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George E. Boyhan, Ray Hicks and C. Randell Hill

This study was undertaken to evaluate natural mulches for weed control in organic onion (Allium cepa) production where current practices rely on hand-weeding or plastic mulch. Three experiments were conducted over 2 years, with two experiments conducted on-farm in different years and one experiment conducted on-station. Treatments consisted of hand-weeding or mulches of wheat (Triticum aestivum) or oat (Avena sativa) straw, bermudagrass hay (Cynodon dactylon), compost, and needles of slash pine (Pinus elliottii) and longleaf pine (P. palustris). All of the mulches with the exception of compost tended to lodge in the onion tops due to their close spacing. Wheat straw and bermudagrass hay reduced plant stand and yield. Compost settled well around the onion plants and initially smothered weeds, but over time the compost treatment became very weedy. Pine needle mulch (referred to as pine straw in the southeastern U.S.) showed the most promise with less stand loss or yield reduction, but did tend to lodge in the tops. None of these mulches were acceptable compared to hand-weeding.

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George E. Boyhan, Raymond Hicks and C. Randell Hill

There has been interest in producing Vidalia onions organically among both conventional and organic growers. In the 2000–01 season we began to look at producing onions organically. Starting with conventionally produced transplants that were transplanted at standard commercial spacings on beds prepared with 10.2–15.2 cm of incorporated compost and 2,802 kg·ha–1 rate of fresh poultry litter. This was sidedressed with an additional 2,500 less/acre (2,802 kg·ha–1) poultry litter. Yields were about half of conventional onion production. In 2002–03, production of organic transplants with 10.2 cm of incorporated compost with 2.24 t·ha–1 rate of poultry litter, which was followed by an additional sidedressing of 2.24 t·ha–1 rate of poultry litter resulted in similar findings. The weight of harvested transplants was about half that of conventionally produced transplants. In the 2002–03 and 2003–04 seasons various natural mulches were evaluated for weed control. They included wheat straw, oat straw, Bermuda hay, pine straw, and compost. None of these performed better than hand weeding and the wheat straw, oat straw, and Bermuda hay actually reduced yields apparently due to allelopathic effects. Finally in the 2003–04 season rates of poultry litter from 0–22.4 t·ha–1 were evaluated for transplant production with rates of 13.4, 17.9, and 22.4 t·ha–1

yielding plants comparable to conventional transplants. Work continues in the area of organic Vidalia onion production. One of the greatest challenge for future work will be finding a cost-effective and practical method of controlling weeds in transplant production.

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George E. Boyhan, David B. Langston, Albert C. Purvis and C. Randell Hill

Five different statistical methods were used to estimate optimum plot size and three different methods were used to estimate optimum number of replications with short-day onions (Allium cepa L.) for yield, seedstem formation (bolting), purple blotch and/or Stemphylium (PB/S), botrytis leaf blight (BLB), and bulb doubling with a basic plot size unit of 1.5 × 1.8 m (length × width). Methods included Bartlett's test for homogeneity of variance, computed lsd values, maximum curvature of coefficient of variation plotted against plot size, Hatheway's method for a true mean difference, and Cochran and Cox's method for detecting a percent mean difference. Bartlett's chi-square was better at determining optimum plot size with transformed count and percent data compared with yield data in these experiments. Optimum plot size for yield of five basic units (7.5 m length) and four replications is indicated using computed lsd values where the lsd is <5% of the average for that plot size, which was the case in both years of this study. Based on all the methods used for yield, a plot size of four to five basic units and three to five replications is appropriate. For seedstems using computed lsd values, an optimum plot size of four basic units (6 m length) and two replications is indicated. For PB/S two basic units (3 m length) plot size with four replications is indicated by computed lsd values. For BLB a plot size of four basic units (6 m length) and three replications is optimum based on computed lsd values. Optimum plot size and number of replications for estimating bulb doubling was four basic units (6 m length) and two replications with `Southern Belle', a cultivar with a high incidence of doubling using computed lsd values. With `Sweet Vidalia', a cultivar with low incidence of bulb doubling, a plot size of four basic units (6 m length) and five replications is recommended by computed lsd values. Visualizing maximum curvature between coefficient of variation and plot size suggests plot sizes of seven to eight basic units (10.5 to 12 m length) for yield, 10 basic units (15 m length) for seedstems, five basic units (7.5 m length) for PB/S and BLB, five basic units (7.5 m length) for `Southern Belle' doubling, and 10 basic units (15 m length) for `Sweet Vidalia' doubling. A number of plot size-replication combinations were optimum for the parameters tested with Hatheway's and Cochran and Cox's methods. Cochran and Cox's method generally indicated a smaller plot size and number of replications compared to Hatheway's method regardless of the parameter under consideration. Overall, both Hatheway's method and computed lsd values appear to give reasonable results regardless of data (i.e., yield, seedstems, diseases etc.) Finally, it should be noted that the size of the initial basic unit will have a strong influence on the appropriate plot size.

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George E. Boyhan, Reid L. Torrance, Jeff Cook, Cliff Riner and C. Randell Hill

Onions (Allium cepa) produced in southeastern Georgia's Vidalia-growing region are primarily grown from on-farm produced bareroot transplants, which are usually sown at the end of September. These transplants are pulled midwinter (November to January) and reset to their final spacing. This study was to evaluate sowing date, transplanting date, and variety effect on yield and quality of onions. Beginning in the first week of November, onions can be transplanted until the end of December with reasonable yield and quality. For example, in the 2003–04 season, total yield of onions transplanted on 22 Dec. 2003 did not differ from any onions transplanted on earlier dates in November or December. In the 2004–05 season, onions transplanted on 20 Dec. 2004, had lower total yield than onions transplanted in November, but were not different from onions transplanted on 4 Jan. 2005. The propensity of some varieties to form double bulbs can be reduced with later sowing and transplanting dates. Sowing the first week of October rather than the fourth week of September and transplanting in December rather than November can reduce double bulbs in some varieties.

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George E. Boyhan, Reid L. Torrance, Jeff Cook, Cliff Riner and C. Randell Hill

Onions (Allium cepa) produced in southeastern Georgia's Vidalia-growing region are primarily grown from on-farm–produced bareroot transplants, which are usually sown the end of September. These transplants are pulled midwinter (November–January) and are reset to their final spacing. This study was to evaluate transplant size and spacing effects on yield and quality of onions. Large transplants (260–280 g per 20 plants) generally produced the highest yield. Medium transplant size in the range of 130 to 150 g per 20 plants produced satisfactory yield while maintaining low numbers of seedstems (flowering) and doubled bulbs, which are undesirable characteristics. Smaller transplant size (40–60 g per 20 plants) have reduced yields and lower numbers of seedstems and double bulbs. Increasing plant population from 31,680 to 110,880 plants/acre can increase yield. In addition, plant populations of 110,880 plants/acre can increase yields compared with 63,360 plants/acre (industry standard), but only when environmental conditions favor low seedstem numbers. Seedstems can be high because of specific varieties, high plant population, or more importantly, in years with environmental conditions that are conducive to their formation. ‘Sweet Vidalia’ was the only variety that had consistently reduced quality and high numbers of seedstems. ‘Sweet Vidalia’ has a propensity for high seedstem numbers, which may have influenced results with this variety. A complete fertilization program that included 133 or 183 lb/acre nitrogen did not affect onion yield, regardless of variety or population density.

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George E. Boyhan, Ray J. Hicks, Reid L. Torrance, Cliff M. Riner and C. Randell Hill

In a 3-year study of poultry litter applications on short-day onion (Allium cepa) production, where rates ranged from 0 to 10 tons/acre, there was an increasing linear effect on total onion yield. Jumbo (≥3 inches diameter) onion yield did not differ with increasing poultry application rates, while medium (≥2 and <3 inches diameter) yields decreased with increasing applications of poultry litter. In addition, organic-compliant fertilizers, 4N–0.9P–2.5K at 150 to 250 lb/acre nitrogen (N), as well as 13N–0P–0K at 150 lb/acre N and in combination with 9N–0P–7.5K totaling 150 lb/acre N were evaluated. Comparison of these commercial organic-compliant fertilizers indicated that there were no differences in total or jumbo yields, while medium yields generally decreased with increased N fertilizer rate.

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George E. Boyhan, Gerard Krewer, Darbie M. Granberry, C. Randell Hill and William A. Mills

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George E. Boyhan, Reid L. Torrance, Ronald H. Blackley Jr., M. Jefferson Cook and C. Randell Hill

Fertilizer rates of N, P, K were evaluated over 4 years (2000–03) as were different sources of experimental and commercial fertilizers. The highest total yields and yields of jumbos (≥7.6 cm) occurred with nitrogen rates of 140–168 kg·ha–1. Neither phosphorus nor potassium rates had an affect on total yield. Phosphorus rates of 0-147 kg·ha–1 and potassium rates of 0–177 kg·ha–1 were evaluated. Increasing nitrogen fertilizer resulted in increasing leaf tissue nitrogen, but did not affect P, K, Ca, or S. Increasing phosphorus fertilizer increased leaf tissue phosphorus only slightly (p = 0.060) with no affect on other leaf nutrient levels. Increasing potassium fertilizer did affect leaf tissue potassium 2 out of 4 years with none of the other leaf nutrient levels affected. Several fertilizers were also evaluated including an experimental fortified peat (10%N), calcium nitrate, ammonium nitrate, diammonium phosphate, 5–10–15 (56 kg·ha–1 N), 18-6-8 liquid, 14–0–12 8%S liquid, 19–8–19 slow-release at rates of 140 and 168 kg·ha–1 nitrogen. All were used to supply 168 kg·ha–1 nitrogen unless noted otherwise. P and K were supplied according to soil test recommendations unless they were part of the fertilizer formulation. There were no differences between the different fertilizer sources for total yield and differences in jumbo yields only occurred one year out of three years of testing and for medium (≥5.1 and <7.6 cm) yields there were differences two years out of three years of testing.

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George E. Boyhan, Juan Carlos Diaz-Perez, Reid L. Torrance, Ronald H. Blackley Jr. and C. Randell Hill

The majority of Vidalia onions are produced as a transplanted crop. Seeding in high density plantings in September is followed 8 to 10 weeks later by transplanting to final spacing. This practice is labor intensive and expensive. Direct seeding would save on labor, cost, and time. Traditionally, transplanting has been done because of better winter survival, more uniform stands, and better irrigation management during seedling emergence. Beginning 5 years ago, we began evaluating direct seeding onions. Initially, seedstems (bolting) and lack of uniform stand establishment were the main problems. Sowing in September resulted in almost 100% seedstems and using a belt planter with raw seed resulted in poor singulation for uniform stand establishment. Mid-October ultimately proved to be the best time for sowing Vidalia onion seed. Earlier sowing resulted in more seedstems and later planting did not give the plants sufficient time to grow resulting in later stand loss during cold winter temperatures. Using polymer coated seed and a precision vacuum planter resulted in uniform, even stand establishment. Fertilizer requirements are almost half with direct seeded onions compared to transplanted onions with a reduction in the need for fungicides and herbicides. We have established direct seeded onions both with drip irrigation and overhead irrigation. There was concern that center-pivot irrigation would not be able to sufficiently irrigate fields during seedling establishment with the frequent hot fall days we experience. Since this work was initiated several growers have successfully produced direct seeded onions under center-pivot systems. Direct seeding Vidalia onions requires attention to detail because there is only one opportunity to get it right. Timing is also critical particularly with planting date and herbicide application.