Normally they weigh several hundred pounds less than the manufacturer advertises. When a dairyman is feeding a small standard bale, I normally allow for a five to ten percent wastage depending upon quality and kind of hay being fed. Often times dairymen report consumption figures equal to the total weight of bales fed divided by the number of cows eating the forage. This is a correct figure to use when determining cost, but it is an inaccurate figure to utilize in determining the nutritional intake due to wastage. You can only count the amount actually consumed by the cow.
Pasture is difficult to determine, particularly if it is a native grass or permanent pasture.
Fall and winter small grain pastures and high quality summer temporary can be estimated more accurately. I normally determine dry matter intake from grain and forage first to provide an estimate of additional intake obtained from pasture. Generally, for good producing dairy cows receiving a grain ration, pasture consumption will vary from 30 to 100 pounds on an as fed basis. The wide variation in consumption is due to quality and quantity available. The season of the year has a great influence on pasture consumption.