AN APPROACH TO ESTIMATING YIELD LOSS OF BARLEY DUE TO CANADA THISTLE

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Experiments were conducted to determine if a relationship exists between yield loss of barley and various indicators of Canada thistle density. Correlation coefficients between barley yields and total numbers of Canada thistle shoots/m2, numbers of shoots/m2 taller than 0.5 m, numbers of flowering shoots/m2 or shoot dry weight (g/m2), using two representations of the density data (untransformed or square root transformation) were significant at P < 0.01. The data for percent yield loss of barley and total numbers of Canada thistle shoots/m2 were analyzed by regression using both representations of the data and yielded the following equations:[Formula: see text]and[Formula: see text]where ŷ = estimated percent yield loss of barley, x = the total number of Canada thistle shoots/m2 and 0.85 and 7.6 were the indices of competition for Eqs. 1 and 2, respectively. Equation 2 probably provides a more accurate estimate of barley yield loss at low levels of Canada thistle infestation [Formula: see text] whereas either equation is acceptable at intermediate-to-high levels of infestation (5–45 shoots/m2). The 95% confidence limits for Eqs. 1 and 2 and for the estimated percent barley yield losses when four different sample sizes (n = 1, 5, 10 or 20) are used to estimate the thistle density mean are presented. The Eq. 2 index of competition is extrapolated to other crops. Comparing a Canada thistle shoot with a wild oat plant in barley, Canada thistle may be about 3.4 times as competitive as wild oats.