The following points highlight the top five factors affecting weed-crop competition. The factors are: 1. Period of Weed-Crop Competition 2. Weed and Crop Densities 3. Weed and Crop Species 4. Cropping Practices 5. Climatic Influences.
Farmers often assume that removing the weeds any time during the growing season shall benefit the crops equally well. Substantial evidence, however, indicates that it is not true, and the time to removal of weeds is as important as their removal itself. A fundamental principle of plant ecology is that early occupants of the ground tend to exclude the later germinating ones. This holds good also in agriculture.
The weeds that germinate either before, or at the same time as the crop, offer serious competition to the crop plants since they get opportunity to establish and accumulate dry matter faster that the crop plants. In maize, for example, it has been found that during the first 2-3 weeks of its emergence, the weeds put forth 15-18% of their growth, while the crop plants attained only 2-3% of it.
Such observations have provided a base in favour of early season weeding to harvest acceptable crop yields. Kasasian and Seeyave (1969) proposed a working hypothesis that most crops required a weedfree respite during the first 1/4-1/3 of their growth period. Longer duration weed control did not give significant additional, economic production.
Conversely, neglecting the early weeding and adopting the late stage weeding resulted in sub- optimal yields. In one experiment, maize plots weeded for the first 30 days after emergence gave 44.4 q ha-1 grain yield, which was only 5% less than the maximum yield of 47.8 q ha-1 obtained from an all season weedfree crop (Table 2.2).
Likewise, little increases in yields of sugarbeat, soybean, fieldbeans, and cotton were obtained when these crops were made weedfree longer than 20, 30, 35, 60 days after crop emergence, respectively. Rogers et al. (1976) found that weeding a cotton crop 30 days after its emergence was late enough to recoup its yield.
Thus, for every crop there is a critical period of weed-crop interference when the removal of weeds should result in the highest economic returns. There are numerous researches conducted in India and abroad on the critical crop growth stages when weeding must be done. By and large, for all practical purposes these results, despite variations, indicate the importance of early weed removal to obtain optimum crop yields.
A very broad-based average period that is crucial for weeding annual crops is the first 20-30 days after sowing or emergence of the crop. However, it may vary somewhat with the crop situation. For example, in Karnataka the critical periods of weeding were worked out as 30-40 d.a.s. in fingermillet; 40 d.a.s. in groundnut, and 20-40 d.a.s. in transplanted rice. In Andhra Pradesh weeding maize plots up to 30 d.a.s. was found absolutely essential.
Even a cursory review of a portion of weed competition literature leads to the conclusion that increasing weed density decreases crop yield. But the weed density and crop yield relationship is not linear. A few weeds per unit area usually do not affect crop yield. Also, the maximum weed effect ie total crop loss, obviously, cannot be exceeded. Commonly the debacle occurs at less than the maximum weed density level. Weed density and crop yield relationship is largely a sigmoidal.
Like the weed density, the crop density also affects the weed-crop competition. Several experiments have been conducted to study the effect of variable seed rates and/or row widths on the yields of crop and the associated weeds.
In general, these experiments revealed that increased crop plant populations obtained by increasing the seed rates tended to reduce the weed growth and increase the crop yield, until the crop plants themselves became competitive with each other. Also, such an affect of crop density was more prominent when the crop seedlings appeared earlier to the weed seedlings in a uniform stand than when the crop seedlings showed later.
Besides the crop density, the plant geometry may also govern the weed growth. Usually, at the same level of crop density the narrow crop rows tend to stress weeds more than the wider crop rows. Using this doctrine, several scientists evaluated the performance of cross-sowing of wheat in terms of weed growth and crop yield. The results have been variable, but considerable number of these experiments favoured cross-sowing in this respect.
Both weed and crop species differ in their competing ability in a community. It is primarily because of differences in their- (i) seedling vigour (ii) canopy development (iii) allelopathic effects, (iv) rooting depth and root volume. Weeds with rapid seedling growth compete with crops better than the slow growing ones.
For example- wild mustard (Brassica spp.) and cocklebur (Xanthium strumarium) reduced the crop yields much more than many grass weeds because these two broadleafs developed extensive root system fast at their seedling stage and reached their canopy formation stage earlier than the grasses. In dryland agriculture, the perennial weeds often reduce crop yield more than the annual weeds by competing for the deep-seated soil moisture more effectively.
Crops, and in many cases their varieties, differ widely in their ability to compete with weeds. For instance, among the winter grains the decreasing order of weed competing ability is: barley, wheat, and oat. Barley develops more extensive root system during its initial three weeks of growth than wheat and oat. Fast canopy forming and tall growing crops suffer less from weed-competition than the slow growing ones.
Cotton, for example- is very slow to close-in while cowpea is fast in this respect. Likewise, groundnut is a slow growing and short stature crop, while clusterbean is fast growing and comparatively a taller crop.
This gives cowpea and clusterbean better competing abilities than cotton and groundnut, respectively. Dwarf and semi-dwarf varieties of crops are usually more susceptible to competition from weeds than their tall varieties. Likewise, longer duration cultivars of rice are often more competitive to weeds than its short duration varieties.
The weed scientists opine that crop varieties may also differ in imarting their allelopathic effects on weed seedlings present in their neighbourhood. Plant breeders had seriously worked on developing crop varieties with strong allelopathic effects on specific weeds, particularly on the parasitic ones. But no commercial success was, achieved. More recently some allelochemicals have been identified in plants and synthesised in laboratories for the control of certain parasitic weeds. The research is still underway.
Cropping practices, such as time and method of sowing of crops, fertilization procedures, irrigation, and the use of soil amendments have pronounce effects, on weed-crop interference. Sowing of wheat after a presowing irrigation, for instance, usually postpones meaningful competition from weeds until the first post-sowing irrigation, which is applied about 25-30 days later.
It is only at this stage that the first flush of weeds appear among the already established wheat seedlings. But if the crop is sown in dry beds and irrigated soon thereafter, the weeds begin to severely infest the crop within 7-15 days of its sowing.
In certain crops it may sometimes be possible to let the first flush of weeds germinate in the finally prepared seedbed and then sow the crop after destroying the weeds with either a light tillage or a non- residual type of herbicide. This technique, called stale seedbed planting, minimizes the initial weed-crop interference in favour of the crop.
The role of fertilization in standing crops in directing the weed- crop competition to their advantage has been investigated extensively. But, unfortunately, in most experiments the fertilizer application stimulated the weeds more than the crop whence weeding became the primary necessity. It was so because fertilization of the weedy crops increased the growth of the crop at lower pace than that of the weeds. However, placement of fertilizers near the crop rows could help in achieving selective stimulation of the crop to some extent.
In paddy fields, puddling and transplanting are very effective cultural practices for discouraging weeds. The direct sown upland rice, which is subjected to alternate drying and wetting, is most susceptible to heavy weed infestations and weed-crop competition.
Abnormal soil reactions favour the growth of specific weeds and reduce the crop growth, tilting thus the weed-crop interference balance in favour of the weeds. The use of proper soil amendments can reverse this situation.
Adverse weather conditions e.g. drought, incessant rainfall and extremes of temperature put the crops under stress without disturbing the weeds. In fact, continuous rain helps the growth of grasses so much that crops are almost wiped out, unless some kind of weed control measures were adopted. Thus, the prevailing weather conditions have to do much with the extent of weed-crop competition, but, unfortunately, the available data to quantify such effects are highly inadequate.