First, let's assume that the grain has been harvested from the fields that all the forages come from. Soybean stubble bales are the least favorite of the three forages listed. Soybeans are a legume, and as they increase in maturity, they increase in ligin. Ligin is not digested by the rumen microbes. Forage tests reveal that soybean stubble that was baled after harvest are about 38 percent TDN and 3.9 percent protein. These numbers are less than wheat straw. If you had alfalfa and wanted to stretch it, then soybean stubble might be an option for cows prior to calving. Forage tests for corn stalk bales have shown a wide variability in quality. Test ranged from 47 percent to 54 percent TDN and 4.5 percent to 6.5 percent crude protein. Fifty-four percent TDN and 6.5 percent crude protein would be similar to average grass hay. The higher the amount of husk and leaf in the bale, the greater the nutrient quality. Some producers may windrow the stalk field, rake, and bale. This would generally decrease quality because of the added stalk (stem) amount in the bale. If the residue that came out of the combine were raked then baled, there would be more husk and leaf in the bale. Baled milo stalks would be similar to corn stalk bales. The more the stalk, the lower the quality. Bottom line, you will need to sample the bales and test for quality -- percent crude protein, percent TDN, and moisture.
If the bales are fed free-choice, the cows will sort out the stalks and cobs. So the diet may be pretty high. If the bales are ground and fed, there will be less sorting, and you may be able to add other feeds to stretch higher quality feed or add other feeds or forages.
As forages mature, nitrates decrease. If nitrates are present, they will be located in the lower portion of the stalk. Nitrate testing corn stalk bales and milo stubble bales is highly recommended.