INDICES OF COMPETITION FOR ESTIMATING RAPESEED YIELD LOSS DUE TO CANADA THISTLE

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Abstract
Data were collected from 9 farm fields over 2 yr to determine the relationship between percent yield loss of rapeseed and density of Canada thistle. Regression analyses of the data for percent yield loss of rapeseed and numbers of thistle shoots per square meter, using 2 representations of the data (untransformed and square root transformation), yielded the following equations: .cxa..gamma. = -3.83 + 1.48x (1) and .cxa..gamma. = -18.63 + 10.42 .sqroot.2 (2) where .cxa..gamma. = estimated percent yield loss of rapeseed, x = the number of Canada thistle shoots per square meter, and 1.48 and 10.42 are the indices of competition for 1 and 2, respectively. Equation 1 provides a more accurate estimate of percent yield loss at all levels of thistle infestation. Equation 2 provides acceptable estimates of yield loss at intermediate levels of infestation (5-40 shoots/m2) and is presented for comparative purposes since the index of competition (10.42) is near the value proposed as a result of extrapolation from earlier studies (10.6) which utilized square root transformation of the data. The significance of this is discussed. The 95% confidence limits 1 for the estimated percent rapeseed yield loss when 4 different samples sizes (n = 1, 5, 10 and 20) are used to estimate the number of thistle shoots per square meter are presented. An example of the utility of the linear equation is presented.

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