Food production on ever-dwindling agricultural land is a severe problem in Ethiopia, necessitating the adoption of more efficient and sustainable land-use strategies to feed the country's growing population. The benefits of intercropping are known to be limited by a variety of factors such as companion crop sowing date and variety. The present study was therefore initiated to evaluate the effect of varieties and planting dates of lupines in bread wheat-lupine under an additive design intercropping system in Northwest Ethiopia. Field experiments were conducted for two years in Adet and Debre Tabor experimental sites. The treatments consisted of four planting dates of lupines and two varieties of lupine. Moreover, the sole crop of bread wheat (Triticum aestivume L.), local lupine (Lupinus albus L.) and sweet lupine (Lupinus angustiflolius L.) were also included. The experiments were laid out in factorial randomized complete block design (RCBD) with three replications. The results showed that at the Adet experimental site, the highest land equivalent ratio (LER) was recorded in the simultaneous planting of sweet lupine and bread wheat (1.3), followed by the planting of sweet lupine two weeks after bread wheat (1.23). While at Debre Tabor experimental site, the highest LER was recorded in planting sweet lupine two weeks after bread wheat (1.67). A greater area time equivalent ratio (ATER) was reported in the simultaneous planting of sweet lupine and bread wheat (1.29), followed by the planting of sweet lupine after bread wheat (1.22) at the Adet experimental site. A greater ATER was also recorded in the planting of sweet lupine after bread wheat (1.53) had the highest ATER in Debre Tabor, followed by simultaneous planting of sweet lupine after bread wheat (1.39) intercropping. This revealed that sole cropping would necessitate 29 and 53% more area, respectively, to attain the same yield as intercropping. In Adet, the simultaneous planting of sweet lupine and bread wheat intercropping, while in Debre Tabor, planting of sweet lupine two weeks after bread wheat boosted productivity, and production efficiency and is a viable option for increasing household food security.

Morningglories (Ipomoea spp.) are among the most troublesome weeds in cucurbits in the United States; however, little is known about Ipomoea spp. interference with horticultural crops. Two additive design field studies were conducted in 2020 at two locations in Indiana to investigate the interference of ivyleaf morningglory (Ipomoea hederacea Jacq.), entireleaf morningglory (Ipomoea hederacea Jacq. var. integriuscula A. Gray.), and pitted morningglory (Ipomoea lacunosa L.) with triploid watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai]. Immediately after watermelon was transplanted, Ipomoea spp. seedlings were transplanted into the watermelon planting holes at densities of 0 (weed-free control), 3, 6, 12, 18, and 24 plants 27 m−2. Fruit was harvested once a week for 4 wk, and each fruit was classified as marketable (≥4 kg) or non-marketable (<4 kg). At 1 wk after the final harvest, aboveground biomass samples were collected from 1 m2 per plot and oven-dried to obtain watermelon and Ipomoea spp. dry weight. Seed capsules and the number of seeds in 15 capsules were counted from the biomass sample to estimate seed production. Ipomoea spp. densities increasing from 3 to 24 plants 27 m−2 increased marketable watermelon yield loss from 58% to 99%, reduced marketable watermelon fruit number 49% to 98%, reduced individual watermelon fruit weight 17% to 45%, and reduced watermelon aboveground biomass 83% to 94%. Ipomoea spp. seed production ranged from 549 to 7,746 seeds m−2, greatly increasing the weed seedbank. Ipomoea spp. hindered harvest due to their vines wrapping around watermelon fruits. The most likely reason for watermelon yield loss was interference with light and consequently less dry matter being partitioned into fruit development due to less photosynthesis. Yield loss was attributed to fewer fruits and the weight of each fruit.

Bioassays using binary mixtures that included a cover crop with known allelopathic potential and a weed species were employed to determine the importance of allelopathy compared to resource competition as interference mechanisms. Responses of weed species germinated with cover crops in a petri dish were measured. Interference between weed and cover crop seedlings was determined in a greenhouse experiment using the additive design, which included partitions to reduce above- and below-ground competition and used capillary mat subirrigation to control moisture and fertilizer availability. Germinating sorghum reduced radicle length of weeds, whereas germinating rye tended to increase weed radicle length. Methods limited above-ground competition, so likely interference mechanisms were below-ground competition and allelopathy. Germination with a cover crop had little effect on germination and shoot length of weeds. Increased density of rye but not of sorghum reduced growth of barnyardgrass seedlings. Reduced number of barnyardgrass leaves in the presence of rye was likely due to allelopathy. Suppression of barnyardgrass dry weight attributed to allelopathic interference by rye was successfully separated and compared to the combined effects of competition and allelopathy.

Intraspecific interference of sicklepod and interspecific interference between sicklepod and soybeans were investigated in field experiments in 1982 and 1983. Sicklepod growth and reproductive parameters were reduced as intraspecific spacing between plants decreased from 90 to 2.5 cm. Sicklepod plants growing intraspecifically were competing at spacings below 90 cm. Soybean plant heights were affected only slightly by sicklepod interference. Soybeans grew little in height from 35 to 84 days after emergence, while sicklepod plants continued to grow until they were 30 to 45 cm taller than the soybeans. Sicklepod plants spaced 10 and 30 cm apart in soybeans reduced soybean yield (25 to 35%) more than sicklepod spaced 90 cm apart. Sicklepod produced 900 to 2000 seeds per plant even when growth was restricted by interference from soybeans.

Field trials were conducted over two spring seasons (2005 and 2006) to investigate the interference between American black nightshade and watermelon in polyethylene-mulched and nonmulched (bare ground) trials. Competition studies were performed with watermelon at 1 plant/m in-row and American black nightshade grown at 0, 2, 4, 6, and 8 plants/m2 in-row. Watermelon yield reductions started to plateau at only 2 American black nightshade plants/m2. Yield reduction was 80, 89, 96, and 98% and 54, 67, 81, and 85% at 2, 4, 6, and 8 American black nightshade plants/m2 in mulched watermelon production in 2005 and 2006, respectively. When watermelon was grown on bare ground, yield reduction was 100% at 2 American black nightshade plants/m2 in 2005 and 68, 81, 89, and 93% at 2, 4, 6, and 8 American black nightshade plants/m2 in 2006, respectively. Watermelon fruit numbers were reduced due to American black nightshade interference, but no significant differences in individual fruit weight were observed.

Bushkiller was evaluated under inter- and intraspecific competition. In experiment 1, bushkiller, trumpetcreeper, and wild grape were greenhouse-grown alone and in two or three species mixtures in pots. Of the three species, bushkiller grew the tallest and had the greatest final biomass when grown alone. When all three species were grown together, bushkiller grew over twice the height of trumpetcreeper, over three times the height of wild grape, and over four times the biomass of either competing species. Plots of height over time showed that competition did not affect bushkiller or wild grape growth rate, but trumpetcreeper growth was reduced when grown with bushkiller. In experiment 2, bushkiller was grown in cultures of one, two, and three plants per pot to determine intraspecific competition effects on growth. Final height of bushkiller was not affected by intraspecific competition; however, bushkiller biomass decreased with increasing competition.

Field experiments were conducted to determine density-dependent effects of eastern black nightshade season-long interference on tomato-yield loss when growing in-row with staked plasticulture tomato. Eastern black nightshade was transplanted at densities of zero, one, two, three, four, or five plants per crop plant hole in the plastic. Eastern black nightshade densities of one to five reduced the number and weight of larger fruit grades (threes, extra larges, jumbos, marketables, totals) similarly but did not reduce yields of smaller fruit grades (culls, mediums, and larges) from the weed-free. Eastern black nightshade reduced percent yield loss of jumbo grade, the premium grade, which could be predicted by a rectangular hyperbola model. The value ($ ha−1) of jumbo fruit and the value of the sum of large, extra large, and jumbo grade was reduced at densities of eastern black nightshade as low as one plant per hole.

Field studies were conducted in 1997 and 1998 at Manhattan and Topeka, KS, to examine the competitive effects of redroot pigweed, Palmer amaranth, and common waterhemp on soybean yield. The experiments were established as a randomized complete block design in a factorial arrangement of three pigweed species, two pigweed planting dates (soybean planting and cotyledon stage), and seven weed densities (0.25, 0.5, 1, 2, 4, and 8 plants m−1 of row, plus a weed-free control). The effect of weed density on soybean yield loss, pigweed biomass, and pigweed seed production were described using a rectangular hyperbola model. Soybean yield loss varied between locations depending on the weed species, density, and time of emergence. Yield loss increased with weed density for each species and location with the first pigweed emergence time. The maximum soybean yield loss occurred at the first planting and 8 plants m−1 of row density, and was 78.7, 56.2, and 38.0% as determined by the model for Palmer amaranth, common waterhemp, and redroot pigweed, respectively. The second planting of pigweed did not significantly reduce soybean yield. The relative ranking of the pigweed species biomass was Palmer amaranth > common waterhemp > redroot pigweed. Maximum seed production for Palmer amaranth, common waterhemp, and redroot pigweed was 32,300, 51,800, and 9,500 seeds m−2. Palmer amaranth produced a larger quantity of seed than did common waterhemp or redroot pigweed at low weed densities (0.25 to 4 plants m−1 of row). But common waterhemp seed production equaled or surpassed Palmer amaranth at high weed densities.

Field studies were conducted in the spring of 1997 and 1998 to quantify the effect of season-long yellow nutsedge interference on watermelon yield. The competitive ability of watermelon with yellow nutsedge was compared in two establishment methods (watermelon transplanted and direct seeded). Critical yellow nutsedge densities and the biological threshold (BT) were used to characterize the competitive ability of watermelon. The critical density in both direct-seeded and transplanted watermelons was 2 yellow nutsedge plants/m2. The BT of yellow nutsedge in seeded watermelons was 37 yellow nutsedge plants/m2, whereas the BT in transplanted watermelons was 25 plants/m2. Transplanting watermelons did not improve their competitive ability with yellow nutsedge. Percent yield loss was similar for both establishment methods at the respective yellow nutsedge densities. Over 40% yield loss was incurred with 12 yellow nutsedge plants/m2 for both establishment methods. Furthermore, it was concluded that watermelons are poor competitors with yellow nutsedge.

Porcelain berry and bushkiller are confamilial, exotic, perennial vines in the Vitaceae family that are considered nuisance/invasive weeds of natural and riparian areas in the eastern United States. To better understand the competitive abilities of these aggressive weeds, greenhouse competition experiments were conducted on cuttings of porcelain berry, bushkiller, and Virginia-creeper, a native member of the Vitaceae family. Plants grown singly or in combination were monitored for stem growth and biomass production. In this research, porcelain berry and Virginia-creeper exhibited similar rates of stem growth, whereas bushkiller grew taller and faster than either of the other species. Porcelain berry stem growth was reduced in competition with bushkiller. All three species exhibited reduced stem biomass when grown with both other species. Root biomass of porcelain berry and Virginia-creeper were not affected by competition, but bushkiller, which produced the heaviest roots, exhibited reduced root biomass when grown with both other species. Porcelain berry root length was reduced by competition with both other species, but neither Virginia-creeper nor bushkiller root lengths were affected by competition. These results indicate that bushkiller is likely the strongest competitor of the three species studied. In these experiments, porcelain berry was less aggressive and vigorous than bushkiller but was similar to Virginia-creeper.

Additive and replacement series studies were conducted to investigate the effect of Glycine max cultivar canopy characteristics, interference, and weed-free period on the vegetative and reproductive growth of Solanum ptycanthum. Solanum ptycanthum established 0, 2, and 4 wk after planting (WAP) did not reduce seed yield of either Group II (Pioneer 9273) or Group IV (Asgrow 4715) varieties. The Group IV variety intercepted a greater percentage of light throughout the season. However, both varieties reduced growth of S. ptycanthum established 2 and 4 WAP, suggesting no disadvantage of Group II in suppressing S ptycanthum growth. The greenhouse and field replacement series experiments indicated that S. ptycanthum plants experienced greater intraspecific than interspecific competition; that is, S. ptycanthum grew more in mixed species competition than in monoculture. Under field conditions, G. max grew equally well in competition with S. ptycanthum or in monoculture. However, in the greenhouse G. max accumulated more biomass and fruit weight in monoculture vs. competition with S. ptycanthum. Relative yield total values (RYT) in the field supported greater productivity per unit area for mixed species competition when compared to G. max or S. ptycanthum grown in monoculture. Data from both studies suggested that S. ptycanthum did not reduce G. max yield when grown in the field. Regardless of maturity group, a fully developed G. max canopy reduced S. ptycanthum growth.