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Agronomy Update

You Want Me to Put How Many Seeds Where?

Published on Tuesday, March 15, 2016

As we get closer to planting, many farmers develop, or consider developing, variable seeding rate prescriptions for their fields. Experience levels vary and, as might be expected, accuracy in prescribing the correct seeding rate in a given field location for any given year varies as well. I thought this would be a good time to share some of my experience as well as my findings from a variable rate corn seeding project I conducted this past growing season.

First, it is important to understand that varying seeding rate is not always the right thing to do. It doesn’t always provide a return on investment, largely because of the year-to-year variability in performance of any given hybrid in any given soil environment. In other words, the weather in a given year, its impact on water movement, transport in a field and on plant available water capacity dictates the economically accurate seeding rate on a given soil/landscape position for that year.

In a study conducted by University of Illinois (Figure 5.10), the same variety was grown on the same part of a field for three years in a row. The optimum harvest population ranged from 25,000 plants/A. to 35,000 plants/A. depending on if the weather was conducive to a 140 Bu./A. year or a 230 Bu./A. year. So, if you can tell me what the weather is going to do, I can tell you what rate to put where!


Image Reference: Pg. 92 of Modern Corn and Soybean Production, Chapter 5. Planting Decisions and Operations. 2000. Authors, Robert G. Hoeft, Emerson D. Nafziger, Richard R. Johnson, and Samuel R. Aldrich

I have observed this same phenomenon in my own experience. I will often “push” corn seeding rates on ridge landscape positions in Marshall, Minden and Macksburg silt loam soil types in west central Missouri. Multiple years of yield data show these soil positions to be higher producing and capable of responding to greater seeding rates.

However, under water and heat stress conditions at pollination, these soil positions tend to dry out in the order of Minden>Marshall>Macksburg. On Minden soils especially, the higher seeding rate at planting hurt yield performance rather than helped it during a stress year. The key is to know your soils, how well they perform in different weather scenarios, and to “tune” your seeding rate recommendations as best as you can to account for variability in performance. One way of tuning in to seeding rate recommendation is to more accurately set the criteria for your zone development.

Setting Your Criteria for Zone Development:

  1. Make sure to use multiple years of yield data for the crop you intend to vary the seeding rate for (corn or soybeans). Don’t mix corn and soybean years in your multi-year yield analysis for seeding rate recommendations. However, it’s ok to do so when developing zones for fertility management or soil testing.Remember that zones are used for different purposes. When looking at seeding rates, keep in mind that corn is much less forgiving to incomplete or insufficient stands than soybeans. Corn is also more sensitive to water and heat stress when the number of plants exceeds available soil water and nutrient supply.

  2. You need at least four years of yield data per crop to have confidently evaluate the year-to-year stability of identified productivity patterns.

  3. Do NOT throw out extreme years in your evaluation. Run an analysis both with an extreme year in it and one without to determine if zone patterns shift. This helps to “fine-tune” your seeding rate zones to account for weather variability. In fact, I often run an analysis on the 2012 yield data alone to fully understand what is going to happen in a high-stress year. It is also useful to look at an extremely good year by itself and compare the geographical delineation of performance areas between the two opposite type of years.

  4. Be more conservative when breaking up yield data into zones. Identify stable zones that consistently yield greater than or less than the field average with less variability. Be willing to recognize that yield response to seeding rate is highly variable and areas with high variability are probably best managed with the average recommended seeding rate for any given hybrid.

  5. Do your best to understand the population response nature of the hybrid that will be planted. From a spectrum of very fixed to very flex, where does it fall? Your Beck’s seed advisor, field agronomist, or product specialist can assist you with this determination.

An Example from Last Year:

Last year was a good corn year for the field show at the right, located in west central Missouri. This field averaged 230 Bu./A. I used five years of corn yield data to assign prescribed rates. I performed a check on my prescription zones by determining the average yield in each zone. Figure 1 on the right shows the average yield in each productivity zone with its corresponding seed planting rate. It looks like we were pretty accurate in determining the higher to lower yielding areas of the field for 2015.


Figure 1.2015 corn yield is on the left side. The right side is the variable rate seeding prescription developed for the 2015 growing season ranging from 27,500 to 34,500 seeds/A.

The question is, did it make a difference to vary the rate within any given zone? Figure 2 shows the average yield’s productivity and prescription zone for proximal rates within a zone. I selected multiple 40 x 40 ft. plots within each zone for two adjoining seeding rates and averaged the corresponding yield in the plots.

What I Learned From This One…

  1. The zones are relatively accurate as far as mean productivity potential.

  2. There was a significant yield advantage pushing the population to 34,500 in the high productivity areas, with a significant yield advantage even over 33,000. The calculated ROI was $36.30/A. to go from the average seeding rate of 31,500 to 34,500 seeds/A. in the high yield zones.

  3. It appears that in the moderate productivity zones, a population of 31,500 seeds/A. was more on-target than 33,000 seeds/A. We need to keep our higher seeding rates in our consistently, stable higher zones and push that ground. The moderate yielding areas of this field are inherently more variable and more challenging to hit the correct seeding rate. The normal seeding rate is the most accurate in this zone.

  4. We didn’t lose much, if any, yield when we dropped to 27,500 seeds/A. in the stable, low productivity zones. If we can maintain yield and reduce seeding rates by 2,000 to 33,000 seeds/A. in the low zones, a calculated potential ROI of $8.81/A. is possible from seed savings.


Figure 2. Mean Yield by proximal seeding rate in each of the three productivity zones

 

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