Weeds are able to survive in the face of many obstacles. These obstacles include natural selection pressures as well as tillage and crop husbandry routinely used to minimize weed competition in crops. Persistence as it applies to weed is a measure of the adaptive potential of a weed that enables it to survive in disturbed habitats. It is important to know why weeds survive on your farm.
Examples of such disturbed environments are crop land, recreational sites, irrigation canals and pastures. Many of the adaptive features or survival mechanisms of annual weeds, such as ability to produce large quantities of seeds, seed dormancy, periodicity of seed germination and short lifespan, have been discussed earlier.
Perennial weeds also have persistence features such as deep rooting, dormancy characteristics of buds on rhizomes and other modified stems, and fragmentation of parts. Crop mimicry by weeds is an example of the extent to which weeds have adapted themselves to survive in such frequently disturbed sites as farm lands.
This is where weeds develop close resemblance to some phases of life history of a crop as to be mistaken for the crop, and thus evade eradication. Modern agriculture has, through land preparation and direct weed control practices, imposed considerable selective pressure on weeds, and also many of them exhibit adaptive strategies which aid their survival.
There are three types of crop mimicry by weeds these are;
- Vegetative mimicry in which close similarity in appearance occurs between weeds and crop sat seedling and vegetative stages.
- Seed mimicry in which the similarity between weeds and crops is seen in seed weight, size and appearance
- Biochemical mimicry in which a weed develops resistance to herbicide previously used for controlling it selectively in a given crop.
Weeds that mimic crops are more difficult to control than that do not. Example of vegetative mimicry are seedlings of perennial wild rice (Oryza longistaminata) in cultivated rice. Wild sorghum (Sorghum halepense) seedlings in cultivated sorghum. Wild cane (Saccharum spontaneum) in sugarcane, and rice mimicry in barnyard grass ( Echinochloa crus-galli). Seed mimicry is illustrated by the similarity in seed size between seeds of upland rice and those of itch grass (Rottboellia cochinchinensis).
Factors affecting persistence
Weed persistence will be affected by climatic, soil (edaphic) and biotic factors. The impact of any of these factors on weed persistence will vary with weed species and prevailing conditions in the habitat. The major climatic factors are light, temperature, water and wind. Light intensity, quality and duration affect growth, reproduction and distribution of weeds.
Photoperiod governs flowering, and consequently affects the latitudinal distribution of weeds and the evolution of ecotypes of a given species. Light exclusion favors colonization of a habitat by shade to learnt weed species.
This is particularly evident in plantation crops. Soil and air temperatures affects latitudinal and elevational distribution of weeds. In addition, soil temperature affects weed seed germination as well as seed dormancy.
Water reaches most habitats naturally in the form of rain, dew and also fog. Irrigation represent purposeful manipulation of a habitat by humans to alter the natural distribution of water. Plants in a given habitat vary in their ability to intercept these forms of natural water supply to meet their growth needs.
The total available water in a particular site is related to the original water supply to the site. And to losses from percolation, runoff, and evapotranspiration. Seasonal distribution of water affects weed species distribution and also a shortage of water at critical times. In the life cycle of weeds may affect seed production.
If the water shortage becomes a recurrent event the survival of some weed species may be threatened. Wind velocity, frequency and direction restrict the occurrence of all plants including weeds. In addition, wind stabilizes oxygen and CO2 distribution, and also affects transpiration losses.