Source:  Nutrient Requirements of Beef Cattle, Dr. David Lalman, OSU: Nutrient Requirements of Beef Cattle | Oklahoma State University
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In the winter months, many people start questioning whether or not they should supplement their livestock with high energy or high protein supplements.  The answer really comes down to your hay quality and the physiological status of the animals you're feeding (i.e. growing, gestation, lactating, or maintenance). 

Beef cattle require a minimum of 7% crude protein in their diet to keep the rumen bugs alive and working.  Below that percentage, digestion slows and the animal gets less nutrition out of every bite.  Thus, technically, as long as your hay has 7% protein or higher, your livestock does not need a protein supplement to keep the gut working.  However, due to physiological demands of keeping warm during cold winter nights and changes in fetal growth throughout the pregnancy, higher protein and energy levels are often needed to maintain body weight and promote fetal development (especially in the third trimester).  Heifers, steers, and yearlings also require added protein and energy to support growth. 

David Lalman, Associate Professor at Oklahoma State University and author of Nutrient Requirements of Beef Cattle E-974, suggests that

Crude protein requirement for beef cows can range from 7.1% to 7.9% during gestation, and up to 10.6% during lactation.   Pregnant yearlings have protein requirements ranging from 8-12.1%, and growing calves need even more protein (8-22%) depending on their body weight.  Bulls, on the other hand, are pretty stable at 7% crude protein. 

Energy requirements also fluctuate with physiological status.  Total digestible nutrients (TDN) is the sum of all the digestible nutrients in a feedstuff and is used as a common measurement for Energy.  Dr. Lelman's report also suggests that

TDN values should range from 50-60% for mother cows; up to 70% for pregnant replacement heifers; up to 80% for growing calves; up to 90% for growing yearlings; and up to 80% for bulls.

With regard to which types of supplements you should use, there are too many types to name.  You could do lose supplements, such as grains, cottonseed hulls, brewer's yeast, or beet pulp.  Alternatively, you could supplement with less wasteful options, such as cake and tubs.   The amount and type of supplement you feed will depend on the nutritional value of that supplement you choose with respect to your desired protein and energy levels. 

Keep in mind that dry matter intake typically ranges from 1.5-3% of a mother cow's body weight.

The need for feed supplements may be especially important for individuals putting up late hay crops because much of the nutrition in the grass may have already leached out.  

Call our office at 970.724.3456, or email middleparkcd@gmail.com, if you would like information on hay testing (to determine the need for supplementation).

Reference

Nutrient Requirements of Beef Cattle, Dr. David Lalman, OSU: Nutrient Requirements of Beef Cattle | Oklahoma State University

You can also check out this article: Supplementing Beef Cows | Oklahoma State University