MODULE 6

• Competition


  A negative form of interaction where individuals make simultaneous demands that exceed limited resources and one individual suffers less
• Competition is what weed control is about. Competition between crops and weeds is why weeds are controlled. If weeds were just there and benign, we wouldn't care as much about them. Because they cause harm to crops by competing with them we are compelled to care and attempt to control or manage them.
• Weeds


  • They have the capacity to obtain more water in the soil because they have more extensive root system that can penetrate deeper layer of the soil to extract water and nutrients
• Leaf area index (LAI)


  The total leaf area per unit ground area, given by the formula above
• Importance of LAI


  • for canopy light harvest, 2) phenology, 3) canopy structure, 4) transpiration, 5) scaling processes, and more. For this discussion, we will mainly focus on the canopy light harvest aspect of LAI (transmission and photosynthesis)
• LAI


  The more leaf material in a canopy, the more capacity there is to absorb light energy from the sun. This light energy is then used to drive plant productivity (primary productivity) through the uptake and conversion of carbon dioxide from the atmosphere into carbohydrates. This is related to biomass accumulation and crop growth.
• Light transmission


  The higher the LAI (meaning more leaf density) the lesser the light transmission
• Factors affecting crop-weed competition

  • For weeds: species, density, distribution, and the length of time that the weed is present in an area

  For crops: density, its distribution (including spacing between rows and spacing in the row), and duration (when thinning/weeding will be implemented)

  All these factors (both crops and weeds) are modified by soil (edaphic) and climatic conditions which in turn determine the degree of competition faced by the crop and weeds
• Differences between weed species

  • Germination patterns differ markedly and sometimes erratically, causing differences in their potential for competition from year to year

  2. Emergence and growth also vary from slow and even, to rapid and almost unpredictable

  3. Different species and biotypes appear to respond differentially to various environmental conditions
• Phenotypic plasticity


  The ability of an organism to change its phenotype in response to environmental conditions
• Weeds that emerge with the crop must be removed before the end of Period 1 (maximum weed-infested period) to prevent them from reducing crop yield. The crop must be kept clean throughout Period 2 (the critical period for weed–crop competition). Later emerging weeds (Period 3) have little effect on crop yield.
• Early literatures assumed that crop yield and weed density was linear. Linear relationship means that with no weeds, crop yield will be maximized, and at high density yield will be zero, which is not correct.
• Zimdahl (1980) proposed that the relationship was curvilinear based on their study. Although it is intuitively logical it is also wrong. The curvilinear relationship fails because it predicts that at a high density crop yield will be reduced to zero, and that does not happen.
• Another proposed relationship between crop yield and weed density is sigmoidal (Zimdahl, 1980). It means that at very low weed densities, there is no effect on crop yield, and as weed density increases, while there may be effect it is not very obvious. As weed density continues to increase, crop yield drops quickly but never goes completely to zero. Even at very high densities, weeds do not eliminate all crop plants.
• Although this sigmoidal relationship represents most data on weed-crop competition, but still it is not correct. Bottom line, it is very difficult to measure the effect of weeds especially in a large area.
• Allelopathy


  First coined by Molisch in 1937. Molisch is considered as the father of allelopathy. The term was refined by Rice (1984) and in 1996, the International Allelopathy Society broaden the definition of allelopathy.
• Crop on crop effects. A sufficient gap must be left before the following crops is sown. Generally, large seeded crops are less affected and transplanted crops are unaffected