Field studies were conducted over a 7-yr period at Lacombe, Alberta, to study the relationship between the duration of Tartary buckwheat interference [Fagopyrum tatarium (L.) Gaertn. # FAGTA] and yield of barley (Hordeum vulgare L.), oats (Avena sativa L.), wheat (Triticum aestivum L.), flax (Linum usitatissimum L.), and rapeseed (Brassica campestris L.). The data were pooled over years and analyzed by multiple regression. The equations were as follows: ŷ = 15.46 + 0.39X1 + 0.00x2 −0.11x3 (barley), ŷ = −15.44 + 0.49x1 + 0.02x2 + 0.08x3 (oats), ŷ = −2.04 + 0.39X1 + 0.05x2-0.03x3 (wheat), ŷ = −4.38 + 1.14x1 −0.04x2 + 0.01x3 (flax), and ŷ = −13.85 + 0.40x1 – 0.01x2 + 0.04x3 (rapeseed); where ŷ was the estimated percent yield loss of the crop, x1 was the duration (days) of the Tartary buckwheat in the crop, x2 was the number of Tartary buckwheat plants/m2, and x3 was the number of crop plants/m2. The time that Tartary buckwheat remains in the crop contributed most to the yield loss observed in all crops. Yield loss between 0.4 and 1.1% per day was attributed to this variable alone. For a given x1, x2, and x3 value the order of percent yield loss was flax>oats>wheat> barley>rapeseed.

The relationship between Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn. ♯ FAGTA) plant density (x) and percent yield loss (ŷ) was expressed by the following linear regression equations for barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.), respectively; ŷ = 0.63 + 2.75 √x and ŷ = 5.04 + 3.05 √x. Tartary buckwheat causes serious yield reduction in barley and wheat. A Tartary buckwheat density at 30 plants/m2 at emergence reduced barley yield by 16% and wheat yield by 22%.