Weed thresholds

The exact nature of weed interference between crops and weeds is still inadequately understood. It has been assumed that direct crop yield reductions from weed presence were the result of competition, of allelopathy, or of these two acting together. Further, many attempts have been made to explain the extent of crop yield reduction in terms of weed thresholds (numbers). Information is presented that suggests that neither numbers of weeds nor their influence through competition and allelopathy adequately explain the effects of weeds on crop yield. A third influence of weeds, what may be termed "direct feedback response to light," is introduced as a possible factor in yield reduction. 

Forcella, F. (1987). Herbicide-resistant crops Yield penalties and weed thresholds for oilseed rape (Brassica napus L.). Weed Res., 27 31-34. 

Duke (1995) contended that resistant weeds have driven growers to develop and use integrated pest management (IPM) or integrated weed management (IWM) methods, such as biocontrol, cover crops, more crop and herbicide rotations, more selective use of herbicides, weed thresholds, etc. 

The assessment of the sustainability of the cultivation of energy crops includes the input and recycling of nutrients, the application of pesticides, the water-use efficiency, the consumption of fossil fuels and the balance of soil carbon. The aim is to recycle the nutrients, which is simple in the case of anaerobic digestion by applying the digestate to the field. If crops are combusted many of the minerals can be returned via the ash. In the case of liquid biofuels, exported nutrients are lost and have to be replaced. The application of pesticides, mainly herbicides, can often be reduced in comparison to food production, but the energy yield per hectare might be reduced if the share of weeds exceeds certain thresholds. Water use efficiency,... 

Sikkema, P., Van Eerd, L.L., Vyn, R.J. and Weaver, S. (2007) A comparison of reduced rate and economic threshold approaches to weed management in a com—soybean rotation. Weed Technol, 21, 647-655. 

Norris RF. 1999. Ecological implications of using thresholds for weed management. Journal of Crop Management 2(1) 31-58. 

Jones, R.E. and R.W. Medd (2000). Economic thresholds and long-term approaches to weed population management. Weed Technol., 14 337-350. 

Norris, R.F. (1999). Ecological implications of using thresholds for weed management. In D.D. Buhler, ed., Expanding the Context of Weed Management. Binghamton, NY The Haworth Press, Inc., pp. 31-58. 

Munier-Jolain, N.M., B. Chauvel, and J. Gasquez (2002). Long-term modeling of weed control strategies Analysis of threshold-based options for weed species with contrasted competitive abilities. Weed Res., 42 107-122. 

Black, I. D. and Dyson, C. D. (1993). An economic threshold model for spraying herbicides in cereals. Weed Research, 33,279-90. 

Crop Protection. Cotton can be affected by insects (30), weeds, diseases (31), nematodes, and mycotoxins. About 90% of the U.S. cotton uses Integrated Pest Management (IPM) practices. This approach optimizes the total pest management system by utilizing all available tools, including rotation, crop residue destruction, maximum crop competitiveness, earliness, pest scouting, action thresholds, releases of beneficial insects, sterile insect releases, and selective crop protection chemistry. 

Not all brassicas affected. Several weeds are hosts to this aphid. Populations vary widely each year due to weather conditions. Threshold for treating winter oilseed rape is 13% plants infected before petal fall. 

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