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Deer are like rats here. . )</font>
Yeah, here too. I have never seen where they did much damage to turnips and collards, though. It might be because we always planted large areas, usually five to ten acres, and the damage wasn't as noticeable as it would be on a small plot. I would suggest getting a bag of rye or wheat seed and scattering it within 25-50 yards of your plantings. I think the deer will leave your greens alone if they have something they like better. Cows will eat turnips and collards, but I have had mine get out of the pasture and walk across a turnip field to get to a rye field.
andrewj is right about the sucking bugs, and sevin is probably the most feasible and economic solution. Remember, though, Sevin has a 14 day waiting period between application and harvest. This won't be that much of a problem with collards, but if you get a good rain on the turnips soon after planting, you should be gathering turnip greens within 5 weeks. They will probably need to be thinned to allow the ones you leave to develop roots.
You will have an insect problem with the collards moreso than the turnips. If you can get the turnips through the stage where the little sucking buggers attack them, you should be home free with them. The major pests here on collards are diamondback moths and cabbage loopers. The best bet there is to start early using Bt (Bacillus Therengensies I think it is spelled) or one of the synthetic pyrethrins such as Ambush. The pyrethrins are pretty expensive, but you can probably get a "homeowner" type container for a plot your size. Be sure that any insecticide you use is cleared for greens, especially if you intend to sell to a grocery store.
You might have a weed problem, again, more in the collards than the turnips. When the turnips start growing, they will outgrow most of the weed pests. Collards are much slower germinating and much slower growing. My daddy always said it took three days to get up a stand of turnips, and three weeks to get up a stand of collards. Don't plant more than you can fertilize and keep the weeds out of. I'll repeat that. DON'T PLANT MORE COLLARDS THAN YOU CAN AFFORD TO FERTILIZE, OR MORE THAN YOU CAN KEEP WEEDED BY HAND.
Most gardeners around here think turnips and collards are easy crops to grow, and that is true if you just have one fifty foot row of each, but when you start trying to do it on a larger scale, they become a very expensive and very labor intensive crop. They also produce a large amount of food on a small plot, and can be a real money-maker. I once kept up with a crop of early turnips I got planted during August. I sold over $5000 of turnips from a one and one-half acre patch.