CJB - A short answer: Deer, like any ruminant, have 4 compartments which makes up the stomach - the rumen, reticulum, omasum and the abomasum. The rumen and reticulum makes up approximately 84% of an adult cow's stomach but a much smaller % of a deer's (sorry - it is late at I cannot currently pull its %). Basically, the rumen (or paunch) is the central holding area for feed and it is where the majority of microbial activity occurs (VFA production - around a half of the starch/sugar is digested here -> energy if you will). The reticulum's (or honeycomb's) purpose is to move the feed back to the rumen until the feed has been broken down into small enough particles to be passed on to the omasum. The omasum (or manyplies) is made up of many folds and its purpose is to absorb water and nutrients. The feed particulate matter then passes to the abomasum (or "true stomach") where hydrochloric acid, pepsin (the enzyme responsible for breaking down protein), lipase (breaks down fat), etc., do their thing prior to the final absorption of the nutrients in the intestinal track. So, as the excess protein passes through to the abomasum where it is directly utilized by the deer, it makes sense to pay attention to protein levels. Hence, the biologists recommend a high protein diet. Also, unlike cattle who are bulk grass/roughage eaters due to their larger rumen/reticulum areas (as a % of body area), deer are considered to be concentrate feeders (as are giraffe). They prefer more easily digested and nutrient rich food products (ie., prefer legumes over a grass pasture - high protein/ starch). Also, they have a limited ability to digest cellulose/fiber when compared to cattle. (It makes sense as they cannot be loaded down with a large belly full of food - nor do they have the luxury of being able to chew a cud for hours after eating). This is probably why you are seeing them in your Dad's soy fields and why they raid the corn fields.
Hope this helps
http://www.thecattlesite.com/articles/2095/understanding-the-ruminant-animals-digestive-system gives a nice breakdown on how all of this functions and explains much more clearly than I have as to why a higher protein diet is beneficial.
