Wheat Variety-Specific Response to Seeding Rate Under Intensive Wheat Variety-Specific Response to Seeding Rate Under Intensive Management Conditions in Western Kansas in 2020–2021 Management Conditions in Western Kansas in 2020–2021

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Introduction
Wheat responses to seeding rate are inconsistent, ranging from quadratic to positive linear, quadratic-plateau, plateau-negative linear, and even inexistent (Jaenisch et al., 2019(Jaenisch et al., , 2022;;Fischer et al., 2019;Lollato et al., 2019).The quadratic response suggests that there is an optimum population to optimize yields.In this case, populations below the optimum may limit crop yields due to sub-optimum stands, and populations above the optimum may limit crop yields due to increased disease pressure, insects, lodging, or insufficient resources such as fertility.Recently, some Kansas evidence suggested that wheat responses to seeding rate were dependent on the level of resource availability of the environment (Bastos et al., 2020).In high-yielding environments (greater than 90 bu/a) where the crop is not limited by resources (including fertility levels, and optimal temperatures and moisture for tillering), crop yield was unresponsive to plant population.Similar results were derived from the Kansas Wheat Yield Contest (Lollato et al., 2019) and from studies with intensively managed wheat in Kansas (Jaenisch et al., 2019) and in Mexico (Fischer et al., 2019).Meanwhile, in average (65 bu/a average) and low (45 bu/a average) yielding environments, wheat responded to increases in plant population up until about 25 to 31 plants per square feet (approximately 1.1 to 1.35 million plants/a), leveling out at greater populations (Bastos et al., 2020).The optimum plant population might also depend on the variety's tillering potential (Bastos et al., 2020), as varieties with greater tillering potential might require less population to maximize yields when compared to varieties with lower tillering potential (Jaenisch et al., 2022).
The majority of the studies evaluating wheat yield response to seeding rate were performed under standard management conditions, not excessively high fertility levels, or other management factors (e.g., Whaley et al., 2000;Lloveras et al, 2004;Bastos et al., 2020).Thus, in this study we aimed to understand wheat response to seeding rate in a scenario with highly available resources.This is relevant in a context in which the increases in food production are needed to feed an increasing global population, especially in regions characterized by actual yields well below the potential yields, such as in Kansas and neighboring states (Jaenisch et al., 2021;Lollato and Edwards, 2015;Lollato et al., 2017;2019;Patrignani et al., 2014).Since resource availability and variety-specific tillering capacity seem to govern wheat yield response to plant population, our objective was to evaluate the grain yield response of different winter wheat varieties to seeding rate, including extremely low seeding rates, in a highly-managed commercial field in western Kansas.

Procedures
A field experiment was conducted during the 2020-2021 winter wheat growing season in a commercial wheat field near Leoti, KS.The research plots comprised of seven 7.5-in.spaced rows wide and were 30-ft long.A two-way factorial treatment structure was established in a completely randomized block design and included four commercial wheat varieties (i.e., Joe, Langin, WB-Grainfield, and LCS Revere) and five seeding rates (200,000, 400,000, 600,000, 800,000, and 1,000,000 seeds/a).All seeds were treated with insecticide and fungicide seed treatment to avoid potential stand losses due to pests (Pinto et al., 2020).The experiments were planted on September 25, 2020, after a long summer fallow in sorghum residue; wheat was the second crop after manure application (5 tons per acre, providing about 150 pounds of N and P).In-furrow diammonium phosphate was applied with the seed at 50 pounds of product per acre.Management of the field consisted of 40 pounds of N per acre, with 3.5 ounces per acre Rave herbicide in February, 180 pounds of N per acre as urea on March 10, and 13 ounces per acre of Nexicor fungicide at heading.Combined with the soil fertility available at sowing, all the manageable stresses were likely reduced.Harvest occurred using a Massey Ferguson XP8 small-plot, self-propelled combine.
A total of 15 individual soil cores (0-to 24-in.depth) were collected from each location and divided into 0-to 6-in.and 6-to 24-in.increments for initial fertility analysis.The individual cores were mixed to form one composite sample, which was later analyzed for base fertility levels (Table 1).In-season measurements included stand count (measured about 20-30 days after sowing) and grain yield at harvest maturity (corrected for 13% moisture content).Statistical analysis of the data collected in this experiment was performed using a two-way ANOVA in PROC GLIMMIX procedure in SAS v. 9.4.Non-linear regression analyses was used to test the grain yield response to plant population, and the residuals from this relationship were subjected to ANOVA to test the effect of wheat variety.

Weather Conditions
The 2020-2021 growing season was dry in the fall (0.4 inch precipitation) and winter (2.3 inch precipitation), with water supply only representing 1 and 31% of crop water demand.The spring, however, had 8.7 inches of precipitation that represented 90% of crop water demand (Table 1).This, together with mild temperatures, ensured high yielding conditions in the experiment location.These mild spring conditions are not typical of the study region, which is usually characterized by high likelihood of water and temperature stresses (Couedel et al., 2021;Lollato et al., 2020;Sciarresi et al., 2019).

Seeding Rate and Variety Effects on Stand Establishment and Grain Yield
There was a significant seeding rate effect on final stand establishment (Table 3).Overall, increases in seeding rate resulted in greater stand count, as expected.However, we note that final populations were closer to the target population at lower seeding rates as compared to higher seeding rates.For instance, the target population of 200,000 plants/a resulted in 252,265 plants/a; while the target of 1,000,000 plants/a resulted in 521,347 plants/a.This is usually observed in seeding rate studies (Bastos et al., 2020).There was also a variety effect on final stand establishment, where Joe resulted in more plants per acre than Langin or WB-Grainfield, and LCS Revere had statistically the same population as all other varieties (Table 3).
Grain yield was affected by seeding rate and by variety independently, with no variety × seeding rate interaction, suggesting that varieties responded similarly to seeding rate (Table 3).Overall, there was a linear-plateau grain yield response to seeding rate, increasing from 89.9 bu/a in the 200,000 seeds/a rate, to anywhere from 97 to 101.3 bu/a in the seeding rates ranging from 600,000 to 1,000,000 seeds/a, with no significant statistical differences among the higher seeding rates.The variety Langin had the highest grain yield (102.1 bu/a), followed by LCS Revere and WB-Grainfield (94.7 to 97.5 bu/a), and finally Joe had the lowest grain yield (90.3 bu/a).
The overall relationship between plant population and grain yield is shown in Figure 1a.Grain yield showed a quadratic relationship as a function of plant population, with the highest yields visually observed between the populations of 300,000 and 550,000 plants/a.Analysis of the residuals of this relationship as affected by wheat variety indicated a significant variety effect (Figure 1b).This analysis evaluates the effect of variety on grain yield when the effect of plant population is accounted for.LCS Revere out-yielded the expected yield for a given population by 8.3 bu/a, while WB-Grainfield and Langin were 0.7 to 3.7 bu/a from the expected yield for a given population.Joe yielded 4.3 bu/a less at a given population than the yield that would be expected for that population level.

Preliminary Conclusions
This trial provided information on wheat response to seeding rate within a highly managed scenario, during a dry growing season.At yield levels ranging between 90 and 102 bu/a, wheat response to seeding rate was independent of variety, and yield maximized at 600,000 seeds/a.Yield increases reported for seeding rates beyond 600,000 seeds/a were not statistically significant.

Figure 1 .
Figure 1.(A) Winter wheat grain yield as function of plant population across all varieties and seeding rates evaluated, and (B) analysis of variance of the residuals of the relationship of grain yield by plant population as affected by winter wheat variety.Data represents one location near Leoti, KS, during the 2020-2021 growing season.

Table 2 .
Weather conditions including average maximum (Tmax) and minimum (Tmin) air temperatures, and cumulative precipitation and reference evapotranspiration (ETo) near Leoti, KS, during the 2020-2021 growing season

Table 3 .
Stand count and grain yield of four winter wheat varieties (WB-Grainfield, Joe, LCS Revere, and Langin) as affected by seeding rate ranging from 200,000 to 1,000,000 seeds/a Significance of fixed effects resulting from the ANOVA as well as post-hoc mean grouping.Means followed by the same letter are not significantly different at P = 0.05.