The effect of soil disinfestation with methyl bromide (MB) or by soil solarization (solar heating) and the fungicides TMTD (tetramethylthiuram disulfide) and fentin acetate (triphenyltin acetate) on the degradation of terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-methylthio)-s-triazine] and atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] were investigated. The degradation of terbutryn appeared to follow first-order kinetics with a half-life of about 2 weeks and was much slower in MB-treated or solarized soils, i.e. half-life of about 11 weeks. Suppression of terbutryn degradation in the MB-treated soil was still evident 8 months after soil fumigation, similar to that found in autoclaved soil. TMTD and fentin acetate at 20 μg/g soil strongly inhibited degradation of terbutryn in soil. Degradation of atrazine was affected to a lesser extent by soil disinfestation. Results from the present study suggest that biocidal soil treatments may slow herbicide degradation. Thus, herbicide dosages in disinfested soil should be adjusted in order to avoid phytotoxicity. Moreover, lower dosages might be sufficient to attain weed control, and combined disinfestation or fungicides with herbicide treatments might be intentionally used to extend herbicide activity.

Field experiments conducted over a 2-yr period demonstrated that common bermudagrass [Cynodon dactylon (L.) Pers. # CYNDA] inhibited growth of newly planted peach (Prunus persica L. ‘Norman’) trees. Common bermudagrass densities of 100, 75, 50, and 25% ground cover reduced tree fresh weight by 86, 64, 43, and 19%, respectively, the first year (1978) and 87, 62, 44, and 28%, respectively, the second year (1979) after planting. Tree trunk diameter relative growth rate (RGR) was reduced by 75 and 100% common bermudagrass ground cover densities at all measurement dates only in 1978. Tree leaf N and K were reduced in both years by common bermudagrass; however, only at the 100% common bermudagrass density in 1978 was N at a deficient level. Leaf chlorophyll was reduced in trees grown in all densities of common bermudagrass only in 1978. Reduced tree growth cannot be explained entirely by competition for essential nutrients; thus an allelopathic effect of the bermudagrass on young peach roots is suspected.

Red rice (Oryza sativa L. ♯ ORYSA) densities of 5, 108, and 215 plants/m2 reduced grain yield of commercial rice (Oryza sativa L.) 22, 77, and 82%, respectively. At a cultivated rice density of 195 plants/m2, red rice at 5, 108, and 215 plants/m2 reduced straw dry weight of cultivated rice 18, 66, and 68%, respectively. At a red rice density of 5 plants/m2, reduction in number of cultivated rice grains per panicle ranged from 8 to 18%, whereas densities of 108 and 215 plants/m2 reduced grains per panicle 56 to 70%. Red rice grain yield was 24 to 33% lower in ‘Mars' rice than in ‘Lebonnet’. Mars, a medium-grain cultivar that matures in 138 days, competed better with red rice than Lebonnet, a long-grain cultivar that matures in 126 days.

Genetic variability for loci governing enzyme/morphological variants and for herbicide response was determined in 10 populations of the slender wild oat (Avena barbata Pott. ex Link ♯ AVEBA), six populations of wild oat (Avena fatua L. ♯ AVEFA), and three populations of godetia (Clarkia williamsonii Lewis & Lewis). The enzyme loci were identified by starch gel electrophoresis and included peroxidase, 6-phosphogluconate dehydrogenase, esterase, and leucine aminopeptidase for the slender wild oat; peroxidase, esterase, leucine aminopeptidase, and malate dehydrogenase for the wild oat; and esterase, phosphoglucoisomerase, leucine aminopeptidase, acid phosphatase, and glutamate oxaloacetate transaminase for godetia. Morphological loci included lemma and leaf sheath hairiness for the oats. For both the enzymatic and morphological loci, levels of genetic variation for each population were quantified by a polymorphic index. The herbicide barban (4-chloro-2-butynyl 3-chlorophenylcarbamate) was used on the wild oats; bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) was used on godetia. Genetic variation for herbicide response was based on genetic variances calculated from phytotoxicity scores. Populations were ranked from highest to lowest for the polymorphic indices and the genetic variances. Concordance between the rankings was tested by rank correlation. Statistically significant relationships were found between the enzyme/morphological characters and herbicide response in the slender wild oat and the wild oat. For some species, the level of genetic variation for response to herbicides appears to be associated with genetic variation for enzymatic and morphological loci.

Wild and cultivated proso millet (Panicum miliaceum L. ♯ PANMI) showed increased height and dry mass with increasing photoperiod and flowered under all photoperiods. Increasing planting density decreased height and dry mass and slowed seedhead development in both types. Shading increased plant height and decreased dry mass of both millets and increased germination compared to unshaded treatments. Planting date did not affect germination of freshly collected seed of either millet. Wild proso millet seed had a low germination rate until 8 weeks after panicle emergence, and the rate increased to 22% after 10 weeks. Germination of cultivated proso millet seed was low for 5 weeks after panicle emergence and increased to 92% after 10 weeks.

Achene germination of common crupina (Crupina vulgaris Cass. ♯ CJNVU) was 86% or greater at day/night temperatures ranging from 15/4 to 29/25 C. Germination was greatly reduced at a day temperature of 36 C, regardless of the night temperature. The time required for maximum germination (9 to 19 days) increased with decreasing night temperature. Maximum leaf area, rosette diameter, and dry-matter accumulation during vegetative growth occurred at 22/18 to 29/25 C, 22/18 C, and 22/18 C, respectively. The plants attained 50% or more of maximum dry-matter accumulation at day/night temperatures ranging from as low as 22/4 and 15/11 C to as high as 36/25 C. Net assimilation rate was relatively insensitive to growth temperature. Rosettes of common crupina grown at 22/11 or 29/18 C exhibited almost complete photosynthetic and respiratory temperature acclimation. Increasing the photoperiod from 10 to 12, 14, or 16 h accelerated flowering. Maximum achene production occurred in the 16-h photoperiod, but viable achenes were produced in all photoperiods. Temperature during vegetative growth influenced subsequent responses to photoperiod. Plants grown for 83 days in a 14-h photoperiod at 15/4 or 22/11 C and transferred to 26/20 C flowered and produced seed in photoperiods of 8 or 16 h. Plants grown at 29/18 or 36/25 C subsequently flowered only in the 16-h photoperiod at 26/20 C. Common crupina has a wide tolerance of variations in temperature and photoperiod. Its range in the United States can be expected to continue to increase.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]-benzenesulfonamide} applied preemergence or postemergence as a soil drench retards the growth of four corn (Zea mays L.) genotypes in soil with 50% inhibition in each case at a rate of approximately 5 g ai/ha. R-25788 (N,N-diallyl-2,2-dichloroacetamide), applied at 1.0 kg ai/ha to the soil preemergence with chlorsulfuron or postemergence 2 days prior to chlorsulfuron treatment, reverses shoot growth inhibition induced by the herbicide in varying degree in ‘XL25A’ and ‘XL72B’ corn hybrids and ‘FRM017’ and ‘FR619’ inbreds, but not in soybean [Glycine max (L.) Merr., var. ‘Amsoy-71’]. Under similar conditions, flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid (phenylmethyl ester)] and CGA-92194 [α-(1,3-dioxolan-2-yl-methoxy)-imino-benzeneacetonitrile] are less effective than R-25788 in protecting corn genotypes from chlorsulfuron injury. R-25788 also partially reverses chlorsulfuron-induced root growth inhibition in XL25A and FR619 corn but not in pea (Pisum sativum L. var. ‘Dwarf Gray Sugar’). Acetohydroxyacid synthase (AHAS) is higher in both activity and the proportion that is very sensitive to in vitro chlorsulfuron inhibition when prepared from XL25A shoots than from the roots. R-25788 does not reverse the in vitro inhibition of corn acetohydroxyacid synthase (AHAS) by chlorsulfuron. Pretreatment of 5-day-old XL25A corn with R-25788 at 24 or 48 μM significantly increases AHAS activity by approximately 25% in both shoots and roots but does not change its in vitro sensitivity to chlorsulfuron. This antidote also increases the glutathione (GSH) content in roots and etiolated shoots. The R-25788-induced elevation of AHAS activity may contribute to its antidotal effect in corn.

Growth chamber and laboratory experiments evaluated the effects of seed-soil microsite characteristics on seed germination. When corn (Zea mays L. ‘Pioneer 3541’), soybean [Glycine max (L.) Merr. ‘Corsoy 79’], velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and giant foxtail (Setaria faberi Herrm. ♯ SETFA) were seeded among soil aggregates 2.50 to 1.25 cm in diameter in the growth chamber, there was decreased seedling emergence with decreasing frequency of irrigation. These same species seeded inside artificial soil aggregates showed increased seedling emergence with decreasing frequency of irrigation. The germination of corn and soybean seed inside 0.4-g, fully moist soil aggregates incubated in the laboratory under high relative humidity conditions was significantly decreased relative to seed incubated in the absence of soil. Velvetleaf and giant foxtail germination was significantly reduced by 0.1-g soil aggregates. Rice (Oryza sativa L.) had 68% emergence from 1.0-g, fully moist soil aggregates, whereas larger seeded corn and soybean had only 20 and 10% emergence, respectively. Germination inhibition of giant foxtail seed by fully moist soil was partially reversed by incubating seed-containing aggregates in an atmosphere of 75% oxygen.

Plant response studies were conducted from 1980 to 1982 under pasture and field plot conditions to determine the sublethal effects of the thistle rosette weevil [Trichosirocalus (Ceuthorynchidius) horridus (Panzer)] on Carduus thistles. Response to weevil damage was dependent upon thistle size and growing conditions. Damage by T. horridus destroyed apical dominance, which altered the thistle growth pattern. Infested plants consistently produced more stems and a larger crown than uninfested thistles. Large thistles were stimulated by weevil damage, producing heavier stems and more heads. Small infested thistles developed more quickly than small uninfested thistles. Small and medium infested thistles were shorter and produced fewer seeds and heads than uninfested thistles. The extent of thistle reduction by this weevil will ultimately be determined by the conditions under which thistles grow.

Field experiments were conducted in 1979, 1980, and 1981 to determine the level of interference five broadleaf weed species exert on soybean [Glycine max (L.) Merr.]. Weed species studied were common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANST), common ragweed (Ambrosia artemesiifolia L. ♯ AMBEL), common lambsquarters (Chenopodium album L. ♯ CHEAL), sicklepod (Cassia obtusifolia L. ♯ CASOB), and redroot pigweed (Amaranthus retroflexus L. ♯ AMARE). The following soybean seed yield reductions were observed with a density of 16 weeds/10-m row: redroot pigweed 22%, common lambsquarters 15%, common ragweed 12%, and sicklepod 5%, respectively. At a density of 8 weeds/10-m row, common cocklebur reduced soybean yield 11%. No single weed growth parameter predicted soybean seed yield reduction for all weed species. Soybean height was reduced by sicklepod competition; was not affected by competition from common lambsquarters, common ragweed, or common cocklebur; and was increased in the presence of redroot pigweed at 12 weeks after planting, when measured 30 cm from the weeds. Leaf area of soybean was higher at greater distances from the weed for all weed species. The range of soybean leaf area reductions occasioned by proximity to individual weed species corresponded fairly well with differences in soybean yield reduction.

Because environmental factors facilitating the breaking of dormancy of seed of catclaw mimosa (Mimosa pigra L. ♯ MIMPI) under field conditions were unknown, the effect of constant temperatures (2 to 44 C) on germination of scarified and unscarified seeds, and the effect of diurnally fluctuating temperatures on unscarified seeds were examined. Temperatures fluctuated diurnally by 10 and 20 C in these latter experiments. Germination was less than 10% at all constant temperatures except 44 C, which had 40% germination. Scarification increased germination to 100% at constant temperatures greater than 12 C. Germination of unscarified seed was significantly lower (0 to 23%) when day/night temperature ranges fluctuated by 10 C than when they fluctuated by 20 C. Optimum temperature for germination was 40/20 C, where 93% germination occurred within 4 days.

A 7-yr study was conducted to investigate successional dynamics of herbaceous vegetation after control of western juniper (Juniperus occidentalis Hook.) trees with picloram (4-amino-3,5,6-trichloropicolinic acid) pellets. Downy brome (Bromus tectorum L. # BROTE) rapidly dominated areas under dead tree canopies with an accompanying yield increase from almost none to about 1400 kg/ha within 4 yr after treatment. In the interstitial zone between tree canopies, a dramatic shift in herbaceous species occurred with tree control. Annual broadleaf species decreased, downy brome increased, and medusahead [Taeniatherum asperum (Sim.) Nevski # ELYCM] gradually became dominant. Total yield of herbaceous vegetation in the interstitial zone increased dramatically after picloram treatment, especially where few shrubs occurred. Available soil moisture, litter accumulation and decay after picloram treatment, and N in the surface layer of the soil affected responses of herbaceous vegetation, especially under dead tree canopies. Responses of annual weedy species to western juniper control emphasize the need for comprehensive manipulation of all components of the vegetation when range improvement is attempted.

Seed dormancy does not play a role in the germination ecology of curly dock (Rumex crispus L. ♯ RUMCR). This study confirms reports that freshly matured seeds are nondormant, and it shows that buried seeds exposed to natural seasonal temperature changes remain nondormant. From October 1981 through June 1983, seeds exhumed at monthly intervals germinated 80 to 100% at all thermo-periods. These results do not support suggestions that seeds of curly dock buried in soil enter dormancy. However, the results do explain why seeds of this species in the Beal and Duvel buried-seed experiments germinated when exhumed at various times during the growing season.

Research was conducted from 1979 through 1982 in east-central Alabama to monitor changes in sicklepod (Cassia obtusifolia L. ♯ CASOB) seed numbers in the soil under various cropping and weed management systems in soybeans [Glycine max (L.) Merr.]. Significant declines in sicklepod seed numbers were attained only when mechanical summer fallow each year prevented replenishment of seed in soil. Repeated disking favored germination and emergence of sicklepod seed and thus caused a reduction in seed numbers. Chemical summer fallow, which relied on postemergence control of emerged sicklepod, did not result in a disturbed seedbed and was less effective in reducing seed numbers. Permitting subcompetitive densities of sicklepod, 0.45 and 0.90 plants/m2, to reach maturity each year resulted in increases in seed numbers in the soil. Increases were more dramatic in conventionally planted soybeans than in no-till culture. Sicklepod produced more pods per plant in tilled culture than in no-till. Sicklepod seed distribution in the upper 30 cm of the soil was not affected by the type of primary tillage.

The herbicide AC 252 214 {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl] 3-quinoline-carboxylic acid} was absorbed by the roots and foliage of soybean [Glycine max (L.) Merr. ‘Williams’], common cocklebur (Xanthium strumarium L. ♯ XANST), and velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and then translocated in the xylem and phloem to meristematic regions. AC 252 214 was metabolized rapidly by soybean and velvetleaf but appeared to be metabolized slowly by cocklebur. The order of tolerance of these three species to AC 252 214 was soybeans > velvetleaf > cocklebur. This order of tolerance was directly correlated in young plants with the half-life of AC 252 214 within the tissue. Velvetleaf exhibited increased tolerance to AC 252 214 with age, which was attributed partially to greatly reduced absorption of the herbicide by older leaves and more rapid metabolism of the herbicide.

The influence of depth (0 to 20 cm) and duration (1 to 31 months) of burial on the deterioration, germination, and viability of common crupina (Crupina vulgaris Cass. ♯ CJNVU) was investigated in the field. Germination and decay of achenes prior to removal from the field were greater at 0- to 10- (7%) and 0- to 15- (21%) cm burial depths than at the 20-cm burial depth for 1980 and 1981 seedlots, respectively. After 25 months of burial, 41 and 26% of the achenes had not deteriorated for the 1980 and 1981 seedlots, respectively. Germination was greatest for nondeteriorated achenes recovered from 3- and 20-cm (63%) and least for the 10-cm burial depth (44%). Common crupina achenes recovered from the field over 25 months of burial exhibited a pronounced periodicity of germination, which was related to induced winter dormancy. The viability of achenes decreased linearly from 90 to near 0% over 31 months of burial.

Common cocklebur (Xanthium pensylvanicum Wallr. ♯ XANPE), velvetleaf (Abutilon theophrasti Medic. ♯ ABUTH), and jimsonweed (Datura stramonium L. ♯ DATST) are weeds that grow taller than soybeans [Glycine max (L.) Merr.] and compete for limited resources such as light. These weeds have an average area of influence of 0.5 m2, thus a density of 1 to 2 plants/m2 forms a full canopy of weed leaves above the soybeans that intercept 44 to 56% of the sunlight. Shade of 44 to 56% without weeds, placed above soybeans late in the season corresponding to times of weed shading, reduced soybean yield 19 to 26%. Since reported soybean yield reductions at weed densities of 0.7 to 2.5 plants/m2 were 18 to 54% for common cocklebur and 12 to 31% for jimsonweed and velvedeaf, it was concluded that most of the interference from jimsonweed and velvetleaf infestations in soybeans could be ascribed to competition for light, but only about one-half the interference was due to competition for light for common cocklebur. Soybean photosynthesis in the field was reduced about twice as much as soybean yield at equivalent levels of shading.