Intensive Grazing/Seasonal Dairying: The Mahoning County Dairy Program

1987-1991

OARDC Research Bulletin 1190

Chapter 11: Soil Pesticides, continued

Separate plots (4 m x 4 m) were established in four different areas of the rotational grazing pastures. Three of the four plots were located in old orchard areas. The plots were established based on the concentrations of DDT and DDE in the soil, which were determined the previous years. The fourth plot was located in an area of pasture shown to be free of pesticide residues. Grasses in 1 m square sub-plots were harvested at two-, four-, or six-week intervals during an 18-week study. These grasses were extracted differentially to determine the amounts of DDT and DDE residues which adhered to the plant surface and those which were associated with plant tissues.

The results of these studies demonstrated that several important occurrences were taking place on the old orchard plots. The grasses did contain significant concentrations of DDE (Figure 11.3) and only traces of DDT. Residues of the DDT and DDE were not adhering to the outer surfaces of the plant. It previously has been shown that hard rains can cause soil to splash onto plants and become a source of contamination. This was not the case in this study. Concentrations of DDE detected in the grass material related to concentrations of residue in the soil of the respective plots. Residues were not detectable in samples of stems when stems and foliage were separated, suggesting that this material was not moving from the root system through the plant. The most logical conclusion was that the residues were volatilizing from the soil and recondensing on the plant foliage. This mechanism was confirmed when ethylene glycol-treated filter paper traps were suspended 15 cm above the soil surface. As with the grasses, the concentrations found in the filter paper traps were related to the concentrations located in the soil of respective orchard paddocks (Figure 11.4).

If volatilization is the major means of movement of the residues from the soil to the plant materials, then meteorological conditions should influence the transfer. Fortunately, an official semi-automated weather station was located approximately 800 m from the experimental pasture plots. The station provided daily weather information on maximum and minimum air and soil temperatures, wind, precipitation, solar radiation, relative humidity, and water balance. Only daily precipitation was related to the movement of residue. In fact, periods of precipitation were responsible for increases in the DDE content of grass (Figure 11.5). Interestingly enough, periods of rain also were associated with periods of highest residue content in the milk during the second experimental year (Figure 11.6).

Figure 11.2. Concentration of DDE in pooled milk samples during the 1989 pasture season.

Figure 11.3. Concentrations of DDE in grasses from three old orchard plots.

The mechanism of volatilization of the pesticides from the orchard soils plus the influence of water content of the soil on volatilization was confirmed in laboratory experiments. In these carefully controlled studies, DDE volatilized from soils taken from the three orchard plot sites. The relationship between soil concentration and the amount detected in laboratory traps was similar to the results of the traps placed on the field plots and residues in grasses. Studies were also conducted where known amounts of DDT and DDE were added to the soil. Only trace quantities of the pesticides volatilized and were detected in the traps when the soils were dry. With the addition of 20% or more of water, DDE was readily volatile.

Other studies have been published that show the water content of the soil was extremely important in order to promote volatilization of pesticides from soil. DDE also has been shown to be eight times more volatile than is DDT. Although there were substantial quantities of DDT in the soil, only DDE appeared in the grasses in significant concentrations. DDT was not identified in the milk of cows but would not be expected to be found, because DDT is rapidly metabolized to DDE in animals.

Figure 11.4. Filter paper/ethylene glycol traps were suspended 15cm above the soil for 14 days. Plots 9, 24, and 26 were within the old orchard plots while plot 28 was a control. See Figure 11.1 for plot locations.

Utilizing the aforementioned equation for the quantity of DDE in the milk fat and the daily dose, the amounts of residue in the grasses accounted appropriately for the amount of residue in the milk fat. Utilizing the herd used for the intensive grazing study, the following calculations can be made. The average body weight of the cows was 550 kg with an assumed daily dry matter intake of 3% of body weight or 16.5 kg. Of that, 6.55 kg were from grain supplement. The remaining 9.95 kg of dry matter intake consisted of grass. Utilizing the aforementioned predictive equation for residue excretion in milk fat, the cows with .3 micro g/g in milk fat were consuming approximately 1.1 mg/ day. That amount of residue was easily obtainable with cows consuming 9.95 kg of dry matter from grass that contained .11 mg/kg of DDE. Following periods of precipitation, that amount of residue was found in the harvested crass (Figure 11.5). Concentrations of .02 to .04 mg/kg in the grass would easily support concentrations of DDE in milk fat of .1 micro g/g. When considering potential contamination of livestock by organochlorine pesticides from soil sources, it is extremely important to account for concentrations in the soil and the effect of precipitation, which can influence the rate and amount of volatilization. Additional studies will be helpful to define precisely this relationship.

Figure 11.5. The relationship between the concentration of DDE in the grass and occurances of precipitation.

Figure 11.6. The relationship between the average concentration of DDE in milk fat and the total precipitation while cows were on an individual paddock. Cows were rotated among uncontaminated and old orchard paddocks 7 and 8.

Contamination of livestock feed with residues from past agricultural practices has often been inconsistent. Feed harvested during one year or season has been relatively free of residue while, at other times, unacceptable residues are present. The serendipity of these experimental plots in close proximity to an official weather station has provided a major key to understanding the role of precipitation in the transfer of organochlorine pesticides from soil to cows via forage. This understanding may play a major role in the management of land that previously had been exposed to residual pesticides so that pesticide residues can be excluded from the human food chain.

Acknowledgements

The authors would like to thank Steve and Dianne Shoemaker for herd management data; technical assistance provided by Leslie Fluharty, David Leighty, and Elizabeth Smith; and the ancillary studies on volatilization of DDE from soil conducted by Lisa Willett.