July 10, 2020 By OMAFRA Field Crop Team
While the management window for many field crops is closing, some late fertility, insect and disease management remains important to stay on top of.
Wheat harvest has started. Most corn and soybean fields have closed canopy and are experiencing rapid growth with the heat. Field crops in many parts of the province are showing signs of moisture stress made worse by the extended high temperatures.
Late nitrogen (N) application in corn is happening now. With the hot dry weather there has been significant visual leaf burn from high clearance spinner spreader applications of dry nitrogen. Injury happens when fertilizer prills land in the leaf whorl. Condensation forms on leaves overnight and moves into the whorl, dissolving the prills and allowing for rapid N uptake by the tender leaf tissue. This N moves with transpiration to the leaf margins, where accumulation causes leaf burn. It is often visually troubling, but yield impacts are generally minimal. The exception might be where fertilizer sources are dusty and do not spread evenly, resulting in excess amount directly behind the applicator.
While split nitrogen applications in corn are a good management strategy, very late applications can create challenges. Nitrogen is primarily taken up by the roots when dissolved in soil water. Lack of timely rainfall after application means that surface-applied N may not move into the root zone in a timely manner and potentially leads to losses. Earlier side-dress timings give surface-applied N a greater chance of receiving timely rainfall to move N into the soil.
While the trend has moved away from side-dress injection, putting N directly in the soil does offer a greater chance of N being available to the crop in a timely fashion. The probability of losing N from the traditional side-dress timing compared to these later applications is relatively low. For producers trying to ensure N supply for post-pollination uptake, the traditional side-dress timing should have the same opportunity as the later applications to make this possible, with more opportunities to get it in the soil.
Insect pests continue to require attention
Hot, dry conditions tend to increase insect issues. There have been significant armyworm reports, with many wheat fields sprayed ahead of harvest. As winter wheat gets harvested, larvae will move to other grassy crops such as spring cereals, grassy forage stands and corn.
Be on the lookout for spider mites in bean crops as hot, dry weather persists. They will move into soybean and edible bean crops as wheat is harvested, and populations can explode rapidly. Scouting is imperative to ensure timely control when warranted. The action threshold is four mites per leaflet or one severely infested plant prior to pod fill. They are very difficult to see without a lens! Damage is stippling (dots) on the upper leaf surface from the chewing mouth parts that collapse individual plant cells, and plants appear sandblasted or dusty on the underside. From the road, a soybean field may look grey if there is an infestation, or brown if the leaves are dropping off and the plants are shutting down and dying.
In alfalfa and dry beans, be on the lookout for potato leafhopper (PLH). Once symptoms of damage appear, yield has been impacted. Scouting is key to staying ahead of PLH. These tiny pests are hard to spot and count because the adults “hop” or fly off the plant. The threshold for control depends on forage height or bean growth stage. In alfalfa, mowing will usually control PLH and encourage new regrowth from the crown. Scout after harvest, as leafhoppers can re-infest and impact the new growth; if PLH are still above threshold after mowing, a spray may be warranted.
For manure applications on harvested forage stands, ensure spreading happens shortly after harvest since the stress of the tanker/spreader weight can be damaging to alfalfa crowns once they have started to regrow. This impacts subsequent forage yield and overwintering tolerance.
Current crop stress offers learning opportunities
When crops are growing under stressed conditions, the impact of management choices becomes very obvious. There are many examples of tremendous crops across the province, but there are also many stressed fields. Taking the time to observe and record differences and their possible causes can help your overall crop management in the future. It’s important to determine if stress response is based on specific management choices or underlying natural variability (i.e. soil texture, soil parent material, etc.).
Ask yourself: why do some fields look better than others? Is it a variety/hybrid issue? Is it the tillage system, crop rotation, or fertility program choices that are causing the differences? It is often a management practice that is the culprit. If your fields are looking rough compared to your neighbour’s, have a chat with them (being mindful of physical distancing guidelines) and see if you can learn some new tricks that can improve your crops in future.
Walk your fields – even taking a walk through tall corn offers many teachable moments. Check out local variety/hybrid trials and other agronomy plots in the neighbourhood. Are some of those new offerings handing the heat and moisture stress better than others? Take some notes and refer to them in the fall when you have trial results. Did a more heat-/moisture-tolerant plant in July result in higher yield at harvest?
Environmental stresses, like insufficient moisture, often magnify other stresses, like soil compaction. Get out in those fields with a shovel and dig up some plants from good and rough areas of the field to compare root growth. Use a tile probe to identify compaction layers. If differences can be detected, explore what might have led to those differences. Work with your agronomists and others to learn as much as you can.
Successful decision-making is done with good data. Take the opportunity to gather all the valuable data that the current conditions offer. This should lead to better outcomes in the future.