In “Secretion of DDT in Milk by Fresh Cows”, Brown, Witt, Whiting, and Stull noted that several studies had considered pesticides used on nearby crops as the main source of pesticide residues found in milk because they contaminated animal feed (Brown, Witt, Whiting and Stull, 1966). However, analysis of pesticide levels from animal feeds did not correspond to the amount of pesticide residue found in milk.
Studies have strived to demonstrate the source of discrepancies in the level of pesticide residues found in milk and what researchers predict should be the quantity of pesticide found in the milk. They have concluded that there might be some relationships between high levels of pesticide residues found in milk and lactation stages in cows. In other words, freshly lactating cows produced high amounts of pesticide residues in milk than cows that were in the late stages of lactation.
Proponents of this idea believe that cows lose significant body weight within a few weeks after parturition, which they believe comes from the stored body fat. According to this idea, cows store insecticide in their body fats. Therefore, the breakdown of body fat results in the release of the stored pesticide residues, which result in large quantities of insecticide in the secreted milk. Researchers have tested this theory by using milk samples from fresh cows and samples from cows that are in the late stages of lactation. These cows are in a similar environment and consume the same diet with the same dietary exposure to pesticides. However, such studies have shown conflicting results in the level of insecticides present in the milk of fresh cows and cows in the late stages of lactation. Hence, it is difficult to determine whether or not fresh cows secreted high amounts of insecticides in their milk. Moreover, Laben and colleagues study showed a reduction in the level of DDT in the milk fat “immediately after parturition for a period of 40 weeks in a sequence of samples taken from the same cows” (Brown et al., 1966).
The hypothesis that fresh cows could be responsible for the disproportionate levels of insecticides in the sampled milk has focused on the possibility of such occurrences in the first two or three weeks after parturition instead of the initial first half of the milking season.
This study aimed to examine the number of insecticides in the milk during the initial periods of lactating through analyzing milk samples to identify ‘short term fluctuation’ caused by weekly changes in the composites. The sample period was 14 days. Data indicated that the level of “DDT and its degradation products in the milk fat ranged from 0.58 to 0.79 ppm” (Brown et al., 1966). However, the result showed a lack of consistency in both downward and upward trends in the amount of DDT and its products within the 59 days of the study. There was a slight but not significant decrease in the daily production of milk fat within the first seven days after parturition. Fluctuation in the level of pesticide and fat production in cows over time did not indicate a consistent trend and were not statistically significant.
The article concluded that there was no sufficient evidence from this study and past studies to support the continued belief that fresh cows contributed to high-levels of pesticide in the milk because of the unique release of the stored pesticide in their body fats. Hence, the problem of pesticide levels in milk does not result from the unique release of stored pesticides in body fats of fresh cows.
Reference
Brown, H. W., Witt, J. M., Whiting, F. M., and Stull, J. W. (1966). Secretion of DDT in Milk by Fresh Cows. Bulletin of Experimental Contamination & Toxicology, 1(1), 21-28.