C.O.R.N. Newsletter: 2024-09

The recent heavy rainfall events have implications on the fate of applied nitrogen (N). In the past two weeks, Ohio has received rainfall ranging from about 1.5 inches in the northwest to 6 inches in the central-eastern counties. Year to date, we are experiencing 2 to 8% higher rainfall than normal. Nitrogen loss is exacerbated under such excessive rainfall scenarios via three pathways: runoff, leaching, and denitrification.

RUNOFF

The amount and intensity of rain received in the past week may have caused substantial soil erosion. Fields with no cover or residue are more susceptible to N runoff. If N fertilizer was applied recently in such fields, especially without incorporation into the soil, there is high likelihood of N fertilizer being eroded out of the field.

LEACHING

The nitrate form of nitrogen is liable to leach down the soil profile. Leaching is more prevalent in coarse textured soils than fine textured soils. In addition to the amount of rain, the amount of N loss via leaching will depend on:

Type of fertilizer used: Leaching loss of N depends on the amount of nitrate present in the soil. Fertilizers that contain N in the nitrate form such as UAN increase the chance of leaching loss because a large portion of UAN exists as nitrate form. According to Havlin et al. (1999), most of the anhydrous ammonia gets converted to nitrate in about 3 to 8 weeks whereas 25% of UAN is in nitrate form on day 1 of application and 100% of it is converted to nitrate within 1.25 to 2.5 weeks. Enhanced efficiency fertilizers that either include nitrification inhibitors or protective covering reduce the leaching potential by slowing the conversion of the source to the nitrate form.

Crop stage: If the field was bare, nitrate leaching potential could have been substantial. The presence of a cover crop or wheat during this time of the year can reduce nitrate leaching. The N uptake of wheat during the dormancy or early green up stage is minimal, thus making nitrate liable to leach down. If wheat was in stem elongation phase, nitrate leaching is less likely due to increased N uptake and prolific rooting system of wheat at this stage.

DENITRIFICATION

When soil is saturated with water during or after the rain event, N can be lost to atmosphere via denitrification process. The denitrification process refers to the conversion of nitrate form into gaseous forms (NOx or N₂). The amount of N loss depends on how long soil stays saturated and soil temperatures during waterlogging, with warmer temperatures accelerating denitrification losses. Table 1 shows the potential loss of N due to denitrification under saturated soil conditions for different periods in days and soil temperatures.

Table 1. Potential loss of nitrogen due to denitrification based on days of saturated soil conditions and soil temperature. (https://extensionpubs.unl.edu/publication/ec155/pdf/view/ec155-2014.pdf).

STRATEGIC PLAN FOR N MANAGEMENT AFTER HEAVY RAINS

If any of the above N loss mechanisms occurred in your field, especially after an N fertilizer application was made, it is important to supplement N to growing/upcoming crop to avoid yield losses. You can follow below given strategies:

Estimate the N loss: The amount of N loss can be estimated based on the fertilizer type, time of application, crop growth stage, and soil water content. Lentz and Culman (2015) outlined a procedure in this article: Concerns for N Loss in Corn from Recent Storms

Determine N in the soil: Pre-Sidedress Nitrogen test (PSNT) can be done to quantify the status of nitrogen in the soil. LaBarge (2022) outlined the procedure to decide if additional N would be needed in this article: Estimated and Soil Test Methods to Determine Supplemental N need after Flooding

Look out for plant N deficiency symptoms: Yellowing of lower leaves or inverted yellow V pattern on leaves could be a sign of N deficiency in crops. If such symptoms persist after soils are no longer waterlogged and temperatures are not on cooler end, it might be worthwhile to consider side-dressing N. You could also utilize sensors to determine NDVI to calculate N fertilizer requirement. The description of some Apps is outline by Logan et al. (2018) in this article: Crop Nutrition Apps

References:

Havlin, J.L., J.D. Beaton, S.L. Tisdale, and W.L. Nelson. 1999. Soil Fertility and Fertilizers. An Introduction to Nutrient Management. 6th ed. Prentice Hall. Upper Saddle River, NJ.

Spring means rapid forage growth, but it also means rapid weed growth. Due to the variability of spring weather, there are often only a few opportunities to control emerging summer annual weeds, winter annuals missed in the fall, and biennials that are small enough to effectively control. To manage weeds before they become a problem in forages, it is important to scout and plan accordingly. Forage is a broad category, and the spring weed control plan can look very different between species and operations. The problem weeds and whether control is necessary are going to be different between permanent pasture systems and alfalfa fields, and highly dependent on the consequences of specific weeds.

In established alfalfa, the decision for weed control of some winter annuals like henbit and field pennycress will depend on the severity of the weed presence, the age of the stand, and the end purpose of the forage. If the weed pressure is high, the stand is young, or the lower forage quality of the weeds interferes with the goal of producing dairy-quality hay, the weed control treatment may be worth the associated cost. In a grazing system, it may be more pertinent to control weeds in the spring to ensure weeds that aren’t grazed don’t go to seed. Numerous weeds can be a problem in forage systems. Reference the 2024 Weed Control Guide for specific recommendations following this general overview.

Summer annuals are most effectively controlled when they are small in the spring. An example weed is common cocklebur, a summer annual that is competitive in forage stands. The seeds of common cocklebur are toxic to livestock and maintain toxicity when cut as part of hay.

In established pastures, there are many effective control options for summer annual broadleaf control, including:

• 2,4-D
• Crossbow
• PastureGard
• GrazonNext

Common Cocklebur Seedling Source: Iowa State Extension

Winter annual weeds are most effectively controlled in the fall when they are small and have not lived through the winter, but there is a window in the spring to control anything missed. A winter annual that is always a concern come spring is cressleaf groundsel, due to its toxicity to livestock. Cressleaf groundsel should be actively managed; if an infestation builds up to where mature weeds with yellow flowers are visible, herbicide control is no longer an option, and the weed maintains its toxicity even after cut.  The first cutting of an area heavily infested with common groundsel will likely have to be discarded instead of sold or fed. There are limited herbicide options for control; in pastures and grass hay fields 2,4-D is an effective choice, but will kill any desired legumes present. Weeds can commonly be seen as pretty flowers by neighbors and left to bloom, be sure to scout property edges to lower the risk of a weed infestation jumping into the field the following spring.

In alfalfa and legume hay, some of the chemical control options include:

• 2,4-DB (Butyrac)
• Pursuit
• Raptor
• Glyphosate (Roundup Ready alfalfa or spot treatment only)

Cressleaf Groundsel Source: Ohio State Weed Science

Another group of weeds that we have a chance to control in the spring are biennials. Poison hemlock is a biennial that commonly finds its way into forages, moving in from field edges and fence rows. Poison hemlock is toxic when ingested by livestock and can also cause skin irritation. Poison hemlock is most often a problem in pasture systems that are overgrazed and where there is inadequate grass growth. Where other grazing options are available, animals tend to avoid this plant, but control of poison hemlock should still be a priority. Being biennial, it is most effective to control poison hemlock while it is in the rosette stage.

For the control of poison hemlock in pasture systems, herbicide options include:

• Remedy Ultra
• Crossbow
• Dicamba

All the previous examples given were broadleaf weeds, but there is also the opportunity to control grass weeds in alfalfa and legume fields in early spring. Roundup Ready alfalfa makes glyphosate and products such as Extreme an option. In conventional alfalfa fields, products containing clethodim (Arrow, Select Max, etc.) and Poast are effective at controlling grass weeds.  Weed control in forages is highly situational; it is important to read the herbicide label to ensure effective weed control, limit potential damage to desired forages, and to be aware of any potential grazing or harvest restrictions.

Extra Resources:

Managing Cressleaf Groundsel OSU Video

Poison Hemlock OSU Video

Does it seem like a new precision weed management technology is popping up just as frequently as weeds do in your fields? With the rapid pace of innovation in this area, it can be hard to stay informed of the most recent developments. In response to frequent questions about drone technology, the GROW network is hosting its first Farmer Forum of 2024 on Monday, April 15^th at 2 pm ET to provide some insight into how drones are being used for weed management.

Listen in – and bring your questions – as panelists from across the country speak about their experiences managing weeds with drones and following best management practices for this developing industry. Certified Crop Advisor (CCA) CEU credits will be available.

Speakers:

Rick Jordan, Operations Manager, CNY Drone Services, NY

Tyler Mudd, Farmer and Drone Operator, Monroe City, MO

Moderator Dr. Steve Li, Extension Weed Scientist, Auburn University, AL

Registration is free! Sign up here.

GROW, or Getting Rid Of Weeds, is a national network of weed scientists, extension educators, and outreach specialists working together to develop best practices and digital tools to support the use of integrated weed management to combat the herbicide resistance epidemic. GROW Farmer Forums are a series of webinars that allow farmers that are adopting these practices and tools to share their experiences with their peers that might be interested in trying out something new.

To explore a variety of other integrated weed management resources, visit the GROW website, https://growiwm.org/