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Each soybean variety was planted at two planting dates and three populations in the study. Seed size, as a component of yield is not normally influenced by plant population. Since seed size is not greatly influenced by plant population, an ultra-low seed count providing an equivalent of 25,000 seeds/acre was used in the study to increase the likelihood of getting an increase in soybean seed size. (Table 2).
The study was designed as a Randomized Complete Block with split, split plot treatments, replicated five times. The whole plot was planting date, the split plot was soybean variety (maturity), and plant populations was the split-split plot. Planter row width was 30" wide. Plot size was 10' wide by 25' long. The soybean seed was counted for each plot and then planted with a John Deere MaxEmerge planter equipped with cone planting units. Original seed size of the seed beans is in Table 1. Seed bed preparation was accomplished by multiple disking of the plot area prior to the first planting. The second planting was made to a stale seed bed. Weed control was difficult to obtain, especially for volunteer corn. The test site was part of the corn variety testing program in 2001, and corn borer pressure had resulted in very heavy lodging and ear yield loss. The weed control program consisted of the following:
The soybeans were also treated with Lorsban (1 qt/a) on July 30 for control of a light infestation of two-spotted spider mites. Plant counts were taken on June 26 to determine established plants per acre. All plants in the second row of the four-row, twenty-five foot long plots were counted to determine established plant population per acre. The soybeans were harvested with a MF8 plot combine on October 16. This harvesting date was too late for the early planted Vinton 81 variety because it suffered severe shattering prior to harvest. This had an adverse effect on both yield and seed size. No lodging was evident at harvest time. At harvest, seed samples were collected from all plots for determination of seed size. One ounce of clean seed (per seed sample) was counted for each harvested plot to determine seed size. Seed samples from the first and second replication were used to determine protein content. These samples were sent to Livestock Nutrition Laboratory Services in Columbia, Mo. for determination of protein by the combustion method. Results and Discussion: The date of planting for the soybeans in this study varied from the dates planned due to wet weather conditions in the spring. The planned and actual planting dates are in Table 3.
The mid-June planting date resulted in a better stand of soybeans, increased soybean yield, larger soybean seed and lower protein content than beans planted in late May (Table 4).
Seedling establishment was less than anticipated for both planting dates, ranging from 41 to 65 percent of seed planted. The mid-June planting date provided better overall stand of established plants than the late-May planting (Table 5 & 6). The lowest planting rate of 25,000 seeds per acre resulted in the lowest soybean stand, the lowest soybean yield and the largest soybean seed size. The data does suggest that seed size can be influenced by the stand of soybeans obtained, however the reduction in overall seed yield due to a reduced stand of soybeans is quite substantial. A soybean stand of 46,507 plants per acre provided soybean yields that were statistically insignificant from stands that were higher (Table 5). Protein content of the soybean seed was only slightly affected by stand establishment, and the trend was for protein content to increase with planting rate and stand establishment (Table 5).
Fontanelle 12-HC31 and Neco 53-3559A tended to provide a higher percentage of established plants than Vinton 81 and Fontanelle 12-HC29 (Table 6). The later-than-planned planting dates actually worked to the advantage of soybean yield this year (2002). When averaged over all planting rates and varieties, the later planted soybeans yielded 10.3 bu/acre more than the early planted soybeans. The maturity group 3 soybean varieties provided higher yields than the maturity group 1 and 2 soybean varieties. Yield by variety and planting date are shown in Tables 7 & 8.
Seed size was affected by planting rate (Table 9). There was a statistically significant difference in seed size between the soybeans planted at 25,000 seeds per acre and those planted at 150,000 seeds per acre. This was regardless of planting date or variety. However, the difference in Vinton 81 soybean seed size in relation to plant population was directly opposite the other three soybean varieties. Fontanelle 12-HC29, Fontanelle 12-HC31 and Neco 53-3559A seed decreased in size as plant population increased. The only explanation for this is that the Vinton 81 soybeans had suffered moderate to severe shattering prior to harvest, and it is likely that the pods with the largest seed size would be the first to shatter.
Protein content of the seed was influenced by soybean variety. Vinton 81 was the containing the most protein while Fontannell12-HC29 was the variety containing the least amount of protein. There was a trend for the earlier planted soybeans to contain higher protein than the later planted soybeans. Plant population had little to no effect on protein content (Table 10). Fontanelle 12-HC31 and Neco 53-3559A provided better stands of higher yielding, larger seeded soybeans than Vinton 81 or Fontanelle 12-HC29 soybeans (Table 11). Summary: Higher soybean yields were achieved from the June 14 planting date this year. Soybeans planted two weeks earlier yielded 10.3 bu/acre less than the later planted beans. Soybean yield varied by variety. Maturity group appeared to be a factor in soybean yield. The earlier maturing soybeans yielded less than the later maturing soybeans. Very low plant populations (14,000 plants per acre) had an adverse effect on soybean yield. There was no significant improvement in yield at soybean plant populations over 46,500 plants per acre. The late planted soybeans yielded the largest seed size this year. Seed size was also impacted by plant population, but it took very low plant populations to yield a larger bean. The variety of soybean used in the study made a significant difference in seed size. Neco 53-3559A produced the largest bean and Fontanelle 12-HC29 produced the smallest bean. The two maturity group 3 soybean varieties produced seed of larger size than the two varieties in the study that matured earlier. Soybean protein content ranged from 37 to 40% and varied by variety in the study. Vinton 81 contained the highest protein content and Fontanelle 12-HC29 contained the lowest protein. There was no trend apparent relative to protein content in relation to maturity group The earlier planting date yielded soybeans with 1% higher protein than the earlier planting date. Protein content was lowest with the lowest plant population, and highest with the highest plant population.
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