage guidelines or have more of a history of causing injury. Having said this, in our research we have really not experienced injury from small grain herbicides applied per label.
- With regard to efficacy of burndown herbicides and cold weather, some of the same principles apply. Applying herbicide from Wednesday through the weekend, when weeds are not actively growing, is not recommended due to the likely loss of activity. Susceptible weeds metabolize herbicide slowly anyway, so the issue is a lack of translocation within the plant and the inability of herbicide to do it’s thing at the active site when plant processes are shut down. This is the type of cold weather we referenced in the recent article about dandelions, when we have observed control of this weed to plummet. We have also observed extremely slow control of overwintered annual weeds during cold weather.
- We would recommend going ahead with burndown herbicide applications on Tuesday, prior to the cold. As with wheat, weeds are actively growing under favorable weather so we assume herbicides will work. It’s still a bit of a guess, but it could be a while before field conditions and weather are suitable for application again.
- This is the type of scenario that makes us want to remind everyone again that a few dollars of herbicide in the fall can help avoid some of the nasty burndown issues that develop when spring conditions are less than optimum. Just saying.
Alfalfa Weevil – It’s Closer Than You Think
Though the current cold snap has caught our attention, we are actually ahead on heat units compared to this time last year, and we’ve accumulated enough degree days to see potential outbreaks of alfalfa weevil in some locations. Weevils have already been spotted in northwest Ohio. Overwintered adults begin laying eggs when temperatures exceed 48°F. Peak larval activity and feeding damage occur between 325 and 575 heat units (based on accumulation of heat units from January 1 with a base of 48°F). Current (Jan. 1 – Apr. 15, 2021) heating units range from around 200 in northeastern Ohio, over 200 across much of the state, and over 300 units in central and southeast Ohio.
In short, now is the time to start scouting. Alfalfa fields should be scouted weekly for weevils until at least the first harvest, which is approximately 3 to 5 weeks away at this point. Don’t let your guard down with the recent turn to cooler weather! We’ve seen significant weevil infestations in past years when early warm weather pushed weevil development earlier than normal, followed by cooler weather later that slowed alfalfa growth. The result was weevil larvae reaching stages when a lot of feeding occurs and the slowed alfalfa growth not staying ahead of their feeding damage. Follow-up scouting may be needed after the first harvest in heavily infested fields.
Spot problem fields early by checking alfalfa tips for feeding damage – small holes and a tattered appearance. Fields that have a south facing slope tend to warm up sooner and need to be checked for weevil earlier.
Here is a video about scouting weevils in alfalfa: https://forages.osu.edu/video.
Scout for alfalfa weevils by collecting a series of 10 stem samples from various locations. Place the stems tip down in a bucket. After you’ve collected 10 stems, shake the stems vigorously into the bucket and count the larvae. Divide this number by 10 to get the average number of larvae per stem. Do this procedure at least 3 times (for a grand total of 30 stems, in 10-stem units). Alfalfa weevil larvae go through four growth stages (called instars). The shaking will dislodge the late 3rd and 4th instar larvae which cause most of the foliar injury. Close inspection of the stem tips may be needed to detect the early 1st and 2nd instar larvae. Also record the overall height of the alfalfa. The treatment threshold is based on the number of larvae per stem, the size of the larvae and the height of the alfalfa according to the following table. When alfalfa is around 12-16 inches in height, growers can consider an early harvest rather than spraying, if they feel the current growth is sufficient to justify the cost of harvest or if spraying can’t be done for some reason (e.g. organic production). When alfalfa stem height is over 16 inches, we would always recommend an early cutting. In those fields which are cut early for alfalfa weevil, the regrowth should be checked closely to make sure weevils that are still alive do not prevent good regrowth.
Table 1. Action thresholds relevant to stand height, tip feeding, and density of larvae per stem.
Stand Height Inches
Indication of Problem % Tip Feeding
Problem Confirmation Larvae per Stem
Recheck in 7 days
When harvested early due to weevil, check within one week for regrowth.
For more information about alfalfa weevil, visit our factsheet at https://ohioline.osu.edu/factsheet/ENT-32 If you are interested in a more detailed treatment of how growing degree days can be used in management decisions for alfalfa weevil, visit this website from the University of Kentucky https://entomology.ca.uky.edu/ef127
Black Cutworms and True Armyworms are ArrivingAuthor(s): Andy Michel, Kelley Tilmon, Curtis Young, CCA, Clifton Martin, CCA, Lee Beers, CCA, Beth Scheckelhoff, Eric Richer, CCA, Mark Badertscher, Cindy Wallace
We have begun collecting two important pest in our expanded trapping this year—true armyworm and black cutworm. True armyworms feed on wheat before moving on young (typically late-planted) corn. Black cutworm can feed on young corn and even completely cut plants. The moths of these pests migrate from the south and lay eggs in April and May. True armyworms prefer to lay eggs in wheat or even grassy cover crops like rye whereas black cutworms tend to lay eggs in weedy fields, especially those with chickweed or purple deadnettle. However, infestations are really hard to predict, and the best way to prevent damage is by scouting.
Although we have found moths in our traps, the overall number is relatively low (14 total for armyworm and 17 for black cutworm). We expect activity of both moths to increase over the next few weeks, and then larval activity later in May. For the most up to date trap numbers, please monitor updates at: https://docs.google.com/spreadsheets/d/1yl_FeI5IJKkBVfdvKJuE6pWkd58OVhCyHeZ6SZM-ePE/edit?usp=sharing.
New FactSheet is published on Nutrient Removal for Field Crops in OhioAuthor(s): Harold Watters, CPAg/CCA
An update for nutrient recommendations for Ohio's major field crops (corn, soybean, wheat, and alfalfa) was published in November 2020 as the Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa. The call came in shortly after suggesting that we grow several other crops in Ohio that were not included in this update.
With information from the Tri-State update, discussion with state specialists, and a review of the literature, a FactSheet was created to offer assistance for these other agronomic crops beyond corn, soybean, wheat and alfalfa, titled “Nutrient Removal for Field Crops in Ohio”: https://ohioline.osu.edu/factsheet/anr-96 found on OSU Extension’s OhioLine.
The current philosophy in Ohio for crop nutrient management is to apply phosphorus (P) and potassium (K) fertilizer or manure equivalent at crop removal rates. Additionally, if soil test levels of P and K are below the critical level, then a "Buildup" recommendation would be considered to increase soil test levels into the "Maintenance" range.
Tables provided in the new fact sheet include crop removal values for 18 crops, our new tri-state maintenance ranges, and recommendations for N, P & K for these crops. Critical soil test levels have not been recently evaluated for the additional crops beyond corn, soybean, wheat, and alfalfa; however, it is reasonable to assume that critical soil test values used for crops listed here, and in the updated Tri-State Fertilizer Recommendations, can be used as guidance for the small grains and forage crops.
No information is given in this fact sheet for vegetable crops. Please use the information found in the References section for information on where to find out more to develop nutrient recommendations. Consider for these many crops the same philosophy stated here and in the Tri-State update to only apply maintenance (or crop removal) applications of P & K.
Will Forage Stands Be Damaged by Predicted Freezes?Author(s): Mark Sulc
The weather forecast this week is indeed concerning for forage stands in general and especially for alfalfa and red clover. The low night temperatures in the forecast may potentially cause severe frost injury to both annual forage crops (e.g. winter rye and winter triticale) and perennials forages.
Growers should scout and evaluate their forage stands several days after the cold nights because predicting freeze damage is difficult to impossible. Freeze damage and plant recovery from it are influenced by many factors, including the absolute minimum low air temperature, soil temperature during the freeze event that can moderate near-surface air temperatures in the canopy, field topography, snow cover during cold nights (that provides insulation), age and stage of plant growth, and stand health and vigor as influenced by soil fertility and prior cutting management.
The overall vigor of the stand will determine the tolerance to freezing and recovery from freeze injury. Vigorous stands that were not cut in the fall and with good soil pH and fertility will tolerate and recovery from freezing the best.
What to look for in established stands: Damage will initially appear as a wilting of the forage legume leaves and stems, with the top of stems bending over into a “shepherd’s hook” appearance (see Figure 1). This can usually be seen within 24 hours in legumes. This initial wilting might be a little harder to see in perennial grasses. This wilting damage is either temporary if the freeze damage is not too severe, or it is the initial symptom of more severe damage that will appear in the next several days.
Several days after a severe freeze event, leaf and stem death will become obvious, as seen in Figure 2. The shoots and their growing points might be completely killed, but it is rare that the entire crown is killed unless the stand vigor is very poor.
Several days to a week after the freeze event(s) evaluate established stands to determine the damage.
If a third or less of the tops are damaged, do nothing as the remaining undamaged stems will provide enough growth and yield. There may be some yield reduction, depending on the stand vigor and health. There will likely be some delay in growth resulting in a later first cutting which will help the stand recover more fully.
If most, but not all, of the stem tops are damaged and the stand is less than 10 inches tall, it should recover in time. New existing buds in the axils of leaves in the lower canopy will grow and new crown buds might be initiated and grow as well. Mowing existing top growth will not improve the recovery.
If most of the stem tops are damaged and the stand is more than 12 inches tall, harvest the forage and allow it to regrow.
If all stems are frozen back with severe plant necrosis, the plant is probably dead. If a large portion of the plants in the field exhibit these symptoms, it would be best to plant an emergency forage or interseed the stand with an annual grass forage crop. This scenario is most likely for stands that were abused, older, were not healthy and vigorous, were cut in the fall, or have inadequate soil pH and fertility.
What to look for in new seedings: New seedings made this spring might survive a cold night better than you might think due to the seedlings being close to the warmer soil surface. Seedlings are more freeze tolerant in the cotyledon to first or second trifoliolate leaf stage than at later growth stages. Most new seedings planted in the last few weeks are in these very early stages of development.
If death of the cotyledons and all leaves occurs, the seedlings are probably dead and will not regrow. A good indicator of survival potential is if at least one set of leaves survives the freeze. A companion crop like oat planted with the forage seeding will help protect the newly emerged perennial seedlings.
In summary: Be prepared to assess your forage stand, especially new legume stands, in the days and week following our cold nights that are forecast for later this week. Reach out to your county extension educator with any questions and observations you have about your forage stands. We will be assessing any damage across the state and will follow up with articles in this newsletter in the coming weeks if necessary.
Check-Out Virtual Crop Scouting SchoolAuthor(s): Laura Lindsey
We’ve wrapped up our extension winter programming, but are you looking for more? If so, check out the 2021 Virtual Crop Scout School to help get ready for the growing season. There are 22 free webinars with a range of topics, including two speakers from Ohio State- Laura and Alex Lindsey. (Yes, we need to get out of the house more often.) Please see the press release below for more information:
The 2021 Virtual Crop Scout School is now available and is free to the general public. The scout school consists of 22 webinars from crop protection specialists at eleven Midwest Universities and is offered through the Crop Protection Network (CPN).
Crop scouts, farmers, and other users can pick and choose from a variety of diverse subjects to help them become more knowledgeable on crop scouting. Topics are split into digestible bits so crop scouts can interact with subject matter in a way that best suits their time and interest.
Crop scouting in an important part of integrated pest management (IPM) that can help farmers obtain higher yields and increased profit per acre. Scouting gives farmers and agronomists a "heads-up" about what is happening in the field, allowing preemptive action and appropriate management decisions to be applied. The field scout gathers information on the crop condition of a field, which can help in discerning which of the various management tools to use.
“The Virtual Crop Scout School is coming on the heels of a web book on crop scouting released by CPN earlier this year,” said Daren Mueller, Extension associate professor at Iowa State University. “Crop scouting can do a lot to bring greater yields to farmers’ fields.”
CPN has partnered with Universities all over the Midwest to make these webinars a reality. This work is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
CPN is a multi-state and international partnership of university and provincial Extension specialists, and public and private professionals that provides unbiased, research-based information. CPN’s goal is to communicate relevant information to farmers and agricultural personnel to help with decisions related to protecting field crops
CFAES Ag Weather System 2021 Near-Surface Air and Soil Temperatures/Moisture
Figure 1 shows that two- and four-inch soil temperatures cooled over the past week in response to a more seasonally average weather pattern. Daily average soil temperatures are now running in the upper-40s to mid-50s across the region. A much colder pattern is expected to move into Ohio mid-week this week, with sub-freezing overnight temperatures and accumulating snow possible, especially across our northern stations. Slightly warmer, but below average conditions, will resume for the upcoming weekend.
Figure 2 (left) shows that precipitation was light across Ohio this week, below to much below average for the state. The heaviest totals (0.50-1.0”) fell across the southern and northeastern counties (light green). Although precipitation over the last couple of weeks has slowed the progress of drought conditions across northwest Ohio, much of the northern tier of the state remains abnormally dry over the last 30-90 days. The top 40 cm of the soil remains dry relative to historical conditions (red shading in Fig. 2 – right). As a result, the U.S. Drought Monitor continues to depict approximately 16% of the state with Moderate Drought conditions.
For more complete weather records for CFAES research stations, including temperature, precipitation, growing degree days, and other useful weather observations, please visit https://www.oardc.ohio-state.edu/weather1/.
Crop Observation and Recommendation Network
C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.
The information presented here, along with any trade names used, is supplied with the understanding that no discrimination is intended and no endorsement is made by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.
CFAES provides research and related educational programs to clientele on a nondiscriminatory basis. For more information, visit cfaesdiversity.osu.edu. For an accessible format of this publication, visit cfaes.osu.edu/accessibility.