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Benefits of Plant Spacing Uniformity

Tuesday, October 1, 2024
filed under: Optimizing Plant Development/Yields

Recent Research Seeks Hard Numbers

Sunflower has long been appreciated for its ability to compensate when the field’s plant stand has doubles, skips or gaps. Does attention to stand pay off in the end, or is it not a big deal for growers?
        An article in the January 2008 edition of The Sunflower stated the following:
        “Any sunflower producer or seed company knows first-hand the benefits of a consistently spaced plant stand:
        “More-efficient use of moisture, sunlight and nutrients.
        “Improved weed suppression (superior plant canopy).
        “Similar pace of plant development (aids in insect management).
        “Uniformity of head and seed size.
        “More-even plant drydown across the field.
        “Better final yield and crop quality.”
        So in the end, is plant spacing a significant issue for sunflower producers? The above 2008 statements suggest it is, as do numerous anecdotes from producers and agronomists. And yes again, suggest data from recent National Sunflower Association-sponsored biennial crop surveys. In the 2019, 2021 and 2023 surveys, plant spacing was judged to be the number-one yield-limiting factor in 12, 10 and 12%, respectively, of surveyed North Dakota fields. For South Dakota, those numbers were 28, 8 and 15%, respectively.

        A recent research project, supported by the National Sunflower Association, sought to put some hard numbers to the question of plant spacing’s impact on sunflower. The project’s results have been published in Agronomy Journal under the title “Canopy Closure, Yield, and Quality Under Heterogeneous Plant Spacing in Sunflower.” The article’s authors include the project’s principal investigators: Neil Olson, Calvin Trostle, Ron Meyer and Brent Hulke.*
        The authors note that back in the early 1980s, Robert Robinson, University of Minnesota agronomist and pioneer sunflower researcher, reported that spacing heterogeneity in sunflower could lower yields by up to 31%, with an average reduction of 10%. Concurrently, lodging was also greater under nonuniform population densities, Robinson noted — which would have contributed to yield loss.
        During that same era, Manitoba scientists H.C. Huang and J.A. Hoes investigated the impact that sunflower density had on the incidence of Sclerotinia stalk rot. They found that closer plant spacings, due to irregular seed placement, resulted in a higher rate of disease spread and less time required for infection by Sclerotinia.
        Much has changed during the past four decades, the Agronomy Journal article notes, “including the widespread adoption of vacuum planters and precision planting controls and monitors that very accurately control and report seed spacing (including skips/doubles and gaps), at least when properly calibrated and monitored.” Also, the article affirms, “the genetics of sunflower hybrids have changed considerably and may exhibit different tolerance of plant spacing than previously observed.” Finally, the authors point out, “we have more tools to study the effects of plant spacing heterogeneity, including remote sensing capabilities that allow for simplified, repeated measures of plant canopies throughout the season.”
        Those factors provided the impetus for this latest research on planting spacing’s effects on sunflower production.

        Field experiments were conducted in Minnesota (Moorhead), Texas (Dalhart) and Colorado (Burlington) during the 2019 and 2020 growing seasons. The research group’s objective was to “evaluate the effects of plant density heterogeneity on yield, quality and canopy closure” in both confection and oilseed sunflower, using modern assumptions and conditions. They controlled the frequency of skips and doubles “while maintaining the same total plant population to simulate unequal seed distribution that could be encountered with an improperly calibrated vacuum planter.” They also employed gap treatments to simulate planter malfunctions, as well as damage from wildlife and cutworms.
        The oilseed hybrid utilized at all sites in both years was Nuseed’s N4H521 CL; the confection hybrid was Red River Commodities’ RRC 2215.
        The treatments were verified within one month of application to remove any late-emerging plants. Weeds in the plots were manually removed. Also, the Colorado and Texas sites received supplemental irrigation as needed.
        Prior to harvest, plots were scouted for disease and lodging issues, and affected plants in the center two rows were counted. The middle two rows were harvested by hand and threshed to measure yield and seed quality. Additional measuring was carried out to determine test weight, oil percentage on the oilseed hybrid, and seed size on the confection hybrid. A UAV was used to determine canopy cover at the Moorhead, Minn., research site at multiple times during both the 2019 and 2020 sunflower growing seasons.

So what did this research project’s results show?
        First, the experiments revealed that yield was occasionally influenced by plant spacing heterogeneity, with more-uniform stand density yielding higher. However, substantial changes in plant density often resulted in no yield differences, the authors note. In the confection plots, the sunflower “compensated for yield losses in thinner stands mainly by producing larger seeds.” The oilseed hybrid increased seed production per head (as reflected by absence of test weight differences across treatments within the same environment).
        Plants that were unharvestable — for any cause, but mostly due to head rot disease or lodging — were more prevalent in less-uniform stands. Canopy closure occurred faster among uniformly spaced plants. “The treatments with significantly worse canopy closure than the control suggest that under certain, less-optimal conditions, we may see greater yield loss with more-extreme gaps and stand heterogeneity,” the investigators note. However, they add, it’s clear that the considerable deviation from properly singulated populations represented by such treatments “would be uncommon except under extreme circumstances (i.e., very high pest pressure or extreme drought at planting).” They also note that producers with today’s modern planting equipment “would quickly observe planter issues that would reduce population (i.e., cause gaps due to mechanical malfunction) and also notice deviation from single seed drop.”
        In conclusion, the authors state that while sunflower is very resilient to stand uniformity issues, “these findings underscore the importance of uniform sunflower plant spacing for effective weed suppression and crop performance.” When yield was lower due to irregular plant spacing, it seemed to be most correlated to the rate of canopy closure.
        “Further physiological research into sunflower yield compensation due to disruptions at different time points throughout the growing season (e.g., mid-season hail damage) will help us understand the limits of sunflower adaptability for plant spacing issues through breeding efforts,” the researchers add. — Don Lilleboe

* Neil Olson was a North Dakota State University graduate student in agronomy at the time of this research project’s implementation. Calvin Trostle is professor and extension agronomist with Texas A&M University, based at Lubbock. Ron Meyer is area extension specialist with Colorado State University, based at Burlington. Brent Hulke is sunflower research geneticist with the USDA Agricultural Research Service, based at Fargo, N.D.

The complete Agronomy Journal article can be accessed free of charge at: https://acsess.onlinelibrary.wiley.com/doi/10.1002/agj2.21655