Jursík M., Soukup J., Holec J., Andr J., Hamouzová K. (2015): Efficacy and selectivity of pre-emergent sunflower herbicides under different soil moisture conditions. Plant Protect. Sci., 51: 214-222.We ranked the most frequently used pre-emergent herbicides in sunflower (Helianthus annuus) according to their efficacy and selectivity under different soil moisture conditions within 2008-2011. The efficacy of oxyfluorfen, aclonifen, acetochlor, dimethenamid, and propisochlor on the majority of weeds (Chenopodium album, Echinochloa crus-galli, Amaranthus retroflexus, Mercurialis annua, and Solanum physalifolium) was only slightly affected by the soil moisture and these herbicides can be used in arid and semiarid regions. The efficacy of linuron, prosulfocarb, and pethoxamid was strongly affected by soil moisture and was insufficient under dry conditions. The majority of herbicides showed good selectivity for sunflower. Crop injury rate of 5-15% was recorded after application of flurochloridone and acetochlor. For flurochloridone, the phytotoxicity increased due to irrigation after herbicide application. The highest sunflower injury rate (27-35%) was recorded after application of oxyfluorfen.

Small plot trials were carried out in years [2001][2002][2003] with sugar beet. In the treatment without weed control, dry weight of sugar beet top and LAI of sugar beet were very low (approx. 50 g/m 2 and 0.5 m 2 /m 2 , respectively). Yield loss of sugar beet was 80-93%. Dominant weeds were Chenopodium album, Fumaria officinalis and Galium aparine. In the treatments where weeds were removed (by hand) until 4 leaf stage of sugar beet, dry weight of sugar beet top and LAI of sugar beet at first increased normally, but were markedly decreased from the half of the vegetation period. Yield loss of sugar beet was 54-28%. Dominant weed in this treatment was Amaranthus retroflexus. The development of sugar beet top dry weight and LAI of sugar beet was practically identical in the treatments where weeds were removed until 8-10 leaf stage of the crop and in those where weeds were removed during the whole vegetation period (500-900 g/m 2 , or 4-7 m 2 /m 2 , respectively). No yield loss of sugar beet was recorded. Dry weight of weeds did not exceed 30 g/m 2 and LAI 0.1 m 2 /m 2 . A. retroflexus and Mercurialis annua were the most frequent weeds in this treatment.

Competitive ability of six summer catch crops (Brassica napus, Lolium multiflorum, Lolium perenne, Phacelia tanacetifolia, Sinapis alba and Trifolium incarnatum) in volunteer winter wheat based on field trials was experimentally tested during the years 2004-2007 in central Bohemia (Czech Republic). The production of aboveground biomass and plant cover of sown catch crops, volunteers and weeds was assessed on experimental plots. General linear models revealed significant influence of catch crops, year and their interaction on dry-mass of the volunteers. The lowest average values of volunteer biomass at the end of growing season (average from [2004][2005][2006][2007] were recorded on plots sown with S. alba (124.7 kg/ha) and P. tanacetifolia (186.3 kg/ha). The average biomass of volunteer plants in stands of S. alba was significantly lower than the biomass of volunteers in stands of L. perenne, L. multiflorum and T. incarnatum. The lowest average biomass of weeds was recorded also in S. alba stands. In the context of our study, catch crop is a crop sown between seasons of regular plantings to make use of temporary idleness of the soil.

Laboratory experiments were carried out with seeds of Galinsoga ciliata and Chenopodium album in 1998, 2000 and 2001. The study involved germination of non-dormant seeds in the light and the dark at 5-30°C, the germination energy at 15, 24, and 33°C, and the primary dormancy of seeds matured during the period from July to October. Both weeds germinated better in the light compared to the dark. Seeds of G. ciliata germinated at 10-33°C. Germination exceeding 85% was recorded at 12-28°C. Ch. album germinated in a wider temperature interval (5-33°C). Maximum germination (75%) was obtained at 18°C. A very high germination energy of G. ciliata seeds was found at 24°C. At this temperature, 89% of seeds germinated during the second and third day of the germination test. Seeds of Ch. album had a low germination energy at 15 and 24°C. The germination energy was high only at 33°C, however, the total germination reached at this temperature was only 26.5%. Both G. ciliata and Ch. album formed primary dormant seeds during all three testing years. The length of primary dormancy varied from 10 to 100 days depending on the date of ripening and on the year. The longest primary dormancy was found with early ripened seeds (July and August).

Four efficacy and selectivity small plot field trials were carried out in four sunflower hybrids at two localities in Middle Bohemia during 2008 and 2009 with herbicides containing the active ingredients flumioxazin and fluorochloridone. For Chenopodium album, an efficacy of 97% was found after pre-emergent application of fluorochloridone + acetochlor; the efficacy of oxyfluorfen was 88-95%; the efficacy of flumioxazin was below 50% at both tested application rates (30 and 60 g/ha) and application timings (2 sunflower leaves and 4 to 6 sunflower leaves); efficacy of flumioxazin increased to 67% and 69%, when surfactants were added (isodecyl alcohol ethoxylate and heptamethyltrisiloxan). Full control of Amaranthus retroflexus was found after all pre-emergent and early post-emergent herbicide treatments; efficacy of late post-emergent herbicide treatments was below 93%. Full control of Mercurialis annua was found only after application of oxyfluorfen. Thlaspi arvense was fully controlled by fluorochloridone + acetochlor (pre-emergent) and oxyfluorfen (in both application terms). Selectivity of the four tested sunflower hybrids to fluorochloridone + acetochlor, bifenox and flumioxazin was acceptable (phytotoxicity 6 to 27%). Higher phytotoxicity (23-45%) was recorded when surfactants were added to flumioxazin. The highest phytotoxicity (68-81%) was recorded after the application of oxyfluorfen.