Monocot weeds

The example compounds had more post emergence activity than pre emergence and were more active on dicot than on monocot weeds. 

Heibicides group Sites of action Example heibicides Dicots weed Monocots weed Total... 

Compared with alkylphosphonates IC and monosalts IIA-IIE, several series of monosalts of alkylphosphonic acids IIF-IIH demonstrated higher inhibitory activity against monocotyledonous weeds in jxjst-emergence application. IIF-1, IIG-1, and IIH-1 could provide 50-80 % control at 150 g ai/ha for the monocot weeds. However, their corresponding phosphonate IC-22 and monosodium salt IIB-2 showed no activity under the same test conditions. We considered these alkali metal salts of alkylphosphonates or alkylphosphonic acids HA-IIH was a new OP herbicide lead which should be worth further optimization. 

Mono alkali metal salts of l-(substimted phenoxyacetoxy)alkylphosphonic acids was beneficial to post-emergence herbicidal activity against monocot weeds. IIG-1 (R =Me, Yn=2,4-Cl2), IIG-7 (R =thien-2-yl, Yn=2,4-Cl2), IIH-1 (R =Me, Yn=2,4-CI2), and IIH-7 (R =thien-2-yl, Y =2,4-Cl2) display 50-60 % post-emergence herbicidal activity against monocot weeds. As shown in Table 3.31, HF-1 (R =Me, Yn=2,4-Cl2) provided 60-80 % control at 150 g ai/ha for the monocot weeds. However their corresponding phosphonate IC-22 (R =Me, Yn=2,4-Cl2) and mono... 

SAR analysis indicated that post-emergence herbicidal activity against monocot weeds could be enhanced by replacing both MeO with MO and HO in the structure of phosphonate IC, but the same replacement had little effect on post-emergence herbicidal activity against dicotyledons. 

Compared with clacyfos (IC-22, HW02), IVD-12 exhibited a shghtly higher herbicidal effect against broad-leaved weeds, leaf mustard or common amaranth, and also showed much higher herbicidal activity against monocot weeds than that of clacyfos. However, cyclic 1-hydroxyalkylphosphonates IVA and IVB had no significant herbicidal activity (Sect. 5.1.5). All cyclic phosphonates IVC were also not comparable to clacyfos. 

As shown in Table 5.32, IVG-2, IVG-4, and IVG-6 exhibited good post-emergence herbicidal activity against tested dicotyledonous plants, especially IVG-4 displayed a 100 % inhibition effect against all tested broad-leaved weeds at 150 g ai/ ha. However, there was no activity against monocot weeds. IVG-2, IVG-4, and IVG-6 will be further tested for their post-emergence herbicidal activity at a lower rate. 

In some triazine-resistant species where resistance is due to more rapid metabolism of the herbicide, the weeds develop resistance gradually and may be only slightly resistant. This is especially true with some of the monocot or grass weeds that are already partially inherently resistant to atrazine (Thompson et al. 1971 Gressel et al., 1982, 1983). DePrado et al. (1995) found that fall panicum has the capacity for rapid detoxification, which is slightly greater in plants from fields that have been repeatedly treated with atrazine. 

In Table 11.2, a listing of known herbicide-resistant biotypes is given according to the type of weeds (dicots and monocots), and herbicide class. 

Herbicide class/example WSSA3 code HRACb code Dicots Resistant weed species Monocots Total... 

In our studies we used model systems to elucidate detoxification capacities of weeds and crops for benzoxazolin-2(3H)-one. Model systems have the advantage of demonstrating biochemical events that may not show under complex natural field conditions. The purpose of these studies is to gain insights in the possible participation of detoxification in the often observed difference of monocots and dicots against allelochemicals. Second, we questioned whether detoxification capacities reflect the membership of species to defined plant communities. 

Table IV. Distribution of Triazine-Resistant Monocot (Grass) Weeds (as of December 1988)...

According to the International Survey of Herbicide Resistant Weeds (www. weedscience.org) report, there are currentiy 434 imique cases of herbicide resistant weeds globally, involving 237 species (138 dicots and 99 monocots). Herbicide resistant weeds have been reported in 82 crops in 65 coimtries. Multiple resistances also have been observed. Weeds have evolved resistance to 22 of the 25 known herbicide sites of action and to 155 different herbicides. The evolution of herbicide-resistant weeds for selected herbicides of six leading sites of actions is shown in Table 1.4. 

It was found that most alkylphosphinates IIIC showed higher inhibition activity against monocot species such as crab grass and barnyard grass than their corresponding alkylphosphonates IC, IE, or IF. Several alkylphosphinates however exhibited a higher selectivity between wheat and weeds than their corresponding... 

IC-22 showed much higher herbicidal activity than that of those repoted plant PDHc inhibitors, acylphosphinates and acylphosphonates[l, 27]. Those compounds exhibited 80-100 % inhibition against weeds at 2.8 kg/ha but at this rate they had shown unacceptable phytotoxicity to the crops. Compared with acylphosphinates and acylphosphonates, IC-22 exhibited promising herbicidal activity and selectivity for development as a selective post-emergence herbicide which may be used for broadleaf weed control in monocot crop fields. 

The above results suggested that when 2,4-Cl2 as on the phenoxy-benzene ring was kept constant, the replacement of the phosphonate moiety by the phos-phinate moiety had a favorable effect on the inhibition against monocot and a higher selectivity between wheat and weeds. 

More than 40 field trials in different regions in China showed that clacyfos exhibited potent activity against annual and perennial broad-leaved and sedge weeds in lawn, wheat and maize fields by post-emergence application. Clacyfos was safe for monocot crops or monocot turf grass at the recommended rates of... 

As shown in Table 8.24, HWS exhibited excellent herbicidal activity against broad-leaved weeds by post-emergence application. Dicotyledonous crops such as sugar beet was susceptible to HWS at a rate as low as of 50 g ai/ha, whereas the monocot crops, maize, and rice displayed high tolerance to HWS even at 1.2 kg ai/ha. HWS was safe for maize and rice, and can be used as abroad-leaved weeds control agent in monocot crop fields. 

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