Many studies find differences between organic and conventional foods, with more favorable results for organic foods (e.g. better nutritionally, lower pesticide residues). A study by Emory University researchers that analyzed samples of conventional and organic milk from different regions of the US adds to the list. They found that the samples of conventional milk contained pesticides, antibiotics, and synthetic growth hormones, but none of these were found in organic milk.
The pesticide levels in the conventional milk varied among the samples, but included frequently used pesticides such as atrazine, permethrin, cypermethrin, chloroyrifos, and diazinon. Chlorpyrifos, found in 59% of the conventional milk samples, is the pesticide that scientists absolutely want banned because of its neurotoxic effects, especially on developing babies and children. They also found that antibiotic residue levels in conventional milk samples surpassed federal limits for amoxicillin (3%), and illegal sulfamethazine (37%) and sulfathiazole (26%). Yikes!
These are important findings because milk is a staple in the diet of many people, especially children. By the way, international milk also can have pesticide and drug residues (e.g. Israel).
But not all organic milk is equal. Unfortunately a number of big so-called organic dairy farms (15,000 to 20,000 cows) are basically factory farms (e.g. Aurora Organic Dairy, Horizon) - they exploit loopholes in organic regulations, as well as deliberately not follow some organic standards. However, even low-quality organic milk has been shown to contain no residues of antibiotics and toxic pesticides - it's just that their milk nutritional profile is different than that of grass-fed organic cows.
Organic milk cows are supposed to be outside grazing during the growing season, at least 120 days a year - thus real organic milk is from "grass-fed" cows. Grass-fed cows (but not conventional and factory-farm organic cows) tend to produce milk with elevated levels of two types of fat: conjugated linoleic acid (CLA) and omega-3 fat known as alpha-linolenic acid. Both have been associated with health benefits in humans, although the amounts found in milk are relatively small. Another type of fat called linoleic acid, (an omega-6 fat), tends to be lower in milk that is from pasture-fed cows.
The Cornucopia Institute has dairy scorecards and rankings of major organic milk brands, as well as other organic foods (e.g. poultry, cereal, eggs). Cornucopia is an organic agriculture watchdog group - an excellent resource to help you choose organic foods.
Some excerpts from the Jean A. Welsh et al. study in the journal Public Health Nutrition: Production-related contaminants (pesticides, antibiotics and hormones) in organic and conventionally produced milk samples sold in the USA
Conclusions: Current-use antibiotics and pesticides were undetectable in organic but prevalent in conventionally produced milk samples, with multiple samples exceeding federal limits. Higher bGH and IGF-1 levels in conventional milk suggest the presence of synthetic growth hormone. Further research is needed to understand the impact of these differences, if any, on consumers.
A total of sixty-nine samples were collected (thirty-four organic and thirty-five conventional). Eight samples from seven of the nine study regions were available for analysis. This included three different brands of organic 2 % milk, three different brands of conventional 2 % milk, and one brand each of organic and conventional whole milk.
Although no detectable levels of current-use pesticides were found in any of the organic milk samples, residues of three legacy pesticides were found in nearly all samples, organic and conventional. This included the organochlorines ppDDT, ppDDE (a metabolite and environmental degradate of ppDDT) and hexachlorobenzene, with levels of the latter two significantly higher in conventional compared with organic milk. While the US Department of Agriculture standards for organic feed production prohibit the use of synthetic pesticides on the land for 3 years prior to certification(46), the half-life of some of these pesticides, such as the once commonly used organochlorines, is 15 years(47). This persistence in the soil on which food for organic milk-producing cows is grown could be the mechanism through which they are exposed to these pesticides.
As growth hormones are produced naturally by dairy cattle, some level of bGH is to be expected in all milk samples, whether produced organically or using conventional methods. We found that the levels of bGH and IGF-1 in conventional milk were significantly greater than those in the samples produced organically.
For an in-depth discussion, from Beyond Pesticides: From Udder to Table: Toxic Pesticides Found in Conventional Milk, Not Organic Milk