lecture 06

lecture 06 - Early Weed Compe)on Cri?cal Period for...

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Unformatted text preview: 2/3/11 Early Weed Compe))on Cri?cal Period for Weed Control: 1. the ?me span when weeds present from the beginning of the crop cycle must be removed or 2. the point aIer which occurring weed growth no longer affect crop yields. This cri?cal ?me can also be defined as the early weeks of crop growth when weeds need to be controlled to prevent yield reduc?ons. Weed Compe))on and Allelopathy BTNY 304 Some photographs were taken off various websites and are intended for instruc?onal purposes only In general, corn and soybeans will not have any measured compe))on with weeds for the first 2 to 4 weeks if the crop and weeds emerge together. ACer this )me, maximum compe))on damage is measured in the next 2 to 4 weeks (this becomes a factor in )ming postemergence weed control programs). If a crop is weed ­free for the first 4 to 6 weeks,  ­ weed compe))on is minimized, even if the weeds become established aCer this period and remain for the en)re season. It is beJer to control the first flush of weeds and leave the last than to wait for the maximum number of weeds to emerge and miss the first flush. This does not mean that late ­emerging weeds will not interfere with harvest efficiency and cause yield loss. Weed Compe))on in Soybeans Weed Pigweed Density 1/C 1/8 ­C added harvest loss 2/C 4/C added harvest loss 1/C 4/C Loss 45% 30% 0% 20% 10% 43% 25% 57% 10% Broadleaf vs. Grasses Weed Density Pigweed 1/8 ­C 30% Morningglory 1/C 52% Loss Jimsonweed Cocklebur Velvetleaf 30% 1/C 12/C if emerge 10 days later if emerge 20 days later Foxtail 12/C 18% ____________ 25% 8% 1 2/3/11 Weed Interference Interference describes the total impact of one plant on another plant, or the ability of weeds to adversely affect crop growth. Compe))on – mutual struggle for growth factors Allelopathy – inhibi)on through biological toxins Parasi)sm – special cases Absolute separa)on of allelopathy and compe))on is difficult. A given area (1 ­square foot or 1 acre) will produce a given amount of biomass  ­ it can be crops, weeds or both. But two plants cannot occupy the same space at the same )me. The ability of weeds to compete successfully with crops for water, light, and nutrients depends on  ­ the )ming of weed emergence in rela)onship to the crop emergence,  ­ the growth form of the weed, and  ­ the density of the weeds present in the crops. Timing of Weed Emergence The first plant to effec)vely obtains  ­ light,  ­ water,  ­ nutrients from a site and becomes established at that site has the compe))ve advantage over plants that develop later Weed compe))on is site specific Effects of weed compe))on are greatest when the crop is young Crop Response Threshold – the level of weed infesta)on (in terms of weed biomass, weed number, or period of weed infesta)on) at which a measurable effect of the crop begins Economic Threshold (for the current crop)  ­ the level of weed infesta)on at which the increase in crop produc)on from weed control will pay the cost of the control The economic threshold is always higher than the crop response threshold by the amount of crop yield (value) to the cost of weed control Allelopathy Allelopathy – reduces development of another plant more than is normally expected from compe))on alone  ­ plant’s ability to eliminate compe))on by producing toxins that prevent normal growth of another plant Examples of plants that exhibit allelopathy Walnut trees Quackgrass Small grains – wheat, rye Sunflower Yellow and purple nutsedge Garlic mustard 2 2/3/11 Parasi)sm Parasi)sm – plants form protoplasmic connec)ons with their host plant Examples of parasi)c plants Witchweed Dodder  ­ nonphotosynthe)c Broomrape Mistletoe 3 2/3/11 4 ...
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