Peanut (Arachis hypogaea L.) yields were not influenced by weeds that were removed within 3 weeks after planting and each week thereafter, but a yield reduction did occur when weeds were not removed until 4 to 8 weeks after planting. Peanuts kept weed free at least 6 weeks after planting showed no yield loss due to competition from weeds emerging later. As weed competition increases, peanut seed yield, forage yield, and soil moisture content decreases.
Winter wheat (Triticum aestivum L. em Thell.) plots that had received various conventional soil fertility treatments continuously for 47 years were evaluated as to the weed species present, and were compared to other cropping situations. This was done to evaluate the influence of continuous fertility treatments on weed types and populations. Replicated soil samples were collected, the indigenous weed seeds were germinated in greenhouse flats, and the species identified. Six species of weeds predominated. Generally, the lowest total numbers of weeds were observed on the plots which had not been fertilized, and populations increased as various nutrients were added. Highest total weed numbers were present on the plots which had received applications of N, P, K, and lime. Most individual species followed the trend of the total plant numbers except for evening primrose (Oenothera laciniata Hill), carpetweed (Mollugo verticillata L.), and henbit (Lamium amplexicaule L.). Highest numbers of evening primrose occurred on the unfertilized plots with decreasing populations as fertility treatments became more complete. Carpetweed and henbit numbers were greatest in the plots receiving P alone or N plus P. Similar species were found on nearby soils which had grown cotton or winter wheat commercially for 25 and 40 years respectively. High numbers of purslane (Portulaca oleracea L.) were observed on both of these soils which were not present in any of the long term fertility treatments. Many more plant species and higher total plant numbers were found in soil from a native grass prairie site.
The concentration of three sulfonylurea herbicides in soil was determined by a modified petri dish bioassay procedure. The method involved planting pregerminated seed of selected species in petri dishes containing 65 to 100 g of treated soil and measuring the radicle lengths after 24 h. Chlorimuron was detected in two soils at 0.002 μg g–1using either corn, sorghum, or sicklepod as the assay species. Chlorsulfuron and CGA-131036 were detected in soils at 0.001 μg g–1using corn, and chlorsulfuron was detected at the same level using sicklepod. In contrast to other methods, the bioassay procedure described could be completed in 48 h, including pregermination of the seed, growth of the plants on treated soil, and plant measurements.
Research was conducted to compare the relative phytotoxicity of trifluralin (a,a,a‐trifluro‐2,6‐,dinitro‐N,N‐dipropyl‐p‐toluidine) with several analogues, and to establish an indices for comparing alternative structural substitutions. GR50 and visual injury values were used to evaluate relative phytotoxicities. There appeared to be a direct correlation between phytotoxicity and selected structural changes on the dinitroaniline ring. The dinitroaniline ring substitutions in order of decreasing phytotoxicity to bioassay indicator plants were substitutions in the one and five positions, one only, and four only. Least phytotoxic was the combined one and four position substitution on the ring.
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