A 2 PARAMETER MODEL FOR PREDICTION OF CROP LOSS BY WEED COMPETITION FROM EARLY OBSERVATIONS OF RELATIVE LEAF-AREA OF THE WEEDS

The recently introduced empirical model for early prediction of crop loss by weed competition based on the relative leaf area of the weeds shortly after crop emergence (Kropff and Spitters, 1991), assumes a maximum yield loss of 100% at high weed infestations. This is biologically not realistic. If weeds have a shorter life cycle than the crop, when they emerge much later than the crop or when they are unable to overtop the crop, maximum yield loss at high weed infestations is expected to be less than 100%. A two parameter model is introduced in this paper that accounts for a variable maximum crop yield loss by weed competition. The parameters are the ''relative damage coefficient'' q, and a parameter that describes the maximum yield loss caused by the weeds (m). The one and two parameter models were evaluated with data on the effects of weeds on rice, sugarbeet and tomato. Both models were able to describe the data on the effect of different weed densities and periods between crop and weed emergence fairly well. The introduction of the second parameter resulted in a significantly better description of observed data in five out of nine datasets. The value of the parameter q depends on the time of observation of the relative leaf area. The change in the value of q in time could be estimated from the difference in relative growth rate of the species. The accuracy of this estimation is strongly determined by the variability in the relative growth rate of the leaf area of single plants. Possibilities for scientific and practical application of the model are discussed.