Hydrolyzable carbohydrates in pasture, hay, and horse feeds: Direct assay and seasonal variation

Carbohydrates may be hydrolyzed or fermented in the digestive tract, and th is distinction is important for the evaluation of the diet of herbivores. B oth hydrolyzable and fermentable carbohydrates are included in the nonstruc tural carbohydrate (NSC) fraction as estimated by difference using proximat e analysis. Our objectives were to measure hydrolyzable carbohydrates in fo rages and concentrates, to compare these values with nonstructural carbohyd rate, to test for prediction of hydrolyzable carbohydrate concentration in forages from its near-infrared spectrum, and to examine seasonal variation of carbohydrates in pasture. Samples of forages (107) and concentrates (25) were collected, dried, ground, and analyzed for NSC (calculated as 100 - w ater - CP - fat - ash - NDF), hydrolyzable carbohydrate (CHO-H, direct anal ysis), and rapidly fermentable carbohydrate (NSC minus CHO-H). Hydrolyzable carbohydrate accounted for 97% or more of the NSC in the concentrates but only 33% in pasture and hay. A two-term polynomial equation fit all the dat a: CHO-H = 0.154 x NSC + 0.00136 x NSC2, R-2 = 0.98, P < 0.0001, n = 132. I n 83 pasture samples, CHO-H concentrations were predicted by near-infrared spectra with a calibration R2 Of 0.97, a mean of 48 g/kg, and a SE of calib ration of 3.5 g/kg DM. In pasture samples collected between September 1995 and November 1996, the coefficient of variation was 31% for both CHO-H and rapidly fermentable carbohydrate (CHO-FR); the largest increments were 31 g /kg of CHO-H from September to October and 41 g/kg of CHO-F-R from February to March. The increased risk of certain diseases, such as laminitis and co lic, that have been previously associated with an abrupt overload of NSC ma y be more precisely attributed to CHO-H in grain concentrates, and to CHO-H as well as CHO-F-R in pastures.