Biochemical and physiological effects of target site resistance to herbicides inhibiting acetolactate synthase (ALS) were evaluated using sulfonylurea-resistant (R) and -susceptible (S) near isonuclear Lactuca sativa ‘Bibb’ lines derived by backcrossing the resistance allele from Lactuca serriola L. into L. sativa. Sequence data suggest that resistance in L. sativa is conferred by a single-point mutation that encodes a proline197 to histidine substitution in Domain A of the ALS protein; this is the same substitution observed in R L. serriola. Kmapp (pyruvate) values for ALS isolated from R and S L. sativa were 7.3 and 11.1 mM, respectively, suggesting that the resistance allele did not alter the pyruvate binding domain on the ALS enzyme. Both R and S ALS had greater affinity for 2-oxobutyrate than for pyruvate at the second substrate site. Ratios of acetohydroxybutyrate: acetolactate produced by R ALS across a range of 2-oxobutyrate concentrations were similar to acetohydroxybutyrate: acetolactate ratios produced by S ALS. Specific activity of ALS from R L. sativa was 46% of the specific activity from S L. sativa, suggesting that the resistance allele has detrimental effects on enzyme function, expression, or stability. ALS activity from R plants was less sensitive to feedback inhibition by valine, leucine, and isoleucine than ALS from S plants. Valine, leucine, and isoleucine concentrations were about 1.5 times higher in R seed than in S seed on a per gram of seed basis, and concentrations of valine and leucine were 1.3 and 1.6 times higher, respectively, in R leaves than in S leaves. Findings suggest that the mutation for resistance results in altered regulation of branched-chain amino acid synthesis.

Jointed goatgrass is a troublesome weed in winter wheat in the Pacific Northwest of the United States. Wheat and jointed goatgrass (JGG) can cross and produce hybrids in the field that can serve as a potential bridge for gene migration between the two species. To determine the potential for gene movement it is important to be able to identify hybrids in the field. To study the effect of wheat genotype on hybrid phenotype, reciprocal crosses were made between JGG and two common wheat cultivars: ‘Brundage 96’, ‘Hubbard’, a common-type advanced breeding line: ‘87–52814A’, and a club wheat cultivar: ‘Rhode’. Hybrids and parents were measured for plant height, spike length, flag leaf length, flag leaf width, and number of spikelets. Reciprocal effects were nonsignificant for all characteristics measured, indicating that hybrid morphology was not affected by the direction of the cross. Hybrids were different from their wheat parents for spike length, plant height, and flag leaf width. Hybrids produced from each of the wheat parents were uniform in phenotypic characters. Spikes were intermediate in circumference (size) from crosses between JGG and common wheat lines; however, club wheat × JGG crosses resulted in spikes that were more similar to common wheat. Spike size and flag leaf width for all hybrids also were intermediate between their parents. Hybrids differed in spike size and awn characteristics because of unique characteristics of the wheat parent. Based on these results, it should be possible to identify hybrids in the field accurately, regardless of the wheat parent or direction of the cross unless the parent is a club wheat.

The effect of planting system and cover crop residues on weed emergence in irrigated vegetable row crops was studied in field experiments from 1995 through 1997. Vegetable crops were either no-till planted (NTP) through cover crop residues or conventionally planted (CP) into soil with cover crop residues incorporated. NTP reduced emergence of hairy nightshade by 77 to 99% and Powell amaranth emergence by 50 to 87% compared with CP. Cover crop treatments were much less important than planting system in regulating weed emergence. Tillage in the spring did not increase the number of viable seeds near the soil surface. Hairy nightshade emergence ranged from 0.6 to 9.8% of the intact seeds in CP compared with 0 to 0.1% emergence of the seeds in the NTP plots. Powell amaranth emergence ranged from 4.9 to 6.5% of the intact seeds in CP contrasted with only 0.4 to 0.9% emergence of the seeds in NTP plots.

The transfer of herbicide resistance genes from crops to related species is one of the greatest risks of growing herbicide-resistant crops. The recent introductions of imidazolinone-resistant wheat in the Great Plains and Pacific Northwest regions of the United States and research on transgenic glyphosate-resistant wheat have raised concerns about the transfer of herbicide resistance from wheat to jointed goatgrass via introgressive hybridization. Field experiments were conducted from 2000 to 2003 at three locations in Washington and Idaho to determine the frequency and distance that imidazolinone-resistant wheat can pollinate jointed goatgrass and produce resistant F1 hybrids. Each experiment was designed as a Nelder wheel with 16 equally spaced rays extending away from a central pollen source of ‘Fidel-FS4’ imidazolinone-resistant wheat. Each ray was 46 m long and contained three rows of jointed goatgrass. Spikelets were collected at maturity at 1.8-m intervals along each ray and subjected to an imazamox screening test. The majority of all jointed goatgrass seeds tested were not resistant to imazamox; however, 5 and 15 resistant hybrids were found at the Pullman, WA, and Lewiston, ID, locations, respectively. The resistant plants were identified at a maximum distance of 40.2 m from the pollen source. The overall frequency of imazamox-resistant hybrids was similar to the predicted frequency of naturally occurring acetolactate synthase resistance in weeds; however, traits with a lower frequency of spontaneous mutations may have a relatively greater risk for gene escape via introgressive hybridization.

The effects of spring tillage sequence on summer annual weed populations were evaluated over two cycles of a 3-yr crop rotation of snap beans, sweet corn, and winter wheat. Continuous no-till (N) planting of vegetable crops each spring (NNNN) reduced summer annual weed density 63 to 86% compared to that of continuous conventional tillage (CCCC), depending upon site and herbicide level. Hairy nightshade populations were reduced by 88 to 96% when spring tillage was eliminated from the crop rotation. The effects of the NNNN spring tillage sequence on weed density were similar at two sites even though the crop rotations at the two sites began with different crops. The rotational tillage sequence of NCNC at the East site, in a crop rotation that began with corn, reduced summer annual weed density by 46 to 51% compared to that of continuous conventional tillage and planting (CCCC) at low and medium herbicide rates, respectively. In contrast, the tillage sequence of CNCN in the same crop rotation and at the same site increased weed density by 80% compared to that of CCCC at a low herbicide rate. The effects of the NCNC and CNCN rotational tillage sequences on weed density were reversed at the West site, and was probably caused by pairing sweet corn with conventional tillage rather than no tillage. The reduction in summer annual weed density caused by reduced spring tillage frequency did not significantly increase crop yields.

Development of herbicide-resistant crops has resulted in significant changes to agronomic practices, one of which is the adoption of effective, simple, low-risk, crop-production systems with less dependency on tillage and lower energy requirements. Overall, the changes have had a positive environmental effect by reducing soil erosion, the fuel use for tillage, and the number of herbicides with groundwater advisories as well as a slight reduction in the overall environmental impact quotient of herbicide use. However, herbicides exert a high selection pressure on weed populations, and density and diversity of weed communities change over time in response to herbicides and other control practices imposed on them. Repeated and intensive use of herbicides with the same mechanisms of action (MOA; the mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide; e.g., inhibition of an enzyme that is vital to plant growth or the inability of a plant to metabolize the herbicide before it has done damage) can rapidly select for shifts to tolerant, difficult-to-control weeds and the evolution of herbicide-resistant weeds, especially in the absence of the concurrent use of herbicides with different mechanisms of action or the use of mechanical or cultural practices or both.