Chemical weed control in rainfed upland rice in Nigeria

Abstract

Two field experiments on chemical weed control in Faro 41 upland rice (Oryza sativa L.) variety were conducted at a rainforest site near the National Cereals Research Institute, Amakama Substation in 1989 and 1990. The herbicides tested were a coformulated mixture of pretilachlor and dimethametryne at 1.5, 2.0 and 2.5 kg a.i./ha, a co‐formulated mixture of piperophos and propanil applied at 3.16 and 3.95 kg a.i./ha, and Oxadiazon at 1.0 kg a.i./ha. Two hand weedings and a ‘no weeding’ treatment were included. The herbicides were applied 5 days after planting. All were safely selective to the crop. Most provided adequate weed control through 8–12 weeks after planting. Weeds controlled included Cynodon dactylon, Commelina benghalensis, Dlgitaria horizontalis, Eleusine indica, Panicum maximum and Pennisetum purpureum as grass weeds. The broadleaf weeds were Emilia sonchifolia, Ageratum conyzoides, Portulaca oleraceae, Richardia brasiliensis and Ipomoea Involuncrata. The sedges Mariscus alternifolius and Cyperus esculentus were encountered in the plots. Pretilachlordimethametryne at 2.5 kg a.i./ha had the best weeding score. Twoyear average grain yields of 1.7–2.6 t/ha were obtained from the herbicide treatments. With only 0.4 t/ha from the unweeded treatment, yield losses of more than 80% were recorded.     

Yield losses in weed-free wheat and barley due to post-emergence herbicides

The tolerance of weed-free wheat and barley to a range of herbicides (bromoxynil, methabenzthiazuron, MCPA, bromoxynil + MCPA. linuron, prometryne, diuron, 2,4-D ester and 2,4-D amine) when applied at the crop three-leaf stage, was examined over the 5-year period 1968–72. All chemicals lowered grain yields to some extent, but the variation between chemicals was greater for wheat than for barley. In both crops, methabenzthiazuron was the safest chemical causing only small (<2·0%,) yield losses at rates up to 2·25 kg a.i./ha. At this rate of application yield losses of up to 36% were recorded for the substituted urea derivatives. Both crops had a low tolerance to 2,4-D ester and amine. Yield losses from bromoxynil, bromoxynil + MCPA and MCPA were similar, varying from 4 to 8% al 0·5 kg a.i./ha to 9–24% at 2·25 kg a.i./ha. Pertes de rendement dans le blé et l'orge provoquées, en l'absence de mauvaises herbes, par des herbicides de postlevée Pendant une période de 5 années (1968–1972) les auteurs ont examiné la résistance du béet de l'orge. Traités, en l'absence de mauvaises herbes, au stade 3 feuilles, avec une série d'herbicidcs (bromoxynil, méthabenzthiazuron, MCPA, bromoxynil-i-MCPA, linuron, prométrync. ditiron, 2,4-D ester et 2,4-D a mine). Tous ces produits ont diminué les rendements en grains dans des proportions variées, mais la variation entre produits a été plus grande pour le blé que pour l'orge. Pour les deux céréales, le méthabenzlhiazuron a été le produit le moins toxique, provotjuant seulement de faibles penes de rendement inférieurcs à 2,0%, à des doses allant jusquà 2,25 kg/m.a/ha. A cette dose d'application, des pertes de rendement allant jusquà 36% ont élé enregistrées avec des urées substituées. Les deux éréles ont manifesté uoe faible resistance au 2,4-D estr et amine. Les pertes de rendement provoquées par le bromoxynil, le bromoxynil + MCPA, et le MCPA ont été analogues, variant de 4 à 8% pour 0,5 kg/m.a./ha, jusqu’à 9 à 24% pour 2,5 kg/m.a./ha. Ertragsverluste durch Nachaufiatifherbizide in unkrautfreiem Weizen und Gerste In unkrautfreien Weizen- und Gerstebestánden wurde in fünfjáhrigen Versuthcn (1968–72) die Toleranz der beiden Arten gegenüber folgenden Herbiziden untersucht: Bromoxynil, Methabenzthiazuron, MCPA, Bromoxynil + MCPA. Linuron, Prometryn, Diuron, 2,4-D-Eister und 2,4-D-Amin- salz. Die Behandlung erfolgte jeweils im 3-Blattstadium. Alle Verbindungen erniedrigten zu einem gewissen Grad den kornertrag. Beim Weizen waren aber die Unterschiede zwischen den verschiedenen Herbiziden grösser als bei der Gerste. Methabenzthiazuron war bei Aufwandmengen bis zu 2.25 kg A.S./ha in beiden Getreidearten das sicherste Herbizid und verursachte nur geringe Ertragsdepressionen (<2%). Bei dieser Aufwandmenge wurden bei den Harn-stoffherbiziden bis zu 36% Ertragsverlust festgestellt. Gegen 2,4-D-Ester und 2,4-D-Aminsalz waren Weizen und Gerste wenig tolerant. Die Ertragsverluste durch Bromoxynil, Bromoxynil + MCPA und MCPA waren etwa gleich und betrugen 4–8% bei 0.5 kg A.S./ha bis 9–24% bei 2,25 kg A.S./ha.     

The Coconut Rhinoceros Beetle Oryctes rhinoceros (L)[Coleoptera: Scarabaeidae: Dynastinae]

Abstract

Field experiments were conducted during two dry seasons in northern Nigeria to identify suitable pre‐transplant herbicides for weed control in irrigated tomatoes. At Samaru in the northern Guinea savanna, diphenamid at 1.5 kg a.i./ha followed by supplementary hoe‐weeding, metribuzin at 0.5 kg a.i./ha, diphenamid at 3.0 kg a.i./ha, metolachlor plus metobromuron at 1.0 + 1.0 kg a.i./ha and two or three hoe‐weedings, resulted in higher tomato yields than the untreated controls, and most of these treatments reduced weed infestations significantly in both years. At Bakura, in the Sudan savanna zone, metribuzin at 0.25 and 0.5 kg a.i./ha, diphenamid at 3.0 g a.i./ha and chloramben at 1.5 kg a.i./ha plus supplementary weeding consistently resulted in yields that were similar to two hoe‐weedings and higher than the untreated controls. Three kg a.i./ha of chloramben depressed yields in all the trials. At both sites hoe‐weeding once six weeks after transplanting gave unacceptably low crop yields, but similar high yields were obtained with two or three hoe‐weedings. Supplementary hoe‐weedings were unnecessary with the application of metribuzin at 0.5 kg and metolachlor plus metobromuron at 1.0 + 1.0 kg a.i./ha. Uncontrolled weed growth resulted in a 53–67% reduction in tomato fruit yield.     

A World-Wide Organization to Promote the Development of Biological Control

Abstract

Three experiments were conducted to evaluate the efficacy of metribuzin in tea fields. These were a field trial in plots infested with a wide range of weeds, a pot experiment using Paspalum conjugatum and a microplot experiment with 100 Ageratum conyzoides plants/plot. Metribuzin at 0.70, 1.05, 1.40 and 2.10 kg a.i./ha was applied in 450 l water/ha. All rates were satisfactory after 1 week but residual activity increased with application rate; metribuzin at 2.10 kg a.i./ha was significantly superior after 6 weeks. Control after 8 weeks was reduced due to the presence of resistant species. Against Paspalum conjugatum metribuzin at 1.40 kg/ha gave good control for 8 weeks but 2.10 kg/ha was necessary for 12 weeks control. All rates compared favourably with dalapon (4.48 kg a.i./ha). Against Ageratum conyzoides metribuzin at all rates gave good control for up to 12 weeks and significantly better than 2, 4-D at 1.12 kg a.i./ha. Most other grass and broadleaved weeds were satisfactorily controlled at 1.40 kg a.i./ha. Metribuzin with its marked post-emergent effect and prolonged soil residual activity from 6-12 weeks after application, depending on weed species, may become an important tool in pre-monsoon weed control programmes in tea fields. It did not induce any phytotoxic symptoms on mature tea.     

Recent Developments in the Use of Herbicides in Rubber and Oil Palm

Abstract

Recent developments in the use of herbicides during the establishment of leguminous cover crops and the mature phases of rubber and oil palm plantings are discussed. Results of 12 semi-commercial and commercial scale trials showed that chemical weeding using paraquat and diuron post-emergence or oxyfluorfen pre-emergence with supplementary manual or chemical control was considerably cheaper than exclusive manual weeding during the six months following the sowing of legumes. Reductions in weeding costs ranged from 6 to 66%. In the mature phase a wide range of herbicide treatments involving extensively used herbicides such as paraquat, MSMA, 2, 4-D amine and sodium chlorate for the suppression of weed growth along rubber tree rows and in oil palm circles and harvesters' paths were compared. Results indicate that there are several suitable alternatives to sodium arsenite, a formerly widely used herbicide which is now banned for general weed control. In both crops, the most cost effective treatments for controlling mixed vegetation were Ustinex Special (MCPA + diuron + amitrole, 2:3:6) at 1.48 kg a.i.ha + Gramoxone (paraquat) at 0.22 kg a.i./ha and Paracol (paraquat + diuron, 1:1) at 0.56 kg a.i./ha + DMA Amine (2, 4-D) at 0.50 kg a.e./ha.     

Plant Diseases

Abstract

Field experiments conducted at Punjab Agricultural University, Ludhiana during 1972–73 and 1973–74 showed that post-emergence application of oxadiazon at 1 kg a.i./ha in transplanted onion gave remarkable weed control and maximum crop yield. Other treatments which performed well include pre-planting alachlor at 2.5 kg a.i./ha plus post-emergence propanil at 1.8 kg a.i./ha and also pre-planting nitrofen at 1.25 kg a.i./ha plus post-emergence nitrofen at 1.25 kg a.i./ha. Herbicides did not significantly influence the bulb quality, soil microflora or the emergence and yield of following crops of okra, bitter gourd and radish.     

Chemical Control of Phalaris minor and Avena ludoviciana in Wheat

Abstract

In field experiments conducted at the Punjab Agricultural University, Ludhiana during 1973–74 and 1974–75, pre-emergence application (one day after sowing) of methabenzthiazuron (1.05 kg a.i./ha), metoxuron (1.6 kg a.i./ha) and chlortoluron (0.75 kg a.i./ha) gave effective control of Phalaris minor and Avena ludoviciana in wheat without any phytotoxic effect on the crop. Methabenzthiazuron, metoxuron and chlortoluron gave 1410, 1420 and 1090 kg/ha, respectively, more grain than the unweeded crop and 590, 600 and 270 kg/ha more than the crop hand-weeded twice. Nitrofen (1.25 kg a.i./ha) applied pre-emergence was effective against P. minor only and gave substantial increase in grain yield. Terbutryne, dichlormate and linuron, while effective against P. minor, also had a phytotoxic effect on the crop.     

Weed control in direct-seeded lowland rice under poor water control conditions

Several herbicides applied singly and in combination were evaluated for weed control and for their effects on crop yield in direct-seeded lowland rice under poor water management conditions characteristic of the conditions in which lowland rice is grown by most farmers in West Africa. Several herbicides including cyperquat at 3.0 kg/ha, a mixture of cyperquat and 2,4-D at 2.0 + 0.5 kg/ha, bentazon at 2.0 kg/ha and avirosan satisfactorily controlled sedges in the 3-year study. Crop yield was consistently high in plots where weeds were controlled with post-emergence application of MCPA + propanil at 1.0+1.7 kg/ha, propanil + thiobencarb 2.2+1.2 kg/ha, oxadiazon 1.5 kg/ha, bentazon 2.0, granular 2,4-D at 1.0 kg/ha, and a pre-emergence application of bifenox at 2.0 kg/ha. Crop yields in plots treated with these herbicides were generally better than with two hand-weedings.     

Insect Pests of Cotton – Western Herbaceum Region of India

Abstract

Pre-plant and pre-emergence herbicides were tested for effective weed control in a seed crop of okra during 1974–75 and 1975–76. All treatments significantly decreased the weed population and increased the seed yield as compared to the unweeded control. Fluchloralin (0.90 and 1.20 kg a.i./ha pre-plant), alachlor (2.50 kg a.i./ha pre-em) and EPTC plus alachlor (3.75 kg a.i./ha pre-plant and 1.25 kg a.i./ha pre-em) proved to be the most effective and economical treatments. No residual effects were observed with any herbicide on the succeeding crops of radish, carrot, turnips, peas and spinach.     

Biological Control of Lantana

Abstract

Experiments conducted at the International Rice Research Institute (IRRI) in the Philippines, showed that all commonly marketed formulations and derivatives of 2,4-D and MCPA were equally effective in controlling barnyard grass [Echinochloa crusgalli (L.) Beauv.] and other annual weeds in transplanted rice (Oryza sativa L). In the Philippines, these phenoxy acid herbicides cost US$2.50/ha as liquid formulations and US$5.50/ha as granular formulations. These low-cost herbicides provide excellent alternatives to hand weeding in transplanted tropical rice. Other herbicides which can control weeds before or after the weeds emerge are more expensive than 2,4-D or MCPA. Some of these selective herbicides, such as butachlor and TCE-styrene + 2,4-D, are in the Philippine market. Their prices vary from US$8 to US$12/ha.

For direct-seeded flooded rice, granular formulations of several new herbicides, such as butachlor, benthio-carb and C-288 (chemistry not yet disclosed), were highly selective in controlling barnyardgrass and other annual weeds under tropical conditions. C-288 controls algae in addition to annual weeds in direct-seeded flooded rice. Of these three promising herbicides, only butachlor is marketed in the Philippines at US$8/ha. For upland rice, the chemicals which look promising are butachlor, benthiocarb and fluorodifen. Butachlor liquid is available to rice farmers in Asia at about US$16/ha. These herbicides provide an excellent alternative to hand weeding. Generally a complete job of weed control in upland rice requires more than one hand weeding.     

Herbicide Performance in Rainfed Cabbage

Abstract

The effectiveness of butralin (2.0 and 4.0 kg a.i./ha pre-plant, 2.0 and 4.0 kg a.i./ha pre-em), chlorthal-dimethyl (10.0 and 12.0 kg a.i./ha pre-em), trifluralin (0.6 and 1.1 kg a.i./ha pre-plant) chloramben (2.0 kg a.i./ha pre-em) and nitrofen (2.0 kg a.i./ha pre-em) in controlling weeds in transplanted ‘Kono Cross’ cabbage and the influence of these treatments on the yield and quality of marketable heads and on the economics of weed control were evaluated in three replicated field experiments on River Estate loam soil in Trinidad during the wet seasons of 1976 and 1977. While herbicides did not affect the size of the cabbage plant population or the date to maturity of the crops, they reduced the cost of weed control by over 30% and increased the net returns relative to the clean-weeded control by over 9%. Butralin at 4.0 kg a.i./ha pre-plant and pre-em and the trifluralin treatments were superior to nitrofen which gave good weed control up to five weeks and higher yields than the clean-weeded control. Chlorthal-dimethyl gave encouraging results but caused slight, temporary stunting. The application of one of these treatments followed by one hand weeding at about four weeks should give full-season weed control and an acceptable yield of marketable heads with increased financial returns. Chloramben was the least effective herbicide tested and its use may not be justified under similar conditions to those of this study.     

Weeding and nitrogen effects on farmers' wheat crops in semi-arid Morocco

The effects of three weeding systems with and without top-dressed nitrogen were examined in 40 experiments on bread wheat and durum wheat planted by farmers in the Chaouia (semi-arid area of Morocco) in 1984–1985 and 1985–1986 growing seasons. In both years, hand-weeding to simulate collection of forage reduced weed numbers by 30% and removed 63% of the weed dry weight. Treatment with 2, 4-D removed 66% of the weeds and reduced weed dry weight by 82%, in 1985–1986. Hand-weeding for forage produced 427 kg ha^?1 weed dry matter in both years, but wheat grain yields were 179 kg ha^?1 lower than with 2.4-D treatments. Over both years, the cost of each kg weed forage was 0·43 kg grain yield loss. Without weeding grain yields were 130 kg ha^?1 lower than 2, 4-D treatments. Weed control with 2,4-D increased wheat grain yields over both non-weeded and hand-weeded treatments by an average of 154 kg ha^?1. Overall, there were no significant effects of nitrogen on wheat or weed yields nor interactions between top-dressed nitrogen and weeding systems. A total of 157 weed species belonging to 29 botanical families were identified on the 40 experimental sites over both years; 89% were dicotyledons. Papaver rhoeas L. (common poppy), 2, 4-D susceptible plant, was the most annual weed in wheat fields in Chaouia.     

Herbicide Research on Groundnut and Sorghum under Farmer Conditions in the Indian Semi-arid Tropics

Abstract

Plots 17 × 5 m were laid out in farmers' fields in three villages. Treatments (replicated thrice) included atrazine (0.5 or 1.0 kg a.i./ha) on sorghum and nitrofen (0.75 and 1.5 kg a.i./ha) on groundnuts with or without hand-weeding or farmers normal weed control. Groundnuts were unfertilized, sorghum received 20 or 80 kg N/ha. Cooperating farmers were allocated plots for traditional weed control, other operations were carried out by research staff. Some plots were discounted because farmers withdrew cooperation or did not participate correctly or because of flooding. In groundnuts nitrofen alone was inferior to nitrofen + one hand-weeding. In sorghum there were no differences between high and low fertilizer regimes. Atrazine gave superior yields to farmers' weed control. All treatments were better than the unweeded control. Generally farmers fields had low weed populations due to good, timely control and low soil fertility. Herbicides are not economic at present. Problems associated with research in farmers fields included: selection of farmers who would participate correctly, variability between different farmers' weeding practices and between different fields due to previous crops and cultural practices; lack of control of farmers weeding operations and the distance of experimental plots from the research station. The authors suggest that small experiments with only 4-6 treatments are easier for participating farmers to comprehend, selecting suitable size and shape of plots is important and that if many farmers are involved the number of replicates can be reduced.     

Insecticide resistance management of permethrin and acephate against the cowpea curculio,Chalcodermus aeneus Boheman (Coleoptera: Curculionidae), a pest of the southern pea,Vigna unguiculata (L.) Walp.

To evaluate strategies for management of potential resistance of the cowpea curculio, Chalcodermus aeneus Boheman, to sprayer‐applied permethrin and acephate, five management regimes were evaluated on early, midseason and late sequential plantings of southern peas, Vigna unguiculata (L.) Walp. during 1991–1993. Management regimes were: (1) consecutive permethrin (0.11 kg a.i./ha early‐midseason‐late); (2) consecutive acephate (1.1 kg a.i.ha early‐midseason‐late); (3) consecutive mixture (0.055kg a.i./ha permethrin and 0.55kg a.i./ha acephate early‐midseason‐late); (4) alternation (0.11 kg a.i./ha permethrin early and 1.1 kg a.i./ha acephate midseason and late plantings); and (5) an untreated check early, midseason and late. The effect of management regime on the toxicities (LC₅₀) of permethrin and acephate was determined by topical application to next generation adult cowpea curculios reared from infested southern pea pods harvested from untreated plants in the management fields. In the field, infestation varied more by planting date than by management regime. Toxicity of permethrin to adults selected in the consecutive mixture regime was significantly lower in 1991 and 1992 late season and in 1993 midseason than in early season for the respective years. There was a seasonal decrease in the laboratory toxicity of acephate to adults with all management regimes.     

Possible Methods for Controlling Ischaemum afrum (J. F. Gamel) Dandy,in Fallow

Abstract

Different mechanical cultivations and herbicide treatments were compared for controlling Ischaemum afrum in a heavily infested fallow area. Of the mechanical cultivations, disc ploughing to a depth of more than 15 cm, following removal of tufts and watering, was found very effective (83% reduction). Incorporated pre-emergence triluralin (1.90 kg a.i./ha) alone or in tank mixtures with oxadiazon (0.95 + 0.95 kg a.i./ha) was also found promising (79 and 76% reduction respectively). Three applications of dalapon (8.09 kg a.i./ha), as post-emergence, at two weekly intervals or two applications followed by another application at 4.05 kg a.i./ha gave almost complete eradication of Ischaemum afrum seedlings for about 12 weeks (99% reduction).     

Effects of herbicides or mowing on musk thistle seed production*

Mowing musk thistle (Carduus nutans L.) within 2 days of the first terminal heads in a plant population showing anthesis eliminated the production of germinable seed from all mowed stalks. Mowing later than 4 days after anthesis led to significant amounts of germinable seed being produced. Regrowth, which produced viable seed, usually occurred from plants mowed before first terminal heads began anthesis. A single mowing usually did not give satisfactory control due to wide differences in the maturity of plants in a natural population. When the terminal heads had reached late bud or more mature stages, 2,4-D ester [isoocyt] ester of (2,4-dichlorophenoxy) acetic acid] at M kg/ha or 2–2 kg/ha did not provide satisfactory control. In general, the most effective herbicide treatments tested at all three stages of maturity were dicamba (3,6-dichloro-o-anisic acid) at 056 kg/ha plus 2,4-D amine (dimethyl amine salt) at M kg/ha, 2,4-D ester at 4–4 kg/ha, or dicamba at 0–56 kg/ha. Inconsistent control of musk thistle results from variability within naturally occurring populations. Uneven development of stands as well as variation among plants are suggested as possible causes.     

An integrated approach to weed management practices in direct-seeded rice under zero-tilled rice–wheat cropping system

An increasing water crisis as well as shortage of farm labor farmers in many Asian regions is forcing a shift from puddled transplanted rice to direct-seeded rice. The weeds, however, are a major constraint to the production of direct-seeded rice. In this perspective, a field study was carried out to evaluate various pre- and post-emergence herbicides and different possible integrated weed management practices in zero-till direct-seeded rice. Weed infestation decreased the rice yield by near about 75%. Co-culture rice with Sesbania followed by (fb) pendimethalin fb 2,4-D effectively reduced the total weed population (65.1%) and biomass (86.7%) at 30 days after sowing (DAS). The lowest total weed dry biomass at 60 DAS was recorded from bispyribac-sodium+azimsulfuron-treated plot and such tank mix application of herbicide performed better against diverse weed flora as compared to a single herbicide. Higher yield and more profit from zero-till direct-seeded rice were obtained with the application of bispyribac-sodium+azimsulfuron herbicides as a tank mixture or an integrated approach through cowpea green-manuring fb 2,4-D+glyphosate fb bispyribac-sodium by effective management of versatile weed flora.     

Soil persistence experiments with [14C]2,4-D in herbicidal mixtures, and field persistence studies with tri-allate and trifluralin both singly and combined

The persistence of [¹⁴C]2,4-D at a rate equivalent to 1 kg/ha was studied in the laboratory on a heavy clay and a sandy loam at 85%of field capacity and 20°C both alone and in the presence of 1 kg/ha dicamba, dichlorprop, difenzoquat, TCA, and 2,4,5-T. The persistence of 2,4,5-T was also monitored in both soils under the same conditions in the presence and absence of [¹⁴C]2,4-D. All soils were extracted at weekly intervals using aqueous acidic acetonitrile and analysed for [¹⁴C]2,4-D remainining radiochemical techniques. The extracts containing 2,4.5-T were additionally analysed gas chromatographically for that herbicide. In each soil type the half-life of the 2,4-D was similar regardless of whether applied singly or in combination with the five herbicides tested. Similarly, [¹⁴C]2,4-D did not affect the breakdown of 2,4,5-T in either soil type. The persistence of tri-allate (1·5 kg/ha) and trifluralin (0·75 kg/ha) both singly and in combination were compared using small field plots at two locations in Saskatchewan. Applications were made during May of 1977 and 1978 and the plots were sampled and analysed for herbicide(s) remaining after 10 and 20 weeks, respectively. The results indicate that within experimental error the loss of both tri-allate and trifluralin from the plots treated with the mixture was the same as from plots treated with the individual compounds.     

Impact of 2,4-D on Ceuthorhynchidius horridus (Coleoptera: Curculionidae) and their compatibility for integrated control of Carduus thistles

Acute and chronic effects of spring application of 2,4-D on adult C. horridus (Panzer) were examined by determining dose-mortality response and adult vitality. LC₅₀ values for males (70.2 kg/ha) and females (61.4 kg/ha) corresponded to 41.7 and 36.6 times, respectively, the recommended application rate of 1.68 kg/ha. Survival of weevils treated with 1.68 kg/ha was not different from untreated controls, but higher doses (16.8–147.8 kg/ha) caused significantly greater mortality. Adult vitality, measured by number of feeding marks per weevil and rate of weight change. was unaffected by the herbicide. When thistle infested fields were sprayed with 1.68 or 2.24 kg/ha of 2,4-D the host plants died, but the survival, reproduction, and population increase of C. horridus were not affected.     

Response of transplanted rice to nitrogen fertilization under different weed management practices

Abstract

Rice yield increased significantly and consistently up to 120 kg N/ha when weeds were not controlled. But when weeds were effectively controlled by two hand weedings (20 and 40 days after transplanting) or by use of thiobencarb at 1–0 kg a.i./ha rice yield increased significantly up to 80 kg N/ha. The optimum dose of N for transplanted rice under mid‐hill conditions of Himachal Pradesh (Palampur) was 82 kg/ha when two hand weedings were done and 86 kg/ha when weeds were managed using thiobencarb. The yield response per kg of N in the respective treatments was 20 and 24 kg of grain. The weed dry weight was significantly lower in thiobencarb and hand weeded plots compared with unweeded check though the difference in weed dry weight due to the former two treatments was not significant.