Weeds reduces the yield and quality of a crop. It also raises the cost of production. In case of paddy, the yield drops by 15-20 per cent and sometimes, even 50 per cent. A crop badly infested with weeds, normally fails in totality. Every year there is huge loss of paddy crop while the quality of produce from weed infested fields is also inferior. These also obstruct harvesting.
Weed-crop competition: Weeds compete with desirable plants. Competition denotes a relationship between the same or different species which leads to the flourishing of one at the expense of the other or at the expense of both. While weed competition with rice does not normally lead to death of either species, it almost results in decreased yields. Weeds are known but the extent of problem they pose is not recognized. Farmers acknowledge the problem in their fields but the high labour cost of hand weeding discourages adequate response.
The competition between weeds and rice depends on the following influences:
i. Relative growth stages of rice and weed.
ii. Nature of stand establishment (transplanting versus direct seeding).
iii. Density of planting.
iv. Rice variety.
v. Moisture and nutrient availability.
In irrigated systems, rice seedlings are transplanted into puddled soil. This gives rice a substantial head-start on weeds and initially, competition is minimal. Competition increases as growth progresses, particularly in stands of direct-seeded rice, since weeds germinate at the same time and compete for light and nutrients with the rice seedlings.
Weed competition generally takes three forms:
Competition for light: Weeds that are shorter than rice crop throughout growth period compete a little or not at all with rice for light. However, weeds that are taller can reduce the light available to rice by as much as 50 per cent. Since sunlight provides the main source of energy utilized by plants for manufacturing food, shading by tall weeds can significantly stunt growth and reduce yields.
Competition for water: Where water is plentiful, competition between rice and weeds is minimal but during shortage the situation is quite different. If weeds consume significant portion of water then tillering, flowering, and grain filling are delayed or impeded.
Competition for nutrients: Weeds have a large nutrient requirement. They are big feeders and can, if left uncontrolled, absorb more of the soil nutrients than the crop. Fertility increases, though fertilization is generally accompanied by increase in weeds, which can result in larger yield reductions.
Reduction of grain quality: Weed seeds in grain reduce the price. Weed seeds in grain can also cause uneven moisture in the grain causing loses in quality due to the formation of molds and/or due to cracking losses during milling.
In addition to competing with rice for sunlight, water, and nutrients, weeds pose another problem. Many weed species act as alternate hosts for insect pests and disease-causing organisms, and their presence among the crop or along bunds and peripheries can increase losses due to insect or disease attack.