A concentrated cattle feed, like a biscuit or hard block, usually made up of various seeds and high in protein. Some common ingredients are linseed, cottonseed, rapeseed, canola, soy, and molassus (to improve the taste, for one). In many cases, such as cottonseed cakes, they're made from the dry by-products remaining after the seeds have been pressed to obtain their oil.
John R. Erickson writes in The Modern Cowboy that cottonseed cakes, consisting of 41% protein, were once the most common protein supplements on American ranches. He mentions, however, they've fallen slightly out of use in modern times with the introduction of alternate feeds and the speculation of animal nutritionists that such a high-protein feed is unneccessary and can't properly be utilized by cattle. Feeds with lower protein counts, which retailed at lower prices than traditional cottonseed cakes, seemed to work just as well and were readily adopted.
During the Second World War, an Allied scheme dubbed Operation Vegetarian planned for an airdrop of five million anthrax-spiked linseed cattle cakes on German pastures, in hopes of hobbling Nazi beef production by as much as 30%. The plan was eventually scrapped, but not until after the production of more than a million of the cakes, which were ultimately destroyed.
While researching this node, I also found another interesting World War II anecdote, from a British woman who was a child in Beaconsfield in Buckinghamshire when the war started and recalled having to eat cattle cakes to supplement her diet when the rationing requirements were particularly strict:
I remember we were rationed and we didn't have ice-cream, sweets, or crisps, and there was no fast food. You didn't have butter on your bread, only jam. We used to eat cattle cake sometimes if we were hungry - it looked and felt llke hard toffee and was quite good! However, I don't think I would eat it now if it were offered to me!
The WWII recollection found at http://www.bbc.co.uk/ww2peopleswar/stories/58/a8899258.shtml