I forgot, 3 oz of cooked beef provides 2.31 mg of iron and the same portion of dark meat chicken provides 1.13 mg of iron. Remember that although plant sources often have higher amounts of iron, they are much less absorbable.

Thanks for the question Franny. To tell you the truth, there isn't much out there in terms of serious nutritional comparisons for conventional beef vs. grass-fed beef, at least beyond the fatty acid content and a few other vitamins such as E and A. Although that seems unfortunate, many people in the grass-fed industry believe that because their beef has been raised in the most natural setting that evolution intended it to be brought up in, there is no need to try to industrialize it by assigning numbers to it.
From what I have found, the USDA figures they have a similar iron content; however, I am weary of how grass-fed their beef was. It may have been grain finished which can have a big effect on the nutrient content that the cow built up during its time in the pasture.
The other reason to believe that their stores of iron will be higher is that if you do an analysis on corn feed vs. grass feed, I will assume that the iron content, per calorie, is much higher in the "leafy green." Just as an example, corn has 1.05 mg of iron per 100 calories and spinach has 5.0 mg of iron per 100 calories. Just a thought..