Because we know key measures of cowherd productivity (weaning weight, calf survivability, rebreeding rate, calving interval) are strongly correlated to cow body condition score (BCS), we often focus feeding programs on managing the amount of condition the cows will carry. And since BCS is essentially a measure of the animal’s stored energy reserves, there is an intuitive sense that feeding extra energy should improve BCS. However, especially with ruminant animals, the combination of feeds being offered can be as important as the nutrient content of those feeds. As changes in diet impact the balance of nutrients available in the rumen, microbial activity and efficiency are affected, and the net flow of nutrition to the host animal can be enhanced or impaired.
With forage-based diets – especially if the grass or hay is lower quality – the nutrient most critical to balancing the diet of the rumen microbes is crude protein, or nitrogen. Research shows that meeting this need, rather than simply trying to provide additional starch (grain) energy directly to the host animal, is the most effective supplementation strategy. In a trial done by Kartchner (1981), weaned pregnant cows were grazed on dormant winter grass, and supplemented with 1.4 lb of soybean meal or roughly 6 lb of cracked barley. With the “protein” supplement, cows ate more hay, digested more of what they ate, and held condition better than the cows receiving grain. On the other hand, the grain supplement substituted for some hay intake, and depressed fiber digestion. The cows were actually better off with no supplement at all!
This is not to say that small, limited amounts of grain can’t be used to increase the energy density of a beef cow diet. Concerns arise when moderate (say, 4-7 lb/hd/day) levels of grain are added to a high-forage diet. This much grain encourages growth and proliferation of the various species of ruminal bacteria that effectively utilize starch – but not fiber. These “bugs” begin to compete with the bacteria and protozoa that do work to break down fibrous feeds...for nitrogen, for protein, for sugar, and for other nutrients. The fermentation activity of the fiber-digesters is in turn inhibited, and we see the “negative associative effects” of reduced intakes and less efficient forage digestion. Research done in Oklahoma (Chase and Hibberd, 1987) clearly showed the impact of increased levels of supplemental corn. Cows received prairie hay and graded levels of corn, which was mixed with small amounts of cottonseed meal to equalize the protein intake between treatments.
Again, moderate levels of grain fed with a low-quality forage signifi-cantly reduced hay intake and utilization. In contrast, work at KansasState (Koster et al., 1996) fed graded levels of supple-mental protein to beef cows receiving prairie hay.
This is an excellent example of a small amount of supplemental feed being responsible for both doubling the amount of hay consumed, and also allowing the cows to get more out of every bite that is eaten.
It is important to remember that the negative effects illustrated above are due to the starch supplied by grains. Several studies have shown that supplemental energy provided as sugar (as would be contained in molasses-based feeds) has, if anything, a positive impact on fiber utilization. That is because fiber-utilizing bacteria can effectively use sugar for a portion of their energy needs.
It is obvious that effective and efficient supplementation of typical beef cow diets must focus on meeting the needs of the rumen microbes. When degradable protein and sugar (or fiber) energy are provided in adequate amounts, forage intake and utilization increase, and the cow receives not only the nutrients supplied by the supplement, but also significantly greater amounts of protein and energy from the hay portion of the diet. These are the types of supplements that help cows achieve and maintain desirable body condition – not increasing levels of grain.
Source: Dr. Cathy Bandyk, Quality Liquid Feeds