The cost of hybrid sunflower (Helianthus annuus L.) seed provides an incentive for reducing planting rates which, in turn, reduce plant populations. This research was undertaken to determine minimum populations needed for maximum yield and their effect on seed quality and head drying. Populations of oilseed and nonoilseed cultivars of 17, 25, 37, 49, and 62 thousand plants/ha were established at six locations in Minnesota. Soils were Typic Haplaquolls, Aeric Calciaquolls, Typic Eutroboralf and Udorthentic Haploborolls. Both oilseed and nonoilseed cultivars required the same populations for maximum yield. Minimum populations needed for maximum yield varied from 25 to 62 thousand plants/ha among locations. Differences in optimum plant populations among locations were attributed to soil, rain, and temperature. Yields were not depressed by populations up to 62 thousand. Optimum population for oilseed and small‐nonoilseed cultivars was that which gave maximum yield because seed quality factors of test weight and/or oil percentage increased with population. Optimum population for large‐nonoilseed cultivars was often less than that giving highest yield because the percentage of large seeds decreased with increasing population. As plant populations increased from 17 to 62 thousand plants/ha, head moisture percentages decreased from 68 to 50% at early harvest and from 43 to 20% at later harvest. Preharvest desiccant sprays reduced head moisture but did not alter the relationship between increasing population and decreasing head moisture. Increasing row spacing to conserve soil moisture between rows did not increase yield on a sandy soil.
Plant spacing in the row is variable in most sunflower (Helianthus annuus L.) fields. The amount of yield loss from moderately uneven stands of sunflower should be known before undertaking expensive techniques to increase uniformity of plant distribution. The objective of this research was to measure the effects of uniformity of plant distribution on sunflower seed yield, plant growth, and seed quality. Four plant distributions of oilseed and nonoilseed cultivars in rows 76 cm apart and in a population of 49,000 plants/ha were established at five locations in Minnesota in 1979 and 1980. Soils were Typic Haplaquolls, Aerie Calciaquolls, and Aerie Haplaquepts. Plant distributions were: single plants 27 cm apart; paired plants 53 cm apart; five plants each 13 cm apart, 80‐cm space; seven plants each 9 cm apart, 80‐cm space, single plant, 80 cm space. Uniformly spaced, single plants lodged least and produced heads of lowest moisture percentage at harvest and seed of highest yield and oil percentage. Yield reductions from uneven plant distributions ranged from none to 31% and averaged 10%. Greater lodging was the most consistent detriment of nonuniform plant spacing. The magnitude of differences in seed quality among the plant distributions was small and consequently of relatively little economic importance.
Mixtures with noncompetitive perennial grasses have been recommended to reduce weed invasion which commonly occurs in pure stands of birdsfoot trefoil (Lotus corniculatus L.). Our objective was to determine the potential of competitive vs. noncompetitive grass species in mixtures with birdsfoot trefoil to prevent weed invasion while maintaining stands with maximum birdsfoot trefoil composition and forage quality. ‘Leo’ and ‘Viking’ birdsfoot trefoil were established in monoculture and in binary mixtures with nine grass species on a Waukegan silt loam (fine‐silty over sandy, mixed, mesic Typic Hapludolls) in southeastern Minnesota. The same treatments, except one, were also established on a Cowhorn sandy loam (coarseloamy, mixed nonacid sand, frigid Aeric Haploquepts) in northcentral Minnesota. Conventional (when Leo reached first flower) and stockpiled (when Leo reached full bloom) harvest managements were applied for 2 years. Mixtures with high yielding competitive grasses (Phalaris arundinacea L., Bromus inermis Leyss., or Dactylis glomerata L.) had the least weed infestation and greatest weed‐free forage yield, but had consistently lower forage quality than the birdsfoot trefoil monoculture or mixtures with noncompetitive grasses (Phleum pratense L., K2‐106; Lolium perenne L.; or Poa pratensis L.) at the southeastern location. Competitive grasses also maximized birdsfoot trefoil persistence in southeastern Minnesota, while noncompetitive grasses and the birdsfoot trefoil monoculture maximized birdsfoot trefoil persistence in northcentral Minnesota. Relative mixture differences in weed infestation, forage quality, and yield were generally not affected by harvest management (stockpiled vs conventional). Stockpiled forage had lower quality than did conventionally harvested forage at both locations, and greater yields at the southeastern location. Stockpiled mixtures containing Leo had a greater final birdsfoot trefoil composition than did mixtures containing Viking only at the southeastern location. Birdsfoot trefoil cultivar selection did not affect final composition when mixtures were conventionally managed at either location. Because competitive vs noncompetitive grasses led to differing birdsfoot trefoil persistence depending on location, noncompetitive companion grasses may maximize birdsfoot trefoil persistence in mixtures in some environments, whereas competitive grasses may ultimately maximize birdsfoot trefoil in others. If forage quality is the primary concern, noncompetitive grasses should be used; if total weed‐free forage yield is the primary concern, competitive grasses may be advisable.
Because phytophthora root rot (PRR) resistant cultivars of alfalfa (Medicago sativa L.) are new developments, there have not been opportunities to determine what effect PRR resistance may have on forage production in different alfalfa growing areas. This paper presents results from several experiments comparing the performance of PRR susceptible and resistant cultivars grown under a range of environments in Minnesota. Broadcast plots were established in April and May in three different years. Some plots received only natural rainfall; others received an additional period of supplemental sprinkler irrigation during mid‐June each year to simulate a 10 to 14‐day wet period. Natural levels of Phytophthora megasperma Drechs. inoculum were adequate in the soil to cause significant yield losses. PRR resistant cultivars usually out‐yielded susceptible cultivars in plots receiving supplemental irrigation or when natural rainfall was sufficient to induce PRR. The most severe losses occurred during the seedling year. PRR killed seedlings and damaged the normal tap root system of alfalfa. Injured plants usually produced adventitious lateral roots, resulting in a shallow root system. This appeared to make the plants more sensitive to stress, such as drought and frequent harvesting. Resistant cultivars appear to be a feasible method for reducing losses from PRR. A minimum of 60% resistant plants may be necessary in a cultivars to provide an adequate amount of protection under severe PRR conditions.
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