Webster and Coble (1997) reported on weed shifts in major crops of the Southeastern states over a 22-year period (1974–1995) when herbicides were the major means of weed control. Sicklepod and bermudagrass had become the most troublesome weeds. The largest decreases in weed pressure were found with Johnsongrass, crabgrasses, and common cocklebur. Morningglories and nutsedges remained relatively constant. The weeds of greatest importance in soybean, peanut, and cotton are the pigweeds.
Rotational crops (including cover crops) often entail the application of practices which stimulate germination (e.g., tillage and irrigation) as well as those that kill emerged seedlings (e.g., cultivation and herbicides). Rotational crops may result in the germination and death of rice weeds in a manner analogous to the stale seedbed approach described earlier.
Using a model to maximize strategies for herbicide-resistant blackgrass, Cavan et al. (2000) gave estimates on the effectiveness of various strategy options. Based on research with a long-term model for control of blackgrass and annual bluegrass, Munier-Jolain et al. (2002) concluded that threshold-based weed management strategies can be more cost-effective than spraying every year and may enable important reductions in herbicide use. However, the highest long-term profitability was obtained for the lowest weed level threshold tested.
Herbicides and Weed Biology
Homer M. LeBaron , Eugene R. Hill , in The Triazine Herbicides , 2008
Weed seed predation, especially after seeds have been shed on soil, may be an important determinant of seedbank losses ( Davis et al., 2013 ; Westerman et al., 2011 ). Insects and small rodents are the main contributors to weed seed predation, thus manipulation of agricultural habitats as to attract them (e.g., no-till, delayed stubble cultivation, introduction of uncultivated strips within fields or as field margins) is expected to increase the number of weed seeds predated ( Landis et al., 2005 ). Carabid beetles are among the most important consumers of weed seeds. It should be kept in mind that seed consumption by carabids is influenced by several factors, including weed species, seed physiological state, insects gender, activity-density level, and seed burial depth ( Kulkarni et al., 2015, 2016 ).
Figure 5.9 . Number of weed seedlings emerged versus the number of viable seeds found in each of 48 individual soil samples. Note log scales both axes.
Changes in weed species and populations also cause changes in plant diseases and insect pests since certain weeds serve as their hosts ( Bendixen et al., 1981 ; Manuel et al., 1982 ; Weidemann and TeBeese, 1990 ; Norris and Kogan, 2000) . Herbicide-resistant weed biotypes are present in our weed populations, although often at very low frequencies, even when herbicides are not used. Weed species have acquired built-in genetic adaptability to survive most control methods used against them. For example, dandelions usually develop a vertical growth habit when growing wild, but when growing in a frequently mowed lawn, more prostrate or flat-growing biotypes evolve. We should continually add to our weed control technology and keep tools available in order to address the adaptability of weeds to different control methods. For further information on the biological characteristics of weeds, including growth strategies, mimicry with crops, plasticity of weed growth, photosynthetic pathways, weed seed reservoir, and vegetative reproduction see Cousens and Mortimer (1995) and Buhler et al. (1998) .
In addition to the added cost of controlling these weeds, weed scientists in the U.S. have documented herbicide resistance in Palmer amaranth to every herbicide mode of action that can be used in row crop production.
Another step is to clean spreading equipment before moving to a new field.
“This is why it is important to scout fields for these two pigweeds before they become established,” Ikley says. “In many cases where the weeds are spread in contaminated manure, the infestation starts with a manageable level of plants and the population can be managed by hand pulling if correctly identified.”
Due to this extensive seed production, the ability of the weeds to germinate throughout the growing season, and widespread resistance to glyphosate and Group 2 herbicides, herbicide programs for control of severe infestations of waterhemp and Palmer amaranth often will cost two to three times the amount of money spent on a weed control program in fields without these two weeds, he says.
“It is a known fact that weed seeds pass unharmed through the digestive tracts of ruminant animals (cattle, sheep),” says Mary Keena, livestock environmental management specialist based at North Dakota State University’s Carrington Research Extension Center. “This means that whatever weed seeds are in the feed or bedding you’re using are still viable when they exit the animal as manure.
Producers have tools to minimize the amount of viable weed seeds in fresh manure, one of which is composting. Information about composting is available in a self-paced online workshop at https://tinyurl.com/2020CompostingWorkshop. To learn the reasons behind these composting operations, check out https://tinyurl.com/2020CompostProducerOperations.
Producers have a few steps they can take to help mitigate and monitor the potential impacts of these weeds. One is to keep records of where they spread manure so they can monitor that field throughout the growing season.