Weed flora
All together there were 11 species of weed flora that belong to seven different families were recorded in the experimental field. Among them, Echinochloa colona, Echinochloa crussgalli and Cynodon dactylon were grasses, Cyperus rotundus, Cyperus difformis, Fimbristylis miliaceae were sedges while, Eclipta alba, Ludwigia parviflora, Ammania baccifera, Euphorbia hirta, Trianthema portulacastrum were broad leaved weed. However, Echinochloa colona and Echinochloa crussgalli among grasses, Cyperus rotundus among sedges and Eclipta alba and Ludwigia parviflora among broad leaved weed were dominant throughout the crop growth period.

Effect on weeds
Statistically detectable disparities were noticed with respect to total weed density (Table 1) due to various weed control practices. The lowest total weed density among the herbicide treatments was registered with T₄ (pre-emergence application of orthosulfamuron @ 100 g a.i ha^-1), T₈ (pre-emergence application of pendimethalin @ 0.75 kg a.i ha^-1 fb orthosulfamuron @ 100 g a.i ha^-1) and T₁₀ (pre-emergence application of pendimethalin @ 0.75 kg a.i ha^-1 fb  post-emergence application metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1) which were on par with each other as well as with weed free check (T₂) and significantly superior to rest of the herbicide treatments and weedy check (T₁). Better performance exhibited by T₄ and T₁₀ in reducing the total weed density might be due to the reason that it was able to control both grassy as well as broad leaved weeds effectively at 60 DAS and at harvest. Better performance of orthosulfamuron, pendimethalin and metsulfuron methyl + chlorimuron ethyl in reducing total weed density has also been reported by Ramana et al. (2007).

Table (1):
Weed density, weed dry weight, weed control efficiency and weed management index of semidry rice as influenced by different weed control treatments

Data were subjected to square root transformation √x +0.5 except for WCE which was subjected to arc sine trans-formation. Figures in parentheses are original values.

Significantly lower total dry weight of weeds at 30 DAS, 60 DAS and harvest was noticed with weed free check (T₂) which was however, comparable with T₄ (pre-emergence application of orthosulfamuron @ 100 g a.i ha^–1) at 30 DAS and T₁₀ (pre-emergence application of pendimethalin @ 0.75 kg a.i ha^-1 fb post-emergence application metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1) at 60 DAS and harvest. Although pre-emergence application of orthosulfamuron @ 100 g a.i ha^-1 at 3-5 DAS (T₄) exerted significant influence in reducing total dry weight of weeds during the early part of lifecycle (30 DAS) of semidry rice, the gains could not be sustained subsequently (at 60 DAS and at harvest) due to gradual increase in the total dry weight of weeds. Superior performance of herbicides in reducing the weed dry weight due to sequential application compared to alone application has also been reported by Narolia et al. (2014).

Pre-emergence application of pendimethalin fb post-emergence application of metsulfuron methyl + chlorimuron ethyl (T₁₀) registered the highest weed control efficiency (89.7%) which was comparable with weed free check (90.6%). The highest weed control efficiency associated with T₁₀ might be due to the fact that the successive application of two herbicides at an interval of 20 days created an adverse environment to weeds, resulting in reduction in total weed dry matter which was comparable with weed free check (T₂).

Among the weed control treatments, except T₃ (pre-emergence application of pendimethalin) and T₅ (post-emergence application of orthosulfamuron), all other herbicide treatments recorded significantly higher values for weed management index which were comparable with each other as well as with weed free check (T₂). Comparable performance of the herbicide applied treatments except T₃ and T₅ with that of the weed free check (T₂) could be attributed to enhanced weed control efficiency of the herbicides.

Effect on yield attributes and yield
Weed management practices were found to influence the yield attributing characters markedly. The highest number of filled grains panicle^-1 and panicle length were noticed with pendimethalin fb metsulfuron methyl + chlorimuron ethyl (T₁₀), pendimethalin fb orthosulfamuron (T₈) and pre-emergence application of orthosulfamuron (T₄) which were comparable with weed free check (T₂). Thousand grain weight did not alter significantly due to weed management practices. Effective suppression of weed growth throughout the critical period of crop-weed competition might have enabled semidry rice to bear promising architecture for yield attributes, both in T₄ and T₁₀ among herbicide treatments as well as in weed free check (T₂).

Maximum grain and straw yield (Table 2) was obtained with the application of pendimethalin fb metsulfuron methyl + chlorimuron ethyl (T₁₀), which was comparable with weed free check (T₂). Harvest index did not vary to a statistically perceptible magnitude. The increase in grain and straw yield might be due to the reason that the sequential application of two herbicides having distinct modes of action created a rather weed free environment by effectively suppressing a broad-spectrum of weed population and consequently weed dry matter. Prevalence of weed free crop growing environment might have enabled congenial conditions for production of higher growth stature and better yield structure which might have eventually resulted in higher yields in T₁₀ (pre-emergence application of pendimethalin @ 0.75 kg a.i ha^-1 fb metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1 as post-emergence), on par with weed free check (T₂). Similar views were also expressed by Narolia et al. (2014).

Table (2):
Yield attributes and yield of semidry rice as influenced by different weed control treatments

Effect on economics
The highest gross returns were recorded with pre-emergence application of pendimethalin @ 0.75 kg a.i ha^-1 fb post-emergence application of metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1 (T₁₀) which was comparable with weed free check (T₂). As regards net returns, it was maximum with the application of pendimethalin @ 0.75 kg a.i ha^-1 as pre-emergence fb metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1 as post-emergence (T₁₀).While, significantly lower values for gross and net returns were observed in weedy check (T₁). The highest returns per rupee invested (2.05) was observed with pendimethalin @ 0.75 kg a.i ha^-1 as pre-emergence application fb metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1 as post-emergence application (T₁₀). The return per rupee invested (0.20) was lowest with weedy check (T₁). The highest gross returns, net returns as well as returns per rupee (B: C ratio) invested with T₁₀ might be due to higher grain yield, barring labour costs and effective working of pendimethalin fb metsulfuron methyl + chlorimuron ethyl compared to other treatments. Though hand weeding resulted in higher grain yield and gross returns than all other treatments, the B: C ratio remained was only 1.28. The lower returns per rupee invested with weedy check might due to higher labour cost involved in hand weeding and the higher cost involved in manual weeding was not compensated by the additional grain yield obtained in weed free check (T₂) resulting in lower benefit cost ratio (BCR). Similar results were reported by Narolia et al. (2014) and Jayadeva et al. (2011).

Table (3):
Gross and net returns (Rs ha-1) and returns per rupee investment of semidry rice as influenced by weed control treatments

Thus, it could be concluded that, sequential application of pendimethalin @ 0.75 kg a.i ha^-1 at 3-5 DAS fb metsulfuron methyl + chlorimuron ethyl @ 4 g a.i ha^-1 at 20-25DAS (T₁₀) was found to be the most effective weed management practice to achieve broad spectrum weed control and to realize higher yield attributes, grain yield as well as economic returns from kharif sown semidry rice in North Coastal Zone of Andhra Pradesh.