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The Cropwalker - Volume 3 Issue 42

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Corn Rootworm Then, Now and in the Future

The Northern and Western corn rootworms have caused a lot of yield loss in Ontario. Initially we had Northern Corn Rootworm (NCR) but years ago it was displaced by the more aggressive Western Corn Rootworm (WCR). There were some recommendations on how to control it. Things like if you used a rootworm insecticide one year you did not have to use it the next. (WRONG).

This insect lays its eggs in late July at the base of corn plants. Normally it lays eggs near corn plants that it fed on. But it does fly to other fields and lay eggs there. Thus, second year corn is at risk. In the 70’s we used a number of different root worm insecticides. They had various levels of control. Not one of them ever resulted in “no root damage” They were hard on the environment and on the people applying them. Farmers could get a “cholinesterase test” to see how much their system was compromised by using rootworm insecticides. We went from in furrow treatments to a “T-band” treatment. This treatment applied the insecticide in a band over the row. The insecticide in a band above the seed was better than seed placed. But it was left on top of the ground so we had to incorporate it. We used little harrows on the banders to incorporate the insecticide. Often these harrows became twisted and did not do a good job. Back then all corn fields were mould board ploughed and no trash was left on the ground. Now as we plant there is a lot of trash that would quickly interfere with the finger harrows meant to incorporate the insecticide. I don’t think we want to go there. We looked at seed placed liquid insecticides. Results were mixed. The issue is that the insecticide is placed on top of the seed. Root worms do not start to feed until June and feed until August. There is a significant time lapse between the time of application and when the serious feeding starts. During this time the product breaks down and roots grow outside of where the insecticide is placed. Bottom line is I don’t think current insecticides will be of much value if seed placed.

There were a couple of different populations that developed in the US. One population had an extended diapause. This meant that eggs hatched this year that would normally hatch next year did not. They did not hatch until the following year. Thus, fields in a corn soybean corn rotation were at risk. This was minimal.

Another version of rootworms started to lay eggs in soybean fields. Thus, they laid eggs where we weren’t used to them laying eggs. So, if you planted corn after soybeans these corn fields were at risk. There is no known populations of either of these two types in Ontario

We have genetic resistance (Bt-RW) but this has broken down. We now have numerous fields that have populations resistant to current genetic materials.

Living with Resistant Populations

The solution is crop rotation. First year corn is not at risk. (Yet) For those of you with a corn soybean wheat rotation there is no issue. If you are a dairy farmer with forages and corn you have to take a long look at not growing corn after corn. This will mean shortening your forage rotation

For poultry and pork producers who grow corn after corn you have a couple of options. 1) Trade acres with your neighbour. 2) Plant more soybeans and buy corn. 3) Use a Bt-RW hybrid with high rate of a neonic insecticide.

Research done years ago in Ontario found that rootworm damage was less on sandy soils. The sand particles had an abrasive effect on rootworm eggs resulting in lower survival rates. Heavier soils also allowed adults to go deeper to lay their eggs.

In most farms the rootworm issue is not major but will be in a couple of years. When the Western Corn Rootworm came in it only took about 3 years for it to be major and the Northern Corn Rootworm was minor. I think we will have wide spread resistance to current Bt-RW genes.

Scouting fields that are planned for continuous corn is an option. You check for the amount of silk feeding as an indicator of the rootworm population in a certain field. You count adults and if averaging one per plant that field is at risk. The results for predicting fields using this technique is much higher than results of scouting for Western Bean Cutworm. In the US they use sticky traps to get a better handle on adult numbers. Fields that are in corn this year and planned for corn next year are scouted for potential damage next year.

In the future I am sure we will get better genetic resistance. Genetic resistance is a big issue in the US resulting in a lot of research efforts to find new genetic control.

You will be hearing more on this topic this year

Is it profitable to raise my soil test levels?

The following table gives some back ground to the value of raising your soil test levels.

Figure 1 - High soil test P&K Pays Dividends

Liming No-Till Fields

If you have long term no-till fields that you think require liming, but do not want to work the product in, I would recommend sampling at a several depths to determine where the acidity lies within the soil profile. If acidity is mainly at the soil surface, you may be able to make corrections without incorporation. My suggestion would be to pull soil samples at 0-2”, 2-4”, 4-6” and 6-12”, to see where the acidity is within the soil profile. You will have to lime more frequently to adjust soil pH at depth than that of conventional tillage. (Notes from Soil Fertility and Fertilizers - 8th edition)

Did I put on the right nitrogen rate for my corn crop?

Nitrogen rate is a bit of a moving target, and it means having to accept that you will always be wrong with this nutrient, partially because decisions need to be made prior to the sink size being fully decided. The best we can do is to be less wrong through using the 4R framework (right product, at the right time, at the right rate, at the right place). On one side of the equation you have background or baseline fertility (source 1), commercial fertilizer or manure applications (source 2), on the other side you have your corn plant taking up the nutrient (sink). Trying to better match the source to sink needs is obviously the million-dollar question. A few thoughts as your still working at taking off the 2020 crop, and if you have any trials in the field.

One of the biggest drivers that affects soil supplies was very visual this spring in cool season crops. Cold springs can mean less nitrogen mineralization, and this effect is seen throughout the growing season, as soil supplies rarely catch up to normal mineralization rates. The other issue with cold springs with long winters (like 2020) is that there is more nitrogen tie up due to little to no residue decomposition over the winter.

Rain fall and/or plant available (soil) water is one of the biggest determining factors of how big your sink will be. But rain fall also leads to losses through denitrification (standing water, especially on clay soils or depressions), or through leaching, (sandy soils/knolls). Solar radiation during grain fill is also a driver, but other than just measuring it passively based on expected tasseling date and matching it with day length, there isn’t much we can do with this metric unless you are willing to apply nitrogen in-crop post tasseling. (Could perhaps forecast expected tasseling date at time of application, and make a judgement call).

Lastly, I want to point out that putting on “more” isn’t necessarily the answer, run a zero-rate nitrogen trial to understand how much the soil is supplying, and what you need to add for crop response. The difference between soil supplies and supplemental N is your Delta yield. These areas do not have to be “big” in size, even ¼ of an acre in a few spots within the field would provide some insight into soil supplies under your current management. They won’t yield zero, yes, you will give up some yield, but it will also provide some insights on how to better manage your nitrogen applications. For the tuition of less than $500 you will learn something about soil supplied N.

Understanding Delta Yield

To do Variable Rate Seed, you have to break the system

A few thoughts in point format note to myself as a reconcile 2020 scripts, and think about 2021 scripts;

1)    To get to the “right” rate, you will have to fail your way forward. This means pushing areas of the past “optimal” to determine what the maximum or minimum rate should be. Without seeing where the system starts to break down within the field, it is tough to make improvements.

2)    You must build seed scripts that adjust through the field more than error rate of the seeding equipment. This will be higher on units with hydraulic drive vs. those running V-drive/electric drives. Rates should exceed the noise coming from the equipment.

3)    You need to assume the current population being used is likely suboptimal for at least a portion of the field. From experience, those situations will tie back into not having enough or too much plant available water. Dry years seem to be more problematic than wet years.

4)    You need to use a larger enough step between rates that it justifies the incremental increase or reduction in seed costs. See the chart below. The Coles notes is that you need to see at least a 2+/- bu. difference to offset seed cost for every 2000 seeds.

Figure 2 - Delta Yield Required for More or Less Population

5)    Seed guides provide a good starting point as an anchor on suggested populations, clients should adjust populations via the monitor if the intended hybrid doesn’t end up in the field. You can easily adjust populations up or down by 5-10-20% across the whole prescription as needed via the rate factor.

6)    Seems obvious but not all knolls will behave the same. Take a look at the 0-6” sample and the 6 to 24” subsoil to provide some clues. On the flip side, not all depressions behave the same.

Question; I’m new to VR corn seeding, what rate should I start with if I want do VR seeding in 2021? Answer: Like all great agronomy questions, it depends what your existing yield and ears per 1/1000 acre to obtain that. Finally, does the ear flex? Or is it more of a fixed ear that can handle higher populations. A rule of thumb in the past was above 8 bushels per 1,000, you should consider bumping up your populations. On some hybrids that throw smaller cobs around (14-16), or have shorter cob length, that number may be closer 7 bushels per 1,000 plants.

Figure 3 - Yield in Bushels per 1,000 Ears at Various Yield/Populations

Planting into Cold Soil Conditions

It casually came up in conversation last week that just because your first planted corn went in early, does not mean it will be ready first for harvest. Growing Degree Days do not start accumulating until corn has germinated and emerged from the soil. As a result, some growers are finding later planted corn is drier than their first planted fields.

New Fertilizer Products in Ontario

Two new products available to Ontario retailers from Gypsoil 1) SulphurMax, which is 30% Sulphur and 17% Calcium. When applied at 100 lbs./ac Gypsoil suggests that 14 lbs of the sulphur releases early and the remaining 16 lbs. is available later in the season. 2) Gypsoil blendable pelletized gypsum, this would be for producers wanting to add calcium to dry fertilizer blends, i.e. potatoes. The pelletized gypsum is 21% calcium and 17% sulphate sulphur.

“The opposite of a good idea can also be a good idea

- Rory Sutherland