Soltani, N., Shropshire, C. and Sikkema, P. H. 2011. SHORT COMMUNICATION: Influence of manganese on efficacy of glyphosate in glyphosate-resistant soybean. Can. J. Plant Sci. 91: 1061-1064. Four field trials were conducted from 2007 to 2010 in Ontario to evaluate the effect of various manganese (Mn) formulations (Mill, Ecoman 5% Mn; Mn2, MangaMax 5.5% Mn; Mn3, Man Max 5.5% Mn; Mn4, Superman 500 Mn; Mn5, Stoller This 5% Mn; Mn6, Nortrace 6% Mn-EDTA (ethylenediaminetetraacetate); Mn7, Nortrace 22% Mn and Mn8, WolfTrax 33% Mn) applied at 2.0 kg actual Mn ha(-1) on glyphosate efficacy at 900 g a.e. ha(-1) in glyphosate-resistant soybean. The tank mix of glyphosate plus Mn4, Mn6 or Mn8 caused as much as 6, 17 and 4% injury in soybean, respectively. There was minimal crop injury (0-1.4%) with other Mn tank mixes. The addition of Mn4 or Mn6 to glyphosate did not antagonize glyphosate efficacy on the weeds evaluated (AMARE, AMBEL, CHEAL and SETVI). The other Mn formulations antagonized glyphosate efficacy for the control of AMARE, AM BEL, CHEAL or SETVI under some environments. The addition of Mn3 or Mn6 to glyphosate reduced soybean yield as much as 15 and 10% compared with glyphosate alone, respectively. Based on these results, it is recommended that glyphosate and manganese applications be applied sequentially to avoid weed control antagonism and maximize soybean yield.