MSc

A field experiment was undertaken at the College of Agriculture, Vellayani, to find out a scientific and profitable weed management techniques for a Rice – Rice Cropping System, under transplanted condition during the Virippu – Mundakan seasons of 1984 – 85.
The first crop with five herbicides (thiobencarb 1.5 kg, butachlor 1.0 kg, pendimethlin 1.0 kg, nitrofen 1.875 kg and fluchloralin 1.0 kg a.i. per ha) and three controls (hand weeding, completely weed free and weedy check) was laid out in RBD.
Using the same lay out, in the second crop, each herbicide treated main plots was divided in to three sub plots and allotted no weeding, hand weeding and same herbicide treatments and the results analysed as that of a split plot with non – factorial structure. The salient findings of the experiments are as follows.

Firsr Crop
Weed number and competition was severe upto the 40th DT and monocots out numbered dicots. Nitrofen and thiobencarb reduced weed population, weed dry matter accumulation, nutrient removal by weeds and achieved high Weed Control Efficiency than hand weeding.
Nitrofen and thiobencarb improved plant growth characters, enhanced yield components, increased grain and straw yield and accelerated nutrient uptake, followed by butachlor. These herbicides recorded low weed Index values. Application of thiobencarb, butachlor or nitrofen was more profitable than hand weeding twice.
Second Crop
There was no shift in weed species due to herbicides. Weed number and competition were severe upto the 40th DT and monocot weed population was higher than dicots. Though there was a general reduction in the population and dry matter of weeds due to herbicide application of first crop, their residual effect was not enough to keep the field unweeded during the second crop season. Weed number, weed dry matter and nutrient removal by weeds were reduced by the repeated application of thiobencarb or nitrofen or these herbicides – hand weeding combinations, They recorded Weed Control Efficiency of more than 62 percent.
Plant growth characters, yield components, grain and straw yield, and nutrient uptake by the crop were favourably influenced by the above herbicides in combination with hand weeding or when repeatedly applied equal to or better than farmers practice of hand weeding. They recorded low values of weed indices. The residual effect of herbicides applied in the first crop was not significant in the above characters. Thousand grain weight and protin content were not influenced by the treatments.
In the system approach, highest total grain yield was produced by thiobencarb –hand weeding, nitrofen – hand weeding and butachlor – hand weeding where as highest net profit was obtained by the repeated application of thiobencarb and butachlor. Herbicide residual toxicity was not observed to the succeeding cowpea crop.