Effects of starch on ruminal fermentation and detergent fibre digestion in lambs fed bermudagrass hay

Abstract

Twelve cannulated Suffolk wether lambs (30.1 kg) were used in a randomised complete block design experiment to determine the effects of increasing dietary concentrate (corn starch-based) on ruminal digestive kinetics and fermentation characteristics and digestion of bermudagrass hay (BGH) fibre in lambs. Treatments consisted of as-fed concentrate:BGH ratios (C:F) of 10:90, 40:60 and 70:30. Ruminal digestion of neutral detergent fibre (NDF) and hemicellulose (HC) declined linearly ( P < 0.01) with increasing C:F. Quadratic responses to post-ruminal ( P < 0.10) and total tract ( P < 0.05) digestion of HC were obtained with increasing C:F. Total tract digestion of NDF declined linearly ( P < 0.05) with increasing C:F. Extent of acid detergent fibre (ADF) digestion was not affected ( P > 0.10) by C:F at any digestive site. Integrated pH-hours below pH 6.7 declined linearly ( P < 0.01) with increasing C:F and was correlated with ruminal digestion of NDF ( r = −0.75, P = 0.005) and HC ( r = −0.67, P = 0.018). Ruminal volatile fatty acid (VFA) concentrations increased linearly ( P < 0.01) with increasing C:F and were correlated with ruminal digestion of NDF ( r = −0.88, P = 0.001) and HC ( r = −0.82, P = 0.001). Ruminal liquid turnover was not affected ( P > 0.10) by C:F. Ruminal forage particulate passage rate tended ( P = 0.12) to increase with increasing C:F and was correlated with the ruminal digestion of both NDF ( r = −0.67, P = 0.023) and HC ( r = −0.57, P = 0.063). While virtually all potentially digestible ADF (cellulose) was digested in the rumen, ruminal escape of HC increased with increasing C:F. Data suggest that HC is the cell wall fraction of BGH that is the most sensitive to dietary starch level.