Crop Report – September 1, 2021

Figure 1. Curves indicate fitted corn grain yield response to applied N at Elora, Ontario. Points indicate the economically optimal N rates for price ratios (PR) varying from 4.5 to 13.7 lb corn per lb of N. (Figure and caption from Deen et al, 2015)

What Will Be the Corn Nitrogen Story for 2021?

While the 2021 growing season started dry, regular and often heavy rains started for many areas from Southern to Eastern Ontario from late June through July. Some received well above normal rainfall in July. Naturally, conversations began revolving around “are we losing nitrogen (N)” or “will we need more N on corn?”

Optimum N Rate Depends on Rainfall

Not surprisingly optimum N rate can significantly vary year to year even within a relatively small area. Within the same ~4-acre corn-on-corn N trial at Elora, Dr. Bill Deen has shown optimum N rate to vary from 120 to 220 lb-N/ac depending on the year (Figure 1).

Figure 1. Curves indicate fitted corn grain yield response to applied N at Elora, Ontario. Points indicate the economically optimal N rates for price ratios (PR) varying from 4.5 to 13.7 lb corn per lb of N. (Figure and caption from Deen et al, 2015)
Figure 1. Curves indicate fitted corn grain yield response to applied N at Elora, Ontario. Points indicate the economically optimal N rates for price ratios (PR) varying from 4.5 to 13.7 lb corn per lb of N. (Figure and caption from Deen et al, 2015)

Over 10 years, Bill and his grad students have some interesting finds:

  • optimum N rate appeared to be primarily driven by yield – high yielding years required more N, lower yielding years required less
  • Yield potential was most strongly correlated to rainfall from the June 15 to July 15 window (more rain, more yield) while correlations outside of this window were poorer
  • Given yields are driving optimum N rate, optimum N rate is also strongly driven by rainfall during the June 15 to July 15 window
  • Variability in year-to-year soil N mineralization did not appear to influence optimum N rate at Elora – yields of plots with no nitrogen did not vary significantly year to year.
  • Variability in year-to-year N-loss did not appear to influence optimum N rate at Elora – N response curves for preplant N did not differ from sidedress N

While not repeated beyond Elora, it is speculated that at locations where N mineralization plays a more important role (e.g. legume nitrogen credits) or where soils are more susceptible to N-loss (e.g. denitrification on heavier textured or poorly drained soils) that factors other than yield may have more impact on optimum N rate.

N Top-Up Trials

Given observations from this trial and a 2021 corn crop with excellent yield potential and significant rainfall from later June through July, it was looking like an environment where optimum N rates could be higher than normal. It was important to have some data to answer “was more N required for corn in 2021?”. Sixteen corn fields close to rainfall data were selected for top-up N trials. A small top-up N application (50 lb-N/ac at silking) was chosen at a random, representative spot where yields from grower rate and top-up rate treatments will be compared at the end of the year. Some additional information (soil nitrate, leaf tissue N) was also collected to see if these can provide guidance in future years for identifying fields that may or may not respond to top-up applications.

Trial Observations

Despite some areas receiving twice the normal rainfall in July, most top-up fields appeared in good shape for N at grower normal rates. Of 16 fields selected, 14 showed no obvious N deficiency symptoms at silking or dough-stage, with green leaves down to the naturally senescing leaves at the base of the plants.

The two exceptions were fields west of London where the greatest rainfall was received. These fields had clear N deficiency symptoms (yellowing of midrib from leaf tip, senescence of lower leaves) only 4 or 5 leaves from the ear leaf by silking and appeared to be strong candidates for responding to top-up N. Interestingly, visiting again at mid-dough stage in later August after rains ceased, N deficiency did not appear to have progressed any further in these fields, looking similar to where it was at silking. Purely speculation, but begs the question whether N was lost in these fields, or whether frequent excess rainfall was just impairing uptake or N mineralization? Stay tuned for yield results this fall.

What Growers Can Do

It’s often said if you don’t measure, you don’t know. Ideally, the best way to get a handle at year end is to test N rates and keep records on-farm. Some growers will regularly include a couple N rate strips the width of their combine with a higher and lower than their normal N rate (e.g. -30 and +30 lb-N/ac) to have yield records, and an idea for where their normal practice stands.

While N rate strips provide good insight any year, they would be particularly beneficial during years of more extreme N requirements (e.g. exceptional yield potential and/or higher expectations of loss). If lower N rates yield as good as normal N rates during an expected high demand year, perhaps there is room to lower overall N rates without significant risk? N rate strips also provide insight for what farms may be generally more or less responsive to N and allow adjustment of N rates by farm.

Figure 2. Typical N deficiency symptoms in corn – leaf yellowing and senescence along the mid rib in from the leaf tip, starting from older/lower leaves.
Figure 2. Typical N deficiency symptoms in corn – leaf yellowing and senescence along the mid rib in from the leaf tip, starting from older/lower leaves.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If you don’t have N rate strips, you could still scout now for N deficiency observations (Figure 2). Assessing N deficiency progress could provide a very crude report card. It is not unusual to have some leaf firing at the lower part of plants this late in season, and this is generally not expected to indicate a shortage or yield loss. If leaves are green down to the base of the plant at the end of grain fill, there was likely excess N. Like extreme years, it would be good to check areas of the field you think may be more or less responsive to N. If leaves are firing well up the plant, or approaching the ear leaf, perhaps there was a shortage of N, or issues with N uptake? At this point (September 1), many fields across the province are in, or moving into dent stage, so perhaps 2 to 3 weeks from maturity.

References:

Deen, B., K. Janovicek, J. Lauzon and T. Bruulsema. 2015. Optimal rates for corn nitrogen depend more on weather than price. Better Crops. 99:16-18.

Weather Data

Location

Year Weekly August 23 – 29 Accumulated
Highest Temp (°C) Lowest Temp (°C) Rain (mm) Rain (mm) April 1st GDD 0C April 1st  GDD 5C April 1st CHU May 1st
Harrow 2021 32 18 14 452 2706 1969 2903
2020 33 18 45 354 2601 1877 2879
2019 26 10 9 414 2514 1787 2686
Ridgetown 2021 31 16 23 436 2560 1832 2738
2020 32 15 68 383 2471 1762 2709
2019 25 8 11 546 2380 1655 2529
London 2021 31 17 9 351 2540 1818 2705
2020 30 14 40 378 2409 1702 2616
2019 24 9 23 536 2278 1579 2445
Brantford 2021 33 16 15 315 2515 1790 2649
2020 33 13 2 213 2432 1731 2595
2019 26 10 274 2354 1642 2505
Welland 2021 31 16 24 330 2531 1802 2704
2020 30 13 20 292 2467 1760 2713
2019 26 11 3 403 2418 1690 2610
Elora 2021 31 14 20 286 2335 1620 2473
2020 30 9 17 299 2234 1552 2430
2019 23 9 6 417 1875 1237 1976
Mount Forest 2021 30 16 40 401 2338 1625 2497
2020 29 12 32 428 2212 1543 2453
2019 24 7 129 2124 1430 2286
Peterborough 2021 31 13 0 301 2344 1617 2457
2020 31 15 2 221 2108 1458 2249
2019 27 5 4 349 2104 1407 2178
Kingston 2021 29 17 4 252 2413 1687 2596
2020 29 8 64 319 2408 1695 2662
2019 25 10 12 363 2335 1611 2507
Kemptville 2021 34 15 22 288 2511 1778 2616
2020 29 8 69 343 2365 1663 2552
2019 27 8 203 2231 1529 2340
Earlton 2021 30 10 22 475 2177 1470 2247
2020 21 13 1 324 1903 1313 2109
2019 26 5 13 362 1764 1151 1889
Sudbury 2021 30 12 22 388 2236 1529 2352
2020 21 17 19 417 1977 1363 2198
2019 27 7 21 401 1815 1198 1956
Thunder Bay 2021 32 7 34 282 2058 1366 2133
2020 23 15 0 219 1777 1194 1949
2019 23 5 31 322 1707 1086 1807
Fort Frances 2021 24 4 38 253 2150 1451 2265
2020 26 15 0 292 1938 1316 2111
2019 25 4 42 413 1834 1184 1936
Report compiled by OMAFRA using Environment Canada data. Data quality is verified but accuracy is not guaranteed. Report supplied for general information purposes only. An expanded report is available at www.fieldcropnews.com.

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