None, its a closed system (actually its a negative system if no feed comes from off the farm as the cows do retain some of the nutirents .) If the cows graze off the land and eat hay off the land then the only nutrients in the manure are what was there to start with. The cows eat the grass but they arent adding anything.
Now, any nutrients brought into the farm in the form of feed are additions to the system. Thats how dairy cows, chickens, etc add to the total fertility on a farm, so much off farm feed going into them. Most of the nutrients in whatever the animals eat except energy and protein do go out the back end. Any feed that comes in adds its nutirents to the system. If he is buying hay and corn then that does add fertilizer value but it depends a LOT on what they are eating. If the cows are being fed purchased grain then there is a large contribution of phosphorous to the farm. If they are eating alfalfa there is a lot of potassium and calcium going out the back side of the cow. Grass hay, less calcium and potassium but still its a net gain.
With all that said there is an organic nitrogen contribution to the system. But that comes from excess protein that is metabolized by the animal. It still came from origional soil nitrogen in someones soil. My pastures now test high in P and K, and rarely need nitrogen due to the amount of manure they get. Particularly the field where I feed. But its not 'new'. Its simply plant nutrients that come out of another field where the hay is grown. Those nutrients either have to go back as manure or in my case come in a truck from the co-op as fertilizer and synthetic nitrogen.
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Today's Featured Article - New Hitches For Your Old Tractor - by Chris Pratt. For this article, we are going to make the irrational and unlikely assumption that you purchased an older tractor that is in tip top shape and needs no immediate repairs other than an oil change and a good bath. To the newcomer planning to restore the machine, this means you have everything you need for the moment (something to sit in the shop and just look at for awhile while you read the books). To the newcomer that wants to get out and use the machine for field work, you may have already hit a major roadblock. That is the dreaded "proprietary hitch". With the exception of the
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