Field experiments were conducted in 1987 and 1988 at Frederick, MD, to determine established horsenettle (Solanum carolinense L.) interference with snapbean. Horsenettle grown for 3 yr prior to planting snapbeans reduced yield 65% in 1987 and 48% in 1988. Horsenettle established for 1 yr reduced snapbean yield 20 and 18% in 1987 and 1988, respectively. As the distance of the snapbean row from the horsenettle row increased, there was less effect on the yield of snapbean. In 1987, snapbean yield in rows 15, 30, or 60 cm from horsenettle was reduced 43, 29, and 15%, respectively, while during 1988 in the same plots snapbean yields were reduced 38, 26, and 11%. Eight horsenettle planted in a 4.6-m row the first year and 16 horsenettle per row for the second year reduced snapbean yield 36 and 55% respectively. Horsenettle fruit number was 16% lower the first year and 45% lower the second year when snapbeans were grown at a 15-cm row spacing instead of a 60-cm row spacing.

A method is presented to determine which of a set of sampling techniques is most nearly optimum for sampling weed abundance. To employ the method, one needs first to develop a set of sampling alternatives. A pilot survey is then conducted to determine time costs and sampling and measurement error variances associated with each technique. We used this method to determine which of three plot sizes (0.6 by 0.6 m, 0.6 by 1.5 m, 1.2 by 2.1 m) and two types of orchard scans is most nearly optimum for estimating weed abundance in apple [Malus domestica (Bork)] orchards. The results indicate that, for weed species that are fairly apparent, orchard scans are optimal when less than 8 h of sampling effort is expended, but when a high degree of precision is required, or when the weed species is more difficult to detect, plot samples may be the better choice. A size of plot ca. 0.6 by 1.5 m size was found to be most nearly optimum.

Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring.