Predicting small intestinal starch digestibility in the horse using in vivo and in vitro assays.

Abstract

13A Predicting small intestinal starch digestibility in the horse using in vivo and in vitro assays N. McGilchrist, J.B. Rowe and G.N. Hinch Animal Science, University of New England, Ar midale NSW 2351 nmcgilch@metz.une.edu.au The site of starch digestion in the horse is important because an increase in digestion in the small intestine will reduce hindgut fermentation and consequently reduce the incidence of health disorders related to acidosis (Rowe 1999). The assessment of feeds in terms of in vivo small intestinal starch digestion is difficult because techniques have not yet been established. The present experiment examined the value of the glycaemic responses of horses to various grain diets as an in vivo procedure (Pagan 1999), and of an in vitro assay of grains (Bird et al . 1999), as predictors of starch digestibility in the small intestine. Four thoroughbred geldings weighing 480 � 40 kg were each given 8 kg lucerne hay per day and, twice daily, four diets according to a latin square design: steam rolled triticale; expanded tritical; expanded barley; and a totally digestible glucose control diet. Each diet provided the equivalent of 670 grams of starch or glucose per feed. Horses remained on each diet for three days. Blood samples for measurements of glucose concentrations by COBAS autoanalyser were taken on the morning of the third day before feeding, then every 15 min for 1 h following commencement of eating, and then every 30 min for 4 h. The in vitro enzyme digestibilities of the grains (Bird et al. 1999) were determined. Blood glucose peak and average concentration, peak minus basal value and area under the curve were analysed using an ANOVA. Diet, horse and period were included in the model. The glucose diet generated significantly greater peak (P) blood glucose and peak minus basal (P_B) concentrations (P<0.01)than expanded barley; the values for steam rolled triticale and expanded triticale did not differ from the glucose diet. Mean glucose concentrations with the expanded triticale and expanded barley diets were significantly lower (P<0.01) than in the glucose control. There were no significant differences between treatments for area under the curve. Average time to peak concentration was 108 min. There were significant differences between horses (P<0.05) for peak, average and P_B concentrations. Both assays ranked steam rolled triticale as the most digestible grain, and showed that was the processing most ef fective for increasing starch digestibility (Table 1). Digestibility of the expanded triticale was ranked lower by the in vitro assay, and triticale was more digestible than barley both in vitro and in vivo. These techniques are likely to be useful in predicting starch digestibility in the horse small intestine and will assist in improving feed processing techniques and grain selection. Bird, S.H., Rowe, J.B., Choct, M., Stachiw, S., Tyler, P., and Thompson, R.D. (1999). In vitro fermentation of grain and enzymatic digestion of cereal starch. Recent Advances in Animal Nutrition in Australia 12, 53_61. Pagan, J.D. (1999). Recent developments in equine nutrition. Cornell Nutrition Conference for Feed Manufacturers 61, 160_167. Rowe, J. B. (1999). How much acid in the gut is too much? Recent Advances in Animal Nutrition in Australia 12, 81_89. Table 1 Means � SE for plasma glucose parameters area under the cur ve (AUC), peak (P), average (AC) and peak minus basal (P_B) concentrations and in vitro starch digestibility (%). Steam rolled triticale SE 144.9 a a a Glucose Mean AUC (mmol/L*min) P (mmol/L) AC (mmol/L) P_B (mmol/L) 825 10.3 7.8 5.6 100 Expanded triticale Mean 560 8.8ab 6.4ab 4.0ab 54.1 SE 127.1 0.62 0.41 0.60 Expanded barley Mean 509 8.2 6.2 3.5 44.9 b b b Mean 739 9.4ab 6.9ab 4.7ab 77.0 SE 162.4 0.62 0.50 0.56 SE 93.9 0.38 0.26 0.34 1.06 0.72 0.89 In vitro Within rows, values with different superscripts differ significantly (P<0.01) Recent Advances in Animal Nutrition in Australia, Volume 13 (2001)