The effectiveness of single and repeated annual preharvest glyphosate applications (1 to 2 wk before crop harvest) to reduce Canada thistle populations was evaluated in field experiments near Bezanson, AB, Scott, SK, and St. Albert, AB. Single applications of glyphosate at 0.45 to 1.8 kg/ha at Bezanson and St. Albert reduced Canada thistle shoot density by more than 75% but in the three experiments at Scott 1.8 kg/ha was required to consistently obtain a similar reduction. Four consecutive, annual applications of glyphosate at 0.45 kg/ha or more did not eliminate Canada thistle but reduced shoot densities by more than 98% at Bezanson. At Scott, similar reductions occurred in two of three sites following two consecutive, annual preharvest applications of glyphosate at the same rates. In the third Scott site the 1.8 kg/ha rate of glyphosate provided a similar reduction but lower rates did not. At Bezanson, barley yields following the first, second, and third applications of glyphosate at 0.45 kg/ha or more were 14% greater than check plots where MCPA was applied in the spring. Yields of wheat seeded 1 yr after a single application of glyphosate at St. Albert were 26% greater than from unsprayed checks. Barley yields seeded 1 yr after each glyphosate application at Scott were not affected because Canada thistle populations were sparse.

Field experiments were conducted at three locations in Mississippi in 1995 and 1996 to evaluate labeled and alternative herbicides and herbicide combinations for weed desiccation prior to soybean harvest. Weeds evaluated included pitted morningglory, hemp sesbania, spotted spurge, common cocklebur, and sicklepod. Soybean yield and harvestable soybean losses were not affected by preharvest herbicide treatments, spray volume, or surfactant concentration. Soybean moisture was most consistently reduced by glufosinate compared to the untreated and other herbicides and herbicide combinations evaluated. Most desiccation treatments at Stoneville and Brooksville resulted in foreign material similar to the weed-free check. Glufosinate at 0.84, 1.1, and 1.4 kg ai/ha desiccated all weeds evaluated 90% or more with no differences among rates. The addition of 3.4 or 6.7 kg ai/ha sodium chlorate to 0.28 kg ai/ha paraquat, 1.1 and 2.2 kg ai/ha glyphosate, or 0.28 and 0.56 kg ai/ha oxyfluorfen increased desiccation of most weeds evaluated, with no difference between sodium chlorate rates. In some instances, reducing application volume from 281 to 94 L/ha improved pitted morningglory desiccation when 0.28 g/ha paraquat was applied alone. There were no differences between 0.25 and 0.50% (v/v) surfactant for most parameters evaluated.

Studies were conducted to determine effects of preharvest applications of glyphosate on the seed and seedling quality of ‘Alpowa’ and ‘Penawawa’ soft white spring wheat (Triticum aestivum) varieties. Glyphosate was applied at 0.62 or 0.84 kg ae/ha at the milk (Zadoks' scale 70 to 79), soft dough (Zadoks' scale 85), or hard dough stage (Zadoks' scale 87) of wheat development; 7 d following the hard dough treatment; and 1 d prior to wheat harvest. In harvest aid applications, wheat yields were reduced only with glyphosate applied at the milk stage of development. Yield reduction ranged from 20 to 77% depending on the year, variety, and glyphosate rate. Likewise, kernel weight and germination were affected only by glyphosate applications at the milk stage with reductions from 19 to 73% and from 2 to 46% for kernal weight and percent germination, respectively, compared to untreated wheat. Using wheat from harvest-aided glyphosate treatments at the milk stage as seeds the following year resulted in reductions ranging from 28 to 99%, 19 to 39%, and 12 to 97% for seedling density, plant height, and seed yield, respectively, compared to seeds from untreated wheat. In this study, wheat seed and seedling quality following preharvest glyphosate applications were most greatly influenced by crop maturity stage at time of application than by herbicide rate or variety.

Field experiments were conducted in 1994 and 1995 at Vegreville, Legal, and Lacombe, AB, to determine the effects of a preharvest application of glyphosate on seedling emergence and growth of field pea. Glyphosate was applied at 0.9 kg ai/ha at each of the three crop development stages, as determined by seed moisture content (SMC), to determinate (‘Ascona’ and ‘Radley’) and indeterminate (‘Miko’ and ‘Trapper’) cultivars. Applying glyphosate when the SMC was less than 30% had little to no effect on seedling emergence but reduced seedling shoot fresh weight in two of six experiments. Applying glyphosate at SMC above 40% reduced seedling emergence and shoot fresh weight in two and three of the six experiments, respectively. Reductions in seedling emergence and shoot fresh weight were greater from seeds collected from the top than from seeds collected from the bottom one-third of sprayed plants. Differences in response between determinate and indeterminate cultivars occurred, but there was no consistent trend. Given the variable maturity in most fields and on individual pea plants, applications of preharvest glyphosate to peas destined for seed production may decrease seed germination and biomass accumulation.

The effect of two commonly used desiccant tank mixtures on Glycine max seed production and viability when applied at G. max growth stages R5 to R8 (R5 = beginning pod fill, R6 = full green pod, R7 = beginning maturity, R8 = full maturity) was evaluated. Two mid-Group IV G. max cultivars, one conventional (‘Northrup King 4260’) and one glyphosate-tolerant (‘Asgrow 4701RR’), were evaluated. Tank mixtures containing either glyphosate + sodium chlorate or paraquat + sodium chlorate + surfactant reduced yield, seed weight, and subsequent germination, emergence, and seedling growth when applied at R5 or R6 to either G. max cultivar. Glyphosate + sodium chlorate applied at R7 reduced germination of the glyphosate-tolerant cultivar. When applied at R6 or R7, glyphosate + sodium chlorate reduced the next generation's seedling length compared to paraquat + sodium chlorate. Glyphosate + sodium chlorate increased the number of abnormal seedlings produced in the conventional cultivar compared to paraquat + sodium chlorate. These results indicate that preharvest desiccant applications should not be made prior to G. max maturity (R7).