why are some of you discussing minerals? not paying attention to the question is one of the things that really irks me. the original poster was clearly asking about using salt to limit consumption in a self-fed protein supplement of either cottonseed or soybean meal.
the option that one poster mentioned of only putting out what you want them to have is not an option with self-fed supplements. your cows will never satisfy their desire to 'gorge' themselves if you don't keep it in front of them at all times.
to solve your dilemma, first you need to figure out how much protein you need to deliver. in other words, are your cows wet or dry, what is the quality of your roughage, etc. there is no point in trying to cut consumption if you are delivering what the cow actually needs.
if you see that you are delivering more crude protein than the cow actually needs, then you can add salt until you reach a ratio that will deliver what she needs.
you also need to know what ratio of meal to salt you are feeding. some common mixes are 1-1, 2-1, and 3-1 meal-salt.
also you should keep in mind that while the crude protein shown on the analysis/tag might vary only slightly, the actual crude protein delivered to the cow will vary tremendously.
for example, a 1-1 mixture of 41% cottonseed meal to salt has a crude protein analysis of 20.5. a 2-1 mixture has an analysis of 27.3 and the 3-1 mixture has a crude protein analysis of 30.75.
as you can see, there isn't much difference in crude protein of the 2-1 mixture and the 3-1 mixture so it might seem a better value to you to use the cheaper product. but the consumption will vary quite a bit so the actual protein delivered to the cow will also vary quite a bit.
as a general rule of thumb, you can figure that a cow will consume approximately .1% of her body weight in salt if forced to do so to get what she wants/needs from a supplement. hence, a 1300 pound cow will eat 1.3 pounds of salt in a day.
using the .1% as a general rule of thumb, if you are using a 1-1 mixture of 41% cottonseed meal to salt, that 1300 pound cow will consume 1.3 pounds of salt and 1.3 pounds of meal. a 2-1 mixture will deliver to her 1.3 pounds of salt and 2.6 pounds of meal. a 3-1 mixture will deliver to her 1.3 pounds of salt and 3.9 pounds of meal.
so if we assume a 41% protein cottonseed meal, a 1-1 mixture will deliver to your 1300 pound cow approximately .53 pounds of crude protein. the 2-1 mixture will deliver approximately 1.06 pounds of crude protein and the 3-1 mixture will deliver approximately 1.6 pounds of crude protein.
figure out how much supplemental crude protein your cow needs before you start shorting her because you think she is eating too much. the first step in doing that is to have your hay tested.