Combining Bensulide and Pendimethalin Controls Weeds in Onions

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Title Combining bensulide and pendimethalin controls weeds in onions

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Journal California Agriculture, 55(1)

ISSN 0008-0845

Authors Bell, Carl E. Boutwell, Brent

Publication Date 2001

Peer reviewed

eScholarship.org Powered by the California Digital Library University of California Onion field trials comparing bensulide and bensulide plus pendimethalin to DCPA and untreated control. Areas with green seed lines of onion without weeds are either DCPA or bensulide plus pendimethalin. Combining bensulide and pendimethalin controls weeds in onions

Carl E. Bell P Brent E. Boutwell

DCPA was the principal preemer- ry bulb onions (Allium cepn L.) ing does not allow room for cultiva- gence herbicide for controlling D are grown throughout Califor- tion equipment. Any hand weeding weeds in onions until its manufac- nia for fresh consumption and for that is done requires the use of as- ture was discontinued in 1996, processing into dried products. In paragus knives, which are about 1.5 although it may be reintroduced in 1997 there were about 46,000 acres of inches wide, or hand pulling rather 2001. The purpose of this research dry bulb onions in California, worth than hoes. Using hoes would take was to test the effectiveness of a over $140 million. Weed control in out at least as many onions as weeds. onions relies heavily on herbicides Another reason for reliance on her- combination of two herbicides, for several reasons. One reason is the bicides is the slow germination of the bensulide and pendimethalin, as close spacing of the crop. Most on- onion from seed and the slow growth a replacement weed-control treat- ions are direct seeded on the tops of of small onions; both factors allow ment. Results are encouraging; beds spaced at 40 to 42 inches (Voss weeds to get a head start on the crop this combination performed as well and Mayberry 1999a; Voss and (Rubin 1990). A third reason for the re- as DCPA in 12 onion field trials Mayberry 1999b). The top of the bed, liance on herbicides is that onions do conducted in the Imperial Valley. which is about 22 inches wide, has not compete well against weeds, espe- Onion yields in fields treated with four to six seedlines just far enough cially early in the production cycle bensulide and pendimethalin were apart to accommodate the mature (Rubin 1990). Onions have narrow, up- comparable to that of fields treated bulbs. The crop is planted to a stand, right leaves that do not shade the with DCPA. so no thinning takes place. This spac- ground to inhibit competitors.

CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 2001 35 These reasons demonstrate the need soil-active herbicide registered for on- cause none met the criteria for adop- for preemergence herbicides, which ions that is used only after the crop tion cited in the previous paragraph. are used when seeds are sown, to give has emerged. This herbicide is not reg- the onions a chance early in the season istered for preemergence use because Cooperative grower trials against much quicker growing and it has been shown to cause significant All of these experiments were con- competitive weeds. In addition, onion stand loss and injury at rates ducted in cooperative grower fields in postemergence herbicides are used to typically used when growing more tol- the Imperial Valley of southeastern keep fields weed-free in order to pro- erant crops, such as cotton. However, California. In this area, onions are duce optimum yield, quality and ease .a very low rate of this herbicide may planted in the fall, around mid-October, of harvest. be safe enough for the crop while suf- for a spring harvest. The first year of DCPA has been the staple preemer- ficiently improving weed control the study, referred to as 1998, spanned gence herbicide for onions for about 3 when combined with bensulide. the winter season of 1997-1998. The decades. It is a versatile and reliable In 1997 we initiated field research second year, referred to as 1999, herbicide, effectively controlling most to evaluate bensulide - applied alone spanned the 1998-1999 growing sea- annual weeds of onions, except those at 4 and 6 lb ai/acre (pounds of active son. Twelve experiments are reported, grown in soils high in organic matter. ingredient per acre) - and a com- six in 1998 and six more in 1999, each Unfortunately, in 1996, manufacture of bined treatment of bensulide at 4 lb on a different field. Two in each year DCPA herbicide was discontinued, ai/acre plus pendimethalin at 0.25 lb were on fields planted to dry bulb leaving onion growers in North ai/acre as alternatives to DCPA at 9 lb market onions (Imperial Valley America and elsewhere without an ef- ai/acre. For adoption of this treatment Sweets); the rest of the experiments fective alternative. Another manufac- by onion growers in California, a suc- were on dry bulb dehydrator onions, turer has acquired the production and cessful treatment had to include weed those destined for drying and process- marketing rights to DCPA and intends control equivalent to DCPA and ad- ing. We recorded soil type, planting to have it available in the spring of equate crop safety. An important fea- date (which is the same as the herbi- 2001. The only other currently regis- ture of these two alternative herbicide cide application date) and harvest tered preemergence herbicide for on- treatments was that they were both al- dates for each field (table 1). ions is bensulide. Previous experi- ready registered for use in onions in Herbicides were applied by hand ence with bensulide has shown that the United States, including California, with a C0,-pressured sprayer at 30 it does not control nearly as many and only slight changes in label direc- psi, using 8003 flat fan nozzles and weeds as DCPA and that there is tions were required to allow the differ- typically delivering 35 gallons per acre some risk of crop injury at dosages ent uses tested here. spray volume. After the preemergence normally used to achieve optimal A selection of other herbicides, both herbicides were applied, the growers weed control in other crops, such as old and new, was included in this re- did all subsequent postemergence her- melons and lettuce. search. The older herbicides were bicide applications as part of their in- those that had shown some promise in dividual production practices. Herbi- Combining two herbicides previous research in California on on- cides varied by grower and field, but At the suggestion of a local pest ions. The new herbicides were those may have included bromoxynil and/ control adviser (see acknow- that had not been tested on onions, to or oxyfluourfen for broadleaf weeds, ledgements), we conducted experi- our knowledge, but that were already and fluazifop, sethoxydim or cletho- ments to test bensulide at a low rate registered on other crops, such as corn dim for grasses. Fields were visually combined with a very low rate of or soybean. Data on the results of evaluated for weed control and crop pendimethalin. Pendimethalin is a these herbicides were not included be- vigor when the crop had one to two true leaves, before these postemergence herbicides were applied. Stand counts, a measure of onion plant density, were made on all experiments after the crop had finished germinating, generally at the two-to-four-leaf stage of growth. Yield data was obtained from nine of the fields when the rest of the field was being harvested, including all eight of the dehydrator onion fields but only one of the market onion fields. Onion plant density and yield data were subjected to factorial analysis of

36 CALIFORNIA AGRICULTURE, VOLUME 55, NUMBER 1 variance. Because of significant inter- cept for nettleleaf goosefoot, bensulide DCPA and satisfied one of our crite- actions between years, locations and alone did not control these weeds well ria for success. location within years with treatment, enough to be acceptable to typical on- each trial was analyzed separately us- ion growers, especially at the lower Crop safety and yield ing two-way analysis of variance. dosage of 4 lb ai/acre. These results The other important criterion for Treatment means were separated us- are consistent with previous experi- this study was crop safety, which was ing Fisher’s Protected LSD (0.05). ments with bensulide on other crops measured as crop stand (population Some of these treatment means were such as melons and lettuce. The com- density) and yield. DCPA did not ad- also compared using single degree of bination treatment of bensulide plus versely affect onion density compared freedom class comparisons (orthogo- pendimethalin controlled all four to the untreated control in any of the nal contrasts). Visual evaluations were weed species. This was in spite of the 12 field trials (table 3). Bensulide, how- not analyzed statistically. fact that the rates used for each her- ever, did reduce onion density in five bicide were lower than normally fields in the 1999 experiments, but did Weed control used in other row crops. A typical not affect any of the fields adversely in Four weed species - nettleleaf bensulide rate is 6 lb ai/acre, com- 1998. In 1999, bensulide, when applied goosefoot, little mallow, London pared to the 4 lb ai/acre used here, at the higher rate, reduced onion den- rocket and annual sowthistle - were and a rate of 0.75 to 1.0 lb ai/acre is sity 4% to 70% in four of the fields abundantly represented in 4 (annual typical for pendimethalin instead of compared to the DCPA treatment. In sowthistle and London rocket), 6 (little the 0.25 lb ai/acre used in this study. three fields, this rate of bensulide also mallow) or 8 (nettleleaf goosefoot) of This treatment was equivalent to the resulted in crop densities 18% to 67% the 12 experimental sites (table 2). Ex- level of weed control achieved by lower than the untreated control. The

CALIFORNIA AGRICULTURE, JANUARY-FEBRUARY 2001 37 lower rate of bensulide also lowered planations, but no reasonable connec- dency to injure onions and affect on- onion density compared to DCPA in tion was apparent. A reduced crop ion stand. The combination treatment two fields, in one case by 14% and in stand does not always equal a reduced of bensulide and pendimethalin the second case by 53*/0.In one field, yield, since onions compensate to shows promise as a substitute for the lower rate of bensulide reduced some extent by producing larger DCPA. However, these results are density by 48% compared to the un- bulbs. However, there can be an effect currently valid only for onion pro- treated control, but densities were also on quality and marketability of the duction in the low desert areas of higher in two of the fields. bulbs if the stand is uneven. California; they need to be dupli- The combination treatment of In six of the nine fields where we cated in other areas and under differ- bensulide plus pendimethalin also had obtained yield data, there was no sta- ent environmental conditions to en- an effect on onion density in 1999. tistical difference in crop yield be- courage widespread adoption of this Stand counts for the combination tween treatments and the untreated treatment. In addition, several newer treatment were lower than the un- control (table 4). This lack of yield dif- herbicides that have more desirable treated control in fields 3 and 4, and ference may also be due to the fact that environmental and toxicological lower than DCPA in fields 1 through weed densities in these trials were low characteristics than these old herbi- 5. In the worst case, onion density was and/or postemergence herbicide treat- cides as identified by the U.S. Envi- reduced by 31"/0. Onion density was ments were effective enough to pre- ronmental Protection Agency are be- also lower than the low rate of vent yield loss. In 1999, onion yield ing tested in a variety of locations in bensulide alone in fields 1 and 4, but from the two bensulide alone treat- California. higher in field 2. ments in field 2 were significantly Onion density in the untreated con- lower than the other treatments, in- C.E. Bell is WedSciciici> Fnriii AdzJisor trol plots was equivalent to that in the cluding the untreated control. The arid B.E. Boirtruell is StnffRescwrcli As- DCPA treatment in 9 of the 12 field ex- cause of this yield loss may be onion sociate 11, UC Coopcrntiuc Exteiisioii, periments (table 3). Weed densities density reductions caused by the her- Iniperinl Coiiiity. were probably relatively low in these bicide early in the season. Also, in this C. Riclinrd Wncgnrr, Prc7sidciit of fields, or the postemergence herbicide field there were high populations of Rockzilood Clieiiiicnl Coiiipnmy, Brniuley, treatments applied by the growers little mallow and London rocket, CA, siiggestcd tcstiiig tlie effect of the were very effective against the species which were not controlled by the coiribinntioiz of ~'~'izdiiiief11171iiiniid of weeds present. Rubin (1990) noted bensulide. In the other four 1999 ex- beizsiilidc niid nrrniiged for iiiost offli~ that effective weed control accom- periments, herbicide use did not raise field trinl locntioiis. Tlie Anicricari Deli!/- plished early in the growing cycle, af- or lower yield compared to the un- drntor Oiiioii niid Gnrlic Associntioii mid ter onions and weeds have germinated treated control. Gozi~niiCoinpniy proz~iiiedfiriniicinlsiip- but before they start to compete, can Onion yield in the combination port. lose Lnrios of Rogers Foods mid result in normal crop yields. However, treatments of bensulide plus Robcrt Deiie?oi/cr of Gilroy Foods pro- in 3 fields (fields 1,4 and 5) in 1999, pendimethalin was equivalent to that zJided iizzinl iiablrfield nssistnnce. SezJernl onion density was 10% to 28% lower in the DCPA plots in all of the fields in grozuers dorinted portions uf tlieirfields in the untreated control plots than in both years. Lack of a preemergence nt 170 cost for tliesc studies. Tlicse groru- the DCPA treatment. Neither weed herbicide in fields 4 and 5 in 1998 ap- usinclirded Crnig Eliiiore, Dcsert Sky density nor the efficacy of post- parently resulted in reduced yield in Fnrnis; Mark Osterknnip, Ostcrkniiip emergence herbicides in the untreated the untreated control plots, even Fnums; Robert Flenziizg, Spriice Fnrnis; control plots was measured, so the though onion density was not affected Otis Krnnier, Kranier Fnriiis; Cnrl reason for lower onion density is not in these treatments. Riissell, Russell Brotlirrs Fnrnis; nnd clear. These results demonstrate the Mike Sirddiith, Siddiitli Fnrnis. value of a preemergence herbicide in Alternatives to DCPA these cases. Since weed densities and DCPA has been an excellent References species compositions are seldom preemergence herbicide for onions; re- Rubin 6. 1990. Weed competition and known before planting, preemergence sults of these 12 field experiments are weed control in Allium crops. In Rabinowitch herbicides typically have value to help further evidence of that fact. However, HD and Brewster JL. Onions and allied ensure optimal crop density. if it is no longer available, alternatives crops. Volume /I: Agronomy, Biotic Interac- tions, Pathology, and Crop Protection. Boca We cannot explain the reason for are needed. Bensulide has been regis- Raton, FL: CRC Press. p 63-84. stand loss in some of the 1999 fields tered for onions for several years, but Voss RE and Mayberry KS. 1999a. De- and none of the 1998 fields, but use has been limited because of two hydrator bulb onion production in California. weather has typically been the culprit. problems illustrated by these trials. UC-DANR publication 7239. Voss RE and Mayberry KS. 1999b. Soil type, especially texture, and soil One is the lack of broad-spectrum Fresh-market bulb onion production in Cali- salinity were evaluated as possible ex- weed control and the other is the ten- fornia. UC-DANR publication 7242.

38 CALIFORNIA AGRICULTURE, VOLUME 55, NUMBER 1