Crop density

In organic systems the number of hours of hand weeding needs to be reduced, as labour, particularly in the developed countries, is expensive. This can be achieved by competition, the timing of cultivations, adaptation of the rotation nutrient management system and by crop density (Kropff, et al., 2000). Barberi, et al. (2002) experimented with a system of soil disinfection using hot steam to sterilise weed seeds. 

Because parts of the model are used in the Dutch pesticide authorization procedure to estimate worker exposure during re-entry, a study was conducted to validate some of the aspects of the proposed model. Emphasis was put on the relationship between hie applied amount of active ingredient and the resulting increase of DFR in relevant zones (crop heights), as well as determining factors (i.e., application techniques and crop density, or leaf volume index). In addition, the influence of re-entry time and crop density on transfer factors (calculated from levels of re-entry exposure and either DFR or SFR) was studied. 

The results of the DFR assessment of different crop zones indicate that low-volume applications result in a more homogeneous distribution over the crop compared to high-volume applications. A recent study on the interception of high-volume applications in the cultivation of chrysanthemums revealed interception ratios from 0.2 to 1 related to the leaf area index (LAI) (Veerman et al., 1994). In our study, it was not easy to assess the LAI because of the structure of the carnation crop. Estimation of the LAI based on the results of estimation of the crop density (leaf volume index) was not reliable enough and resulted in a large variance of the calculated interception ratio (from 0.4 up to 5). 

Popendorf et al. (1975) suggested a relationship between respiratory exposure and organic foliar dust released from the foliage due to crop disturbance therefore, it was hypothesized that crop density may be associated with both dermal and respiratory exposure. However, in the present study, no such relationship could be observed. This may be due to the large variation in the method used to determine crop volumes. Improvement of the crop-volume/leaf-surface-area method may contribute to the clarification of whether crop density can be considered a determinant of re-entry exposure. 

Half-life estimates of approximately 28 days for thiophanate-methyl indicate a very slow decay compared to methiocarb with an estimate of half-life of about 11 days. The application of a model based on a first-order decay process resulted in fairly high R2 and significant fit. The results suggest that both pesticides are relatively stable compared to other compounds under similar environmental conditions (Brouwer et al., 1994). With respect to the objectives of the study and the proposed model, it can be stated that the results confirm the assumption of a linear relationship between application rate (for both application techniques) and the increase of dislodgeable foliar residue. This relationship holds for modeling purposes. The contribution of the crop density or total crop surface area to the process of interception cannot be quantified with the results of the present study. Because the interception factor ranges from about 0.35 to 0.9 (Willis and McDowell, 1987), the... 

In conclusion it can be stated that the basic assumptions of the re-entry model — a linear relationship between application rate and initial dislodge-able foliar residue and a first-order decay of the DFR — have been confirmed. The relationship between the transfer factor and re-entry time at various DFR levels should be explored further. Including information on foliage surface area or crop density may lead to a refinement of the model however, crop volume estimating methods should be improved before their influence on the exposure processes can be fully evaluated. 

Bierman, E.P.B, Brouwer, D.H., and van Hemmen, J.J. (1997) Measuring crop density comparison of volumetry and stereological methods, Bull. Environ. Contam. Tox., 58 1006-1013. 

Avoid agronomic practices that result in high crop densities and late tillering. 

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