Weed Management in Horticultural Crops

FACTORS AFFECTING CROP WEED COMPETITION/INTERFERENCE

1. Period of weed growth:

It is important to note that when the crop-weed competition is critical; when the weed must be removed and when they may be allowed to grow without fear of any appreciable crop damage. The weeds that germinate and grow at the same time as the crop seeds are most successful competitors. If weeds are controlled at early period of crop growth, the optimum crop yield could be harvested which may be quite close to the maximum. The critical weed free period may, however, vary somewhat with the crop situation. Weeds should be removed at an early stage or at younger stages otherwise they compete severely. A little delay in weeding results in infestation of the crop for major parts of their critical growth periods. In situations, where weeds germinate late (as in dry land wheat/sorghum), late stage weeding is more useful than their early weeding. The same is also true for crops like grams and peas that are irrigated for considerable period after planting. There is less competition when weeds are either removed early or they germinate slowly because crop plant become sturdy and can tolerate competition. On other hand, when the weeds germinate along with crops, the crop plants are usually smothered by the weed completely e.g. direct sown rice suffers severe competition from the weeds than the transplanted crop of rice.

2. Intensity of weeds:

In general, increase in the weed intensity results in reduction of the growth and yield of crops. There has to be some minimum amount of weed growth in terms of either population or yield in the field to exert a minimum biological stress on the crop performance. To maintain zero weed growth is too costly and cannot be achieved.

3. Weed species:

Different weeds differ in their ability to compete with the crop at similar density levels. This is primarily because of difference in their growth habits as well as due to the allelopathic affects which they may exert on the germination and growth for the crop e.g. at early stage of growth, Cocklebur (Xanthium strumarium) is a better competitor of crops than many grass weeds because it posses broad leaves that shade the ground heavily. In dry areas Canada thistle (Cirsium arvense) and Field bind weed (Convolvulus arvensis) are more competitive than annual weeds because of their deep roots and early heavy shoot growth.

4. Soil fertility:

Since weeds compete with crops for mineral nutrients, particularly nitrogen. Addition of fertilizers, manures and other amendments will lessen weed-stress on crop. Methods and time of application of fertilizer to the crop fields are important in determining whether the added fertilizers will suppress or increase weed growth in fields. Band application of fertilizer will also decide the weed growth. Application of fertilizer early in the crop season when weed growth is negligible is more beneficial to crop than when weeds are already grown up. Weeds can grossly alter the fertilizer response curves of the crops.

5. Soil moisture status:

Water supplied to weedy crop fields either through irrigation or through rainfall is equally accessible to both the crop and weeds. Irrigation is expected to increase the yields of both the crops and weed, if the fields are initially under stress. Increase in yield of the crop (weedy and weed free) by rains or irrigation have been found to differ with the crop and weed species, weed growth etc. Higher water use efficiency of the crops has been noted in weed free plots than in the weedy plots because of higher level of crop yield in the weed free plots. Time of rain or first irrigation after planting of the crops is an important factor determining the intensity of the crop weed competition e.g. If a crop is irrigated when the plants are 15 cm tall it can compete with weed that germinated with 1st irrigation or rain, whereas when fields are irrigated immediately after planting of crops it induces early flush of weed which is highly detrimental to the crop growth. In water logged soils, many weeds survive much better than the most crop plants and compete severely with them.

6. Soil Reaction:

Some of the weed species tolerate abnormal soil reactions much better than others. Some weeds grow vigorously on abnormal pH soil, where majority of our crop plant will be stunted in growth. Weed crop competition is more intense on high or low pH soils than on normal pH soils. Addition of soil amendments to raise or lower the pH soils, therefore vigorate crop growth, tilting the balance of weed crop competition in favour of the crops.

7. Climate influence:

Climate and seasonal variations are greatly responsible for inducing wide differences in weed-crop competitions. Adverse weather conditions e.g. drought, excessive rain, continuous rain, extreme of temperature intensify the weed crop interference, since most of our crop varieties are highly susceptible to such climate influence whereas the weeds are tolerant to these stresses. Weeds germinate at different times at different places. Weed emergence in the fields is more rapid in soils kept continuously wet by rainfall than those that dried three days after planting. The differences in germination of weed are narrowed as the temperature increases because the weed seeds absorb enough moisture in the first three days wet period itself before the soil dries up.

1. Cropping practices:
It includes:

I. Date and method of planting of crop
II. Crop density and rectangularity
III. Crop species and varieties.

i) Date and method of planting of crop:

Longer the period or interval between the emergence of crops and weeds lesser will be the weed crop interference. Weed seeds germinate most readily from top 1.25 cm of soil or up to 2.5 cm depth. Only few weeds such as Barnyard grass and cocklebur may germinate even from 10 cm depth of soil. Therefore, a planting method that dries the top 3-5 cm of soil rapidly is enough to deny weed seeds the opportunity to absorb moisture for their germination which usually post pones weed emergence until 1st irrigation is given. By this time, the crop plants are well established to compete with the late maturing weeds. In other cases, when the planting method which leaves the top most soil wet, weed seeds absorb enough moisture to germinate which induces severe weed crop competition.

ii). Crop density and rectangularity:

In low plant population resulting from low seed rate, faulty germination or loss of crop seedlings, weed growth is dense and weed crop competition intense. Wide row spacing may also induce dense weed growth. It can weaken the crop plants and intensify weed crop interference further. Square planting should be ideal in reducing the intra-plant competition in crops. Competition among individuals of same species of plant is comparatively weaker than competition among individual of different species.

iii). Crop species and varieties:

Crops and varieties differ in their habitat often to weeds to grow as well as in their ability to with stand weed competition. Such differences among crop spp. may result from either their inherent morphological and physiological features or from differences in their cultivation requirements e.g. Barley is found more tolerant to interference by weeds than wheat and wheat more tolerant than oats. Linseed is more sensitive to the presence of weed than to other winter grains. High tolerance of barley to crop weed competition may be due to development of extensive seminal and crown roots during its first three weeks after planting. Fast canopy forming and tall crops suffer less from weed competition than slow growing or short stature crop. Dwarf varieties of crops are more susceptible to weed crop competition than the tall because initially they grow slowly.