Yield-losses, crop

Chemical, cultural, and mechanical weed control practices have been relatively successful ia reducing yield losses from weeds (448). However, herbicide-resistant weed populations, soil erosion, pesticide persistence ia the environment, and other problems associated with technologies used (ca 1993) to control weeds have raised concerns for the long-term efficacy and sustainability of herbicide-dependent crop production practices (449). These concerns, coupled with ever-increasing demands for food and fiber, contribute to the need for innovative weed management strategies (450). 

Moore, P.H. Osgood, R.V. (1985). Assessment of sugarcane crop damage and yield loss by high winds of hurricanes. Agricultural and Forest Meteorology, 35, 267-79. 

Halberg, N. Kristensen, I.S. (1997) Expected Crop Yield Loss when converting to Organic Dairy Farming in Denmark. Biological Agriculture and Horticulture, 14, 25-41. 

Fig. 5.4 Long-term impact of reduced nitrogen fertilisation on the yield of whole crop cereals and SRCs on a sandy soil in Germany (relative yield related to the yield of an application rate of 150 kg N ha-1). The reduction of the N application rate by 50% results in a mean relative yield loss of -10% after 15 years for rye and triticale. Non-fertilisation cause significantly higher losses. By contrast, the relative yields of poplar and willow on reduced and even non-fertilised stands do not decrease, but instead increase, although the absolute yields grow over time. One of the reasons for this phenomenon seems to be mycorrhica...

Early work with sulfur dioxide showed a linear relationship between visible injury and reduction in yield for many crop species. The assessment was made that no reduction in yield would be found unless visible injury were noted. Definitive research with ozone, other oxidants, or mixtures of these pollutants with other gases has not been done. Thus, we do not know whether such relationships between visible injury and yield hold for the oxidants, but data in Table 11-3 suggest that for acute exposures there may be good correlations between injury and yield reductions. Many researchers have hypothesized that the oxidants may have an effect on plants that will produce a yield reduction with little or no visible injury. Such studies need to be designed in a more defmitive manner before it is concluded that yield reductions without visible symptoms are clearly acceptable. Projections of yield losses have made use of some of the data reported earlier. ... 

There are many reasons that weeds need to be controlled. The direct competition that leads to yield loss is the most obvious. Others include the contamination of the crop with seeds from aggressive or poisonous... 

Rapid crop growth (3-6 Objective minimise crop yield loss and prevent production of... 

The loss of availability of a major herbicide like atrazine or all triazines would seriously change weed management options. Extra cultivation costs were included in the Battelle (1989, 1993) studies. None of the studies included any costs of drift damage from use of alternative herbicides in close proximity to other, susceptible crops. Drift costs, extra yield losses, and other costs on no-till com were included in the Ciba Crop Protection and Novartis Crop Protection studies described later in this chapter. 

Yield loss in fruit and nut crops caused by weeds that are not adequately controlled by available herbicides or weed management techniques was estimated at 450 million annually in the United States (Chandler et al., 1984). 

Citrus trees are a long-term investment, and growers cannot afford crop damage or yield loss from weed competition, cultural operations, or misapplication of chemicals. The most critical time to control weeds in trees is from planting to early establishment, which spans from 3 to 6 years depending upon soil fertility. 

Atrazine is widely used in corn production due to its efficacy, application flexibility, low cost, and crop safety. Its efficacy ratings for some common weed species are listed in Table 34.1a. Table 34.1b presents estimates of the infestation of these important weeds in Illinois and of the potential yield loss that would occur if these weed species were uncontrolled. These data indicate that atrazine provides significant levels of control for most of the important weed species that are widely distributed and those that would cause significant com yield loss if left uncontrolled. 

Crop yield losses from weed competition can be substantial. The degree of loss depends on, crop and weed species present timing and duration of competitive interactions and resource availability (Agri 21 FAO, IFIC 2004 ). 

Worldwide, the competitive effect of weeds causes a 10% loss in agricultural production. Yield losses in rice and other grass crops in West Africa have been reported to range from 28-100% if weeds such as witchweed (Striga hermonthica) -a parasitic weed-are not controlled the greatest reductions occur on nutrient-poor soils. Left unchecked, weeds cause dramatic reductions in food production that eventually can... 

Crop yield losses due to weeds vary according to the competitiveness of the crop, the weeds present, and the population density of the weeds. Weed control is extremely important to any good program of crop production. Crop loss due to weed competition can be substantial ( 3). As an example, it has been estimated that nearly 100 million bushels of soybeans, or the equivalent of the production from 4,000,000 acres, was lost due to weed competition in the year 1970. 

No method is available for predicting the economic loss to vegetation in the United States from air pollutants. Surveys to date have not found a way to incorporate the impact of pollutants on the growth and yield of crops where visible injury is not found or, if present, is not recognized. If these factors were included with the additional pollutant impact on ornamentals and on natural ecosystems, it would be reasonable to expect... 

Fertilizers containing nitrates, phosphates, and heavy metals are applied to soil to increase yields of crops. Insecticides, herbicides, and fungicides are applied to food crops to reduce losses to pest species. As discussed in Chapter 9, these chemicals may be absorbed into the roots of plants and passed up the food chain to humans by animals that consume such plants. These chemicals may also be directly consumed by humans who eat the treated plants. 

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