Determining an Optimum Seeding Rate for Spring Wheat in Northwest Minnesota
Jochum J. Wiersma, Department of Agronomy and Plant Genetics and Northwest Research and Outreach Center, University of Minnesota, 2900 University Avenue, Crookston, MN 56716
Corresponding author: Jochum J. Wiersma. firstname.lastname@example.org
In wheat (Triticum aestivum L. emend. Thell.), the seeding rate to maximize grain yield can be derived from the parabolic response curve of grain yield versus number of plants per unit area. Since cultivars genetically differ for yield components, individual cultivars need to be tested at a wide range of seeding rates to determine their optimum seeding rate. Seven Hard Red Spring Wheat (HRSW) cultivars were planted from 1996 to 1998 at two planting dates and six seeding rates. The planting dates were 13 May and 30 May in 1996, 10 May and 28 May in 1997 and 1 May and 22 May in 1998. The seeding rates ranged from 5.8 to 63.1 live seeds per ft2. The optimum seeding rate across all cultivars was 45.0 live seeds per ft2 for the first planting date, which corresponded with an initial plant population of 34.5 plants per ft2. For the second planting date, the optimum seeding rate was 49.1 live seeds per ft2, which corresponded with an initial plant population of 35.9 plants per ft2. A significant interaction between cultivars, seeding rate, and planting date was detected for grain yield in 1996 and 1997, but not in 1998. Producers in northwest Minnesota need to adjust their seeding rates upward from the current practices because the initial plant population to maximize grain yield is 10 to 15% higher for the cultivars included in this study when compared to the standard recommendations. More importantly, based on these results, producers should assume when calculating a seeding rate a stand loss of 20 to 25% rather than the 10 to 15% previously assumed. In addition, individual cultivars differ for the optimum seeding rate and variety-specific recommendations can be made.
In wheat, the seeding rate for maximum grain yield can be derived from the parabolic response curve of grain yield versus number of plants per unit area, which increases quickly to a maximum and slowly decreases at higher plant densities (7,12,13,17). Grafius (10) described grain yield as a function of yield components: namely, the number of tillers per unit area, the number of kernels per spike, and the weight per kernel. Subsequent research showed that cultivars differ in these genetically determined yield components.
Significant interactions between cultivars, seeding rates, and planting dates for grain yield have been reported (5). Briggs and Aytenfisu (5) and Faris and De Pauw (9) both suggested that new wheat cultivars, particularly if they differed from existing cultivars, should be tested at a wide range of seeding rates to determine their optimum seeding rate. Delayed planting past the optimum time reduced grain yield and increasing the seeding rate only partially compensated for the loss of grain yield (5,6). Based on the significant interaction between cultivars and seeding date for grain yield, Ciha (6) also suggested that new cultivars should be tested at different seeding dates to determine the optimum seeding rate.
In northwest Minnesota, the recommended optimum initial plant population for all hard red spring wheat (HRSW) cultivars is 28 to 30 plants per ft2 (14). The seeding rate corresponding with this recommended plant population is calculated as a function of percent germination of the seed lot, the number of seeds per pound or seed count, and the expected stand loss using the following formula:
Under good seedbed conditions, the stand loss is generally assumed to be between 10% and 15%. If planting is delayed, the current recommendation calls for increasing the initial plant population by 1 to 2 plants per ft2 per week for each week that planting is delayed after 1 May. Many growers do not calculate a seeding rate for each individual seed lot, but plant approximately 90 lbs of seed per acre.
These recommendations have not changed since they were established well before the introduction of the first semi-dwarf cultivars more than 30 years ago. Furthermore, these recommendations were never intended for use with specific cultivars. As Ciha (6) pointed out, the agronomic performance characteristics of spring wheat have changed considerably with the introduction of the semi-dwarf growth habit. Faris and DePauw (9) found that the two semi-dwarf cultivars included in their study attained their maximum grain yield at higher seeding rates than the standard height cultivar ‘Neepawa’ in the same trial grown under semi-arid conditions in Saskatchewan. Under similar growing conditions, Baker (3) found that the effect of planting date on grain yield was similar between semi-dwarf and standard-height wheat cultivars.
The objective of this research was to determine the optimum initial plant population and corresponding seeding rate of five semi-dwarf HRSW cultivars and two standard height HRSW cultivars for maximum grain yield at both optimum and delayed planting in northwest Minnesota.
Experiments and Results
Seven HRSW cultivars consisting of five semi-dwarf cultivars and two standard height cultivars, were planted at two planting dates and six seeding rates using a split-split plot layout in a randomized complete block design with four replicates. Planting date was used as whole plot, with cultivars as sub plots, and seeding rates as sub-sub plots. The cultivars that were selected represent the gamut of available cultivars that are grown in the region (Table 1). ‘BacUp’ is a special purpose cultivar with a high level of tolerance to Fusarium Head Blight (FHB), but low to medium grain yield. ‘Grandin’ and ‘Gunner’ are recent releases with high grain quality, but medium to high yield potential. ‘Hamer’, ‘Marshall’, and ‘Verde’ are high yielding semi-dwarf cultivars. Pioneer 2375 is semi-dwarf cultivar that has been very popular among producers because of tolerance to FHB, but it is known to tiller poorly and lodge readily (R.H. Busch, USDA-ARS Wheat Geneticist, personal communication).
Table 1. Descriptions of the seven HRSW cultivars evaluated for optimum seeding rates to maximize grain yield under optimum and delayed planting in northwest Minnesota from 1996 through 1998.
The experiment was planted on a Wheatville loam (coarse-silty over clayey, mixed over smectitic, superactive, frigid, Aeric Calciaqualls) at Crookston, MN, in 1996, 1997, and 1998. Fertility was applied according to soil test recommendation with a yield goal of 75 bushels per acre. The planting dates were 13 May and 30 May in 1996, 10 May and 28 May in 1997 and 1 May and 22 May in 1998. The first planting date each year was the first time in the spring that soil conditions were sufficiently dry to allow preparation of the seedbed; the second planting date averaged 19 days later. Spring wheat production in northwest Minnesota is seldom limited by soil moisture. However, maximum temperatures rapidly increase as the growing season progresses, which shortens the development of the crop and reduces grain yield. Planting occurred later than normal in 1996 and 1997 due to cool, wet spring conditions. In contrast planting was ahead of normal in 1998 due to warm and dry conditions. The ideal planting conditions in 1998 were followed by a 6-week period with precipitation at almost 200% of normal.
The seeding rates were based on live seeds planted and equaled 5.7, 17.2, 28.7, 40.1, 51.6 and 63.1 live seeds per ft2. A germination test was used to determine the number of live seeds per pound of seed (2). The plots were planted in 7-inch rows using an Almaco (Almaco, Nevada, Iowa) plot drill with John Deere double-disk openers equipped with depth pans to ensure proper planting depth at 1.5 to 2 inches. The plots were 5-ft wide and 15-ft long. Complete plots were harvested with a Wintersteiger (Wintersteiger GmbH, Ried im Innkreis, Austria) plot combine. The harvested grain was cleaned with a Clipper Model 400 Office Tester and Cleaner (A.T. Ferrell Company, Bluffton, Indiana) and weighed. The recorded plot yield was converted to bushels per acre. The initial plant population was recorded at Zadoks growth stage 12 to 13 (18) as the average number of plants in two 3-ft lengths of row and converted to number of plants per ft2. The stand loss was calculated as one minus the difference between live seeds planted and initial plant population divided by live seeds planted.
For individual years, the analysis of variance (ANOVA) was computed for all traits measured using Statistix 7 (Analytical Software, Tallahassee Florida), assuming planting date, cultivars, and seeding rates as fixed effects and replications as a random effect. Main effects and interactions were tested using the error terms appropriate for the split-split plot experimental design (15). A linear and quadratic contrast for all traits measured was calculated for seeding rate. Regression coefficients were derived from the appropriate sums of squares. Optimum seeding rates were calculated from the first derivative of the regression equation. Before combining across years, Bartlett’s (4) test was used to test heterogeneity of error variance across environments.
Bartlett’s test for heterogeneity of error variance was significant for all traits measured. Thus, the data was not combined across years. For initial plant population no significant interaction was detected between cultivars, seeding rate, and planting date in 1996, 1997, or 1998 (Table 2). However, there was a significant interaction between seeding rate and planting date in all three years. Thus, the initial plant population for each seeding rate was calculated for each planting date and year but averaged across cultivars (Table 3). Initial plant population increased as seeding rate increased, but the proportion of seedlings to live seed planted decreased with seeding rate, regardless of the planting date (Table 3). This decrease in initial plant population relative to the increase in seeding rate was linear for each planting date and year (Table 3). Hanson and Lukach (11) found a similar response in barley and postulated that it was a function of the increased seeding rate itself that caused intra specific competition for moisture during germination and emergence.
Table 2. P-values for the main effect of seeding rate and the interactions of seeding rate, planting date, and cultivar for both the initial plant population and grain yield in 1996, 1997, and 1998.
Table 3. Initial plant population in individual years (1996-1998) and averaged across years for six seeding rates and two planting dates averaged across seven HRSW cultivars in Crookston, MN.
* pd1= first planting date; pd2 = second planting date.
† Only statistically significant results (P = 0.05) are reported.
In 1997 and 1998, the estimated initial stand for the lowest seeding rate was higher than the corresponding seeding rate of 5.7 seeds per ft2. This is likely due to sampling error. At the lowest seeding rate it was noted that the distribution of the seed was not very uniform along the length of the plot and often bunched up. Consequently, the initial plant population showed a very large standard deviation at the lowest seeding rate and the estimated mean may inadvertently have been overestimated. In addition, coleoptile tillers may erroneously have been included in the stand counts resulting in an overestimation of the actual initial plant population.
Using the linear regression equation for each planting date and year from Table 3, the stand losses in this experiment that corresponded with the current recommendation for initial plant population of 30 plant per ft2 were 13, 27, and 28% for the first planting date and 21, 36, and 19% for the second planting date (Table 4). Thus, the average stand loss for the first planting date was 23 and 25% for the second planting date. These averages are a minimum of 7 to 10% higher than the stand loss that is currently assumed when calculating a seeding rate for the current recommendation of 30 plants per ft2 for initial plant population.
Across cultivars, the seeding rate for maximum grain yield equaled 45.0 and 49.1 live seeds per ft2 for the first and second planting date (Table 4). Using the linear regression equations from Table 3, the initial plant population for maximum grain yield across years was 34.5 plants per ft2 for the first planting date and 35.9 plants per ft2 for the second planting date (Table 4). The initial plant populations for maximum grain yield in this study were higher than the current recommendations of 28 to 30 plants per ft2 for planting around May 1 and equal to slightly higher for planting in the third to fourth week of May. However, the corresponding seeding rates needed to attain these initial plant populations were considerably higher than the number of live seeds per ft2 producers routinely plant. Similar to the findings of Cira (6) an increase in seeding rate could only partially compensate for the loss in grain yield when planting was delayed. Using the regression equations in Table 4, the grain yield for the optimum seeding rate was estimated at 64.3 bu per acre for the first planting date and 58.7 bu per acre for the second planting date (Table 4).
A significant interaction for grain yield was detected between cultivars and seeding rates as well as cultivars, seeding rates, and planting dates in both 1996 and 1997 but not in 1998 (Table 2). This is similar to previous work (9,16). Thus, an optimum seeding rate was calculated for each cultivar and planting date for all three years (Table 5). The optimum seeding rates were derived from the regression equation only if the regression of seeding rate and seeding rate2 on grain yield was statistically significant (P = 0.05) for an individual cultivar. In 1998, no regression equation could be fitted for a number of cultivars and the reported averages were calculated using 1996 and 1997 data only.
Table 5. The optimum seeding rate for each planting date in each year (1996-1998) and averaged across years for seven HRSW cultivars in northwest Minnesota.
* pd1= first planting date; pd2 = second planting date.
** optimum seeding rates are only reported if the linear regression equation of seeding rate and seeding rate2 on grain yield for each cultivar and planting date was statistically significant (P = 0.05).
Except for the cultivar Gunner, the average seeding rate for individual cultivars increased from the first to the second planting date (Table 5). Hamer and Verde, two high yielding semi-dwarf cultivars, showed on average the largest increase, while Pioneer 2375 showed the smallest increase. As previously stated, Pioneer 2375 is known as a cultivar that tillers poorly. Thus, it can be postulated that cultivars that do not tiller well are not able to take advantage from an earlier planting date with increased tillering that would allow grain yield to be maximized at a lower initial plant population. However, within years, the optimum seeding rates to maximize grain yield showed much more variation. In 1996, the optimum seeding rate for Bacup, Gunner, and Pioneer 2375 were substantially lower for the second planting date (Table 5). A possible explanation is that in 1996 the months of June and August were dry with total precipitation equal to 37 and 12% of normal for those two months, respectively. This resulted in moisture stress for the second planting date during the critical phases of tillering and grain fill and consequently favored lower plant populations to maximize grain yield.
In 1997, only the optimum seeding rate for the cultivar Grandin showed a decrease for the second planting date (Table 5). In 1998, no regression equation could be fitted for Hamer for either planting date or for Pioneer 2375 for the first planting date (Table 5). It is not clear why no regression equation could be fitted for Hamer or Pioneer or why a number of cultivars showed a lower optimum initial plant population for the second planting date in either of 1997 or 1998.
These seeding rates needed to maximize grain yield are significantly higher than current recommendations for northwest Minnesota, which recommend an optimum initial plant population of 28 to 30 plants per ft2 and assume a stand loss of 10 to 15% when calculating the corresponding seeding rate. The results are also higher than the recommendations in other wheat production areas in the world. Anderson and Barclay (1) reported an average of 12.9 plants per ft2 as optimum for western Australia. In the Netherlands, 23.2 plants per ft2 is considered the optimum for winter wheat as well as spring wheat (8).
Producers in northwest Minnesota need to adjust their seeding rates upward from current practices because the initial plant population to maximize grain yield is 10 to 15% higher for the cultivars included in this study when compared to the standard recommendations. Individual cultivars differ for the optimum seeding rate. More importantly, based on these results, the stand loss producers should assume when calculating a seeding rate should be 20 to 25% rather than the 10 to 15% previously assumed.
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