356 Anomalies of Carbohydrate Digestion and Metabolism by Ruminants

Abstract Carbohydrate, derived either from plant cell walls (neutral detergent fiber; NDF) from fresh or conserved forages consumed by cows and growing cattle or from starch of grains and concentrates in commercial feedlot diets, provides most of the dietary energy for ruminant growth and production. The ruminants’ ability to harvest forage from remote, inaccessible locations, to scavenge energy from NDF of feeds and byproducts of grain largely indigestible by non-ruminants, and to employ non-protein N to form protein-rich, nutrient-dense foods for humans are key ecological advantages over non-ruminants. Outlined below are some topics needing research attention. First, feed composition tables invariably indicate that the digested energy (DE) of carbohydrates is 4.4 kilocalories per gram, yet the gross energy (GE) of purified carbohydrate polymers (starch; cellulose, glycogen) averages 4.18; monomers (hexoses, pentoses) average 3.74. Having DE exceed GE violates the first Law of Thermodynamics! Might this 5 percent discrepancy reflect underestimations of fecal lignin or of protein digestibility due to metabolic fecal N? Favoring feeds rich in carbohydrate, this discrepancy confounds formulation of least-cost diets. Secondly, site of digestion of carbohydrate alters its metabolizable energy (ME). Fermentation, though required to yield ME from NDF, generates methane and heat. In contrast, starches and sugars that escape ruminal fermentation, if digested in the small intestine, circumvent these losses increasing their relative ME. Unfortunately, when ruminally fermented, starch, either directly or via pH depression, reduces NDF fermentation; might ruminal NDF digestion be increased by feeding NDF plus buffer versus concentrate diet components asynchronously? Third, carbohydrate metabolites lost in urine (lactate, pentoses, hippuric acid) need further quantification. Finally, ruminally-formed metabolites (methylglyoxal, methane) adversely affect animal health and energetics. Ruminal modification (site- or time-targeted enzymes or inhibitors; altering the ruminal microbiome with pre- or probiotics, inoculants, nutraceuticals) should help enhance ruminant productivity and sustainability.