Performance of reduced herbicide doses in spring cereals Jukka Salonen Salonen, J. 1993. Performance of reduced herbicide doses in spring cereals.Agric. Sci. Finl. 2: 537-550. (Agric. Res. Centre of Finland, Inst. Plant Prot., FIN-31600 Jokioinen, Finland.) The consequences of dose reduction of three new herbicide formulations were studied for the control of annual broad-leaved weeds in fields ofspring barley (Hordeum vulgare L.) and spring wheat (Triticum aestivum L.). The herbicide formulations were MCPA/mecoprop-P, MCPA/dichlorprop-P and MCPA/fluroxypyr. The efficacy of the lowest recommended dose and a 30% lower rate were tested and compared with the reference herbicide tribenuron-methyl. Trials were conducted at seven sites for three years. Considerable annual fluctuations in weed infestation were recorded. Although the dose reduction occasionally caused considerable decline in control (on %-scale), sup- pression of weed biomass was still satisfactory in most of the trials. On average, a 75% reduction of weed biomass in spring barley and an 83% reduction in spring wheat were achieved with reduced herbicide doses. Use of reduced herbicide doses for three years in the same fields caused neither a significant increase in weed infestation nor changes in the species composition of weed populations compared with treatments at recom- mended rates of application. There was a significant difference in biomass production between weed species. Consequently, the total biomass production of annual dicotyle- donous weeds correlated only weakly (r=0.48) with the total weed density. Even in untreated plots the weed biomass at harvest constituted, on average, only 3.1-3.6% of the total vegetative biomass of crop stands. Thus, the crop yield responses to chemical weed control remained low. Key words; spring barley, spring wheat, broad-leaved weeds, MCPA/mecoprop-P, MCPA/dichlorprop-P, MCPA/fluroxypyr, tribenuron-methyl Introduction Reduced herbicide doses have provided adequate control of broad-leaved weeds in many recent cer- eal experiments (e.g. Baandrupand Ballegaard 1989, Davies et al. 1989, Fogelfors 1990, Kem- mer and Hurle 1990, Proven et al. 1991, Sa- lonen 1992a). At present, political Action Plans stipulate thereduction ofpesticide use in the Nordic countries (Thonke 1991, Ympäristöministeriö 1992). Reduction of herbicide doses is one of the measures suggested and studied to achieve this ob- jective. In the Nordic countries (Denmark, Finland, Nor- way, Sweden), herbicides represent 60-80% oftotal pesticide use (Thonke 1991, Markkula et al. 1990). Herbicides are commonly used to control broad-leaved weeds in fields of small-grain cereals, which represent the most widely cultivated crops. Therefore, special attention is paid to optimization of herbicide use in cereal fields as cereals are prob- ably able to out-compete weeds even at low rates of herbicide application. 537 Agric. Sei. Fint. 2 (1993) https://www.c-info.fi/en/info/?token=80v6wQ7CvDYhFa4M.GuabmBwqYPllGMz4iyHoKQ.SHTUpClWFfLexcQY-sBSJayrh1G8yxnewpgSFdp1BnI2Qc1Mc6Hh5GzZ2gOtQRSOzLjVCyFHu89sjA6Yk_cy8b1r7qha1VOo-7iAAGvzM0IRaQQZWOgp1Cv1JGFBSoaYQcH9whp2TpXLkwhZAICV90sw84Cya1GWzrQHjAAFKg0MPww0nWF12Eh0vGJ--g2-sIKVaCPeGT8bcZVvNe0HNLQ2VIpOEw8cDvRaCWf8ONr94nsIMpT0UYN8DBCen68cRot6ag The recommended herbicide doses given on the product labels are normally suggested by chemical companies and then officially tested and approved by the relevant national authorities. The recom- mended "normal" dose implicitly ensures reliable weed control in most situations. The use of factor- adjusted doses is, however, emphasized by the ex- tension service and computer-based advisory sys- tems (Kudsk 1989, Baandrup and Ballegaard 1989, Jennéus 1991). Formulated mixtures of MCPA/dichlorprop and MCPA/mecoprop are commonly used in spring cer- eal crops in Finland (Hynninen and Blomqvist 1993). To date, the commercial formulations of phenoxypropionic acids, dichlorprop and meco- prop, have been mixtures of two optical isomers, R (+> and S w . However, only the R<+) isomer is an active part of herbicide. Recently, these isomers have been separated, and formulations containing only the active isomer have been developed (Squires et al. 1987). Replacement of conven- tional racemic isomers by the new active isomers, dichlorprop-P and mecoprop-P, results in approx- imately 50% reduction in the use of the active in- gredients, dichlorprop and mecoprop. The first commercial products containing active isomers were registered in Finland in 1992. The purpose of this study was to investigate pos- sibilities for reducing the lowest recommended ap- plication rates of the new cereal herbicides by 30%. The risk of failure was assessed, and the con- sequences of continuous use of reduced herbicide doses on weed infestation were studied. Further- more, crop yield responses to chemical weed con- trol were measured. Material and methods Field experiments were conducted at seven experi- mental stations of the Agricultural Research Centre. Four stations (Anjalankoski (KYM), Jokioinen (RKA), Kokemäki (SAT) and Mietoinen (LOU)) are located in southern Finland and three stations (Mouhijärvi (SAH), Pälkäne (HÄM) and Ylistaro (EPO)) in central Finland. The same trial protocol was used for three years, 1989-1991, in spring bar- ley and spring wheat monocultures in the same field. At each site there was one spring wheat (cv. ’Luja’) trial and at four sites (EPO, KYM, LOU, RKA) there was a spring barley (cv. ’Pohto’) trial. Thus, during the 3-years of experimentation there were in total 21 spring wheat trials and 11 spring barley trials. The experiments were established in 1989 in fields where spring cereals were sown in 1988. The crops were sown at the recommended seed rates: 450 viable seeds of barley and 600 seeds of wheat m 2. Various soil types from ranging from sandy clay to heavy clay were represented. The experi- mental plots (4.0/5.0 mxl2 m) were ploughed to a depth of 20-25 cm every autumn. Commercial herbicide formulations of MCPA/mecoprop-P (270/305 g a.i. I' 1 , ’Duplosan KV-M’) for use in wheat fields and MCPA/dichlor- prop-P (265/285 g a.i. I' 1, ’Duplosan DP-M’) in barley fields were applied at their lowest recom- mended rates and at 30% lower rates. MCPA/fluroxypyr (400/100 g a.i. f l, ’Starane M’) was applied to both crops. In addition, tribenuron- methyl (750 g kg ' granular formulation, ’Express 75 DE’) (Ferguson et al. 1985) was used as a reference herbicide (Table 1). New formulations of phenoxy acid herbicides containing only the optically active isomers of di- chlorprop and mecoprop (Squires et al. 1987) were Table 1.Treatments in the field experiments in spring barley and spring wheat fields in 1989-1991. MCPA/dichlorprop-P was applied only in spring barley and MCPA/mecoprop-P only in spring wheat. Treatment Herbicide dose 1 ha" g a.i. ha" Unsprayed 0 MCPA/fluroxypyr 0.70 280/ 70 MCPA/fluroxypyr 1.00 400/100 MCPA/dichlorprop-P 1.25 331/356 MCPA/dichlorprop-P 1.75 464/499 MCPA/mecoprop-P 1.25 338/381 MCPA/mecoprop-P 1.75 473/534 Tribenuron-methyl 7 g 5.3 11 Non-ionic surfactant (’Citowett’) 0.05% was added to the spray solution (water volume 2001 ha' 1). 538 Agric. Sei. Fin!. 2 (1993) included in the experiments. The objective was to investigate whether the positive results of reducing the recommended doses of racemic mixtures (Sa- lonen 1992a) would apply also to new formula- tions. Fluroxypyr was introduced into the official screening trials in Finland in 1982 as a new herbi- cide for weed control in cereal crops, with particu- lar effect on Galium aparine L. (Paul et al. 1985). Treatments were arranged as a randomized com- plete block design with four replicates. Herbicides were applied at the 3- to 4- leaf stage of the crop (Zadoks’ scale 13-15 (Zadoks et al. 1974)) with a portable van der Weij propane sprayer that deliv- ered 200 1 ha 1 spray solution at a pressure of 300 kPa. Herbicides were applied between the end of May and mid-June, about one month after sowing. The temperature at the time of application ranged from 10 to 25°C, and the relative humidity from 33 to 77%. The emergence of crops and weeds were moni- tored before the herbicide application. Crop devel- opment (growth stages) and weed emergence were recorded. Weeds were assessed in 0.25 m 2 sample plots. Annual dicot weeds were counted 0-1 days before spraying (with some exceptions of 2-4 days delay). Furthermore, the weed infestation (number and air- dry weight per unit area) was assessed one month after spraying and at harvest. The relative number ofemerged weeds at the timeof spraying herbicides was calculated by comparing the number of weeds (No. m 2) at spraying and one month later. Crop yield results are given at 15% moisture content. The impact of different control regimes on the subsequent weed infestation was assessed one year after the 3-year trial period in 1992. Weeds were counted at the timeof spraying herbicides in spring cereal fields. Statistical analysis Analysis of variance was applied to weed and crop data by introducing Year as a within-subject factor and Site , Treatment and Block as between-subject factors. The random factor Block was nested in the site. The biomass ofweeds, as a dependent variable, was transformed with the common logarithm log(y+l) to achieve normal distribution and homo- geneity of variances. Weed density (No. rri 2 ) was transformed with square root. The data from un- sprayed plots were excluded from the final statisti- cal analyses. The effect of herbicide dose reduction was tested with single degree-of-freedom contrasts. The effect ofweed infestation (density, biomass) on crop yield was tested with regression analysis. Stat- istical analyses were done with the General Linear Models procedure of the SAS statistical package (SAS Institute Inc. 1990). Results Occurrence of weeds Weed density at the time of herbicide application varied within therange of7-702 weeds m'2 (Fig. 1). Also, the relative number ofweeds which emerged before herbicide application, compared with the number of weeds per unit area one month later, varied considerably (Fig. 2). On average, 72% of the annual dicotyledonous weeds emerged before spraying. Crop plants usually reached at least the second leaf stage (Zadoks’ scale 12-13) before the first flush of weeds. Most weed seedlings were between the cotyledon stage and the first true-leaf stage at the time of herbicide application. The predominant weed species in the experimen- tal fields were typical ofFinnish cereal fields (c.f. Erviö and Salonen 1987). The weed populations varied between sites (Table 2) and, to some extent, between years at the same site. The most frequent and abundant weed species were Chenopodium al- bum L., Fumaria officinalis L., Lamium L. spp., Stellaria media (L.) Vili., Matricaria L. spp.(in- cluding Tripleurospermum inodorum Schultz Bip.) and Viola arvensis Murray. Volunteer turnip rape (Brassica rapa L. subsp. oleifera DC.) occurred in those fields (LOU, SAT) where there were trials with turnip rape some years before the experiment. The weed biomass (air-dry weight, DW) in the untreated plots ranged from 0.4 (SE 0.2) to 61.5 (SE 539 Agric. Sd. Fin!. 2 (1993) Table 2. Predominant weed species in the experimental fields. Site Weed species" Anjalan- CHEAL FUMOF GALSS POLCO koski (KYM) Jokioinen CHEAL LAMSS STEME VIOAR (RKA) Kokemäki BRSRO CHEAL THLAR VIOAR (SAT) Mietoinen BRSRO FUMOF LAMSS STEME (LOU) Mouhijärvi CHEAL MYOAR TRFPR VIOAR (SAH) Pälkäne CHEAL MATSS STEME VIOAR (HAM) Ylistaro LAMSS MATSS POLCO STEME (EPO) 1 1 BAYER codes for weeds (BAYER 1992): brsro -nnmka rapa ssp. oleifera (volunteer), CHEAL = Chenopodium album, FUMOF = humina officinalis, GAESS = Galeopsis spp., GALS = Galium spp., LAMSS = Lamium spp., MATSS = Matricaria spp., POLCO = Fallopio convolvulus, STEME = Stellaria media, THLAR = Thlapsi arvense, TRFPR = Trifolium pratense, VIOAR = Viola arv Fig. 1.Weed infestation in the experimental fields at the time of herbicide application in a) spring wheat and b) spring barley. The experimental sites are: EPO = Ylistaro, KYM = Anjalankoski, LOU = Mietoinen, RKA = Jokioinen, HÄM = Pälkäne, SAH = Mouhijärvi, SAT = Kokemäki. Fig. 2. Weed emergence in unsprayed plots at the time of herbicide application given as a percentage (five classes) of the weed density (No. m" 2) one month later in 33 spring cereal experiments in 1989-1991. 540 Agric. Sei. Fin!. 2 (1993) 12.2) gDW m 2 one month after herbicide applica- tion, and from 1.8 (SE 0.9) to 116.3 (SE 7.7) g DW m 2 at harvest. Biomass production varied consider- ably between weed species. Consequently, the total weed biomass in unsprayed plots, one month after spraying, weakly correlated (r=0.48) with the total weed density at spraying. The most competitive weed species producing the highest biomass per plant were volunteer turnip rape (0.60 g DW plant '), Galeopsis L. spp. (0.27 gDW plant 1) and Fallopia convolvulus (L.) A. Love (0.15 g DW plant 1). The biomass production of barley averaged 1.10 g plant 1 and that of wheat 1.02 g plant 1 at the four sites whereboth crops were grown in the same field. Herbicide efficacy Generally, all herbicides were effective when ap- plied at the lowest recommended dose, and 30% dose reduction reduced the efficacy, on average, by less than 10 percentage units (Fig. 3). However, a significant (P<0.001) Year*Site*Treatment inter- action was detected (Table 3). This indicates that there were differences in the effectiveness of weed control between sites and between years within a site. Tribenuron-methyl was the most effective herbicide in most trials (Fig. 4), particularly when Matricaria spp. and Lamium spp. (EPO, LOU) were the predominant weed species. The reduction in herbicide efficacy was consid- ered significant if 30% dosereduction caused more than 15% reduction in efficacy (on %-scale) com- pared with the efficacy achieved with the recom- mended dose. In wheat trials such a reduction (>15%) in the biomass-based efficacy occurred in 29% of plots treated with MCPA/mecoprop-P, and in 14% ofplots treated with MCPA/fluroxypyr. The corresponding figures for barley trials were 8% with MCPA/dichlorprop-P and 19% with MCPA/fluroxypyr. To describe the probability of achieving a certain level ofweed control, herbicide efficacy was calcu- lated for each treatment within each replicate and theresults wereranked in four efficacy classes (Fig. 5). Accordingly, treatment with reduced herbicide doses still provided at least 70% control in 70-89% ofplots monitored. At the recommended herbicide doses the 70% efficacy level was reached in 78- 91% of cases. Weed biomass in sprayed crop stands one month after herbicide treatment was less than 15 gDW m 2 in every trial. Dry weight of crop plants in un- sprayed plots averaged 506 (SE 24) g DW m 2 in barley and 482 (SE 15) g DW m : in wheat. Dry weight of weeds was significantly (P<0.01) higher in unsprayed than in sprayed plots. To simplify the ANOVA analyses, data from the unsprayed plots were not included in the final analyses (Table 3). Only in some fields was the weed biomass signifi- Fig. 3. Efficacy of herbicides determined as % reduction of weed density (light bar) and dry weed biomass (dark bar). The mean efficacy and the SE of the mean in a) 12 spring barley trials and b) 21 spring wheat trials during 1989-1991. 541 Agric. Sei. Fint. 2 (1993) 542 Agric. Sei. Fin!. 2 (1993) cantly higher in the plots treated with reduced doses than in the plots treated with normal doses. In gen- eral, weeds produced more biomass in wheat stands than in barley stands (Fig. 4). The dose reduction of MCPA/mecoprop-P de- creased the effect of control particularly against Myosotis arvensis (L.) Hill, Matricaria spp.. Poly- gonum L. spp. and Viola arvensis. Similarly, the dose reduction of MCPA/fluroxypyr significantly decreased (>lO %-units) the efficacy against Fu- maria officinalis, Matricaria spp., Polygonum spp. and Viola arvensis. Conclusions concerning MCPA/dichlorprop-P were not drawn due to the limited number of observations. Crop-weed interactions The yield of spring barley and spring wheat aver- aged 4,900 kg ha 1 and 3,700 kg ha' 1, respectively. In the trial plots treated with herbicides the mean yield of wheat was 1.9% higher and barley yield was 4.0% higher than in untreated plots. The monetary value of such a yield increase ranges from FIM 150 to FIM 300 which is sufficient to cover the average cost of (FIM 100) herbicides for broad- leaved weed control. Herbicide treatment did not reduce crop yield significantly (P<0.05) in any trial. There was no significant difference in the mean crop yield from plots which received a recom- mended dose and those which received a reduced dose of herbicide. Only in one trial from 21 wheat trials did the dosereduction of MCPA/mecoprop-P result in a significantly (P<0.01) lower wheat yield, and once, in the same trial, with a reduced dose of MCPA/fluroxypyr (P<0.02). The proportion of weed biomass from the total vegetative biomass of cereal fields was relatively low (Table 4). Fig. 4. Comparison of the remaining weed biomass in 1989-1991 in a) spring barley and b) spring wheat one month after treatment with different herbicide formulations and doses. The figures in paranthesis indicate the air-dry weed biomass (g m" ) in the unsprayed plots each year. The experimental sites are: EPO = Ylistaro, KYM = Anjalankoski, LOU = Mietoinen, RKA = Jokioinen, HÄM = Pälkäne, SAH = Mouhijärvi, SAT = Kokemäki. 543 Agric. Sei. Fint. 2 (1993) 4 Table 3. Repeated measurements analysis of variance (ANOVA) of weed biomass log(Y+l) in sprayed plots in spring barley at four sites and in spring wheat at seven sites. Trials were repeated for three years. Air-dry weight of weeds was measured one month after herbicide application. Crop Degrees Type 111 F-value F-test of Mean probability Source of variation freedom Square Spring barley Between-subject effect Site 3 3.64 28.83