The constant use of herbicide in the field year after year has been reported to gradually bring about a change in weed flora and the new weeds are often more hardy than the orginal ones. The phenomenon has been named as ecological shift in weed flora. Many examples of ecological shifts in response to repeated use of 2,4-D have been recorded in India. The phenomenon is based on the principle that in mixed population of weed there are always a few plant of one or more weed species which are tolerant to the herbicide applied. These tolerants plants called ecotypes of prevalent weed species, which are then called chemo types.
Weed tolerant species / plants developed from prevalent (existing) weed species due to repeated use of particular herbicide are called chemo Types.
When herbicide is used repeatedly the tolerant weed ( Chemo types) gradually build their population in the spaces vacated by the susceptible weeds. In many rice fields of Punjab, Haryana and West Bengal, where herbicidal control is common has given way certain weeds like Cyperus spp. Scirpus spp. and Eleocharis spp. Similarly in Northern India new problems weeds like Lathyrus, Convolvulus, Medicago etc created serious problems in wheat fields.
To obviate such undesirable ecological shift in weed flora occurring due to use of mono-herbicides ( use of single herbicide), it is now recommended so adopt herbicide rotations and herbicide combinations. In herbicide rotation two or more herbicides are selected for a crop situation and these are used in alternate years, just like we practice crop rotation in the field. E.g In maize field if Atrazine is used year after year, Panicum spp tend to establish. But when Atrazine and Simazine are used in rotation, the Panicum spp. remain under control.
Herbicide combination are effective when their individual components is known to control a different class of weeds in case of crops like sugarcane , maize etc.
Glyphosate, 2, 4-D, Paraquat, 2,4-D , 2,4-D +2,4.5-T.