A heavy harrow as they are called have much longer and larger diameter teeth than the smaller spring tooth harrows. Frames and everything is much heavier too.
A packer harrow bar use regular spring tooth harrows, very similar to mounted harrows on a cultivator. While they do a nice job of leveling loose soil, they are not effective at beating apart sod lumps, nor are they very good for spreading heavy straw.
Adding packers on to the back,well thats pretty self explanatory, and while they will break up some of the dry lumps of soil, they do nothing to sod, other than make it slightly smoother for the next pass. In regular conditions, they also do push in the smaller rocks if the ground is soft enough.
My advice is if you go with a packer bar, if you have rocks make sure you get one that has the harrows then the packers, not the other way around, otherwise all your rocks are on top of the ground. Mine has the P30's, and they do a excellent job and the herringbone pattern left behind certainly does reduce washouts on side hills during downpours till the crop is established.
Not exactly sure what you are referring to when you say "newly broke land", so it makes it a bit difficult to know what kind of work you really want to do with it. I'm guessing you mean tearing up old sod, not actually "breaking" new land as in with stumps and roots.
I'm sure everyone else will tell you differently, because most don't think it's cool to use them anymore, but nothing will break up sod lumps quicker and better than a simple set of 42" diamond harrows. Spring tooth harrows have too much give in them to do a really good job.
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