Effects of NDF level and ruminally degradable starch on performance of lactating dairy cows fed sorghum diets, ruminal parameters and in situ degradation of nutrients

Abstract

Three experiments were conducted to determine the effects of grain processing and NDF levels on performance, ruminal parameters and in situ degradation of nutrients in lactating dairy cows. Higher efficiency was achieved by using SF diets, with major impact on peak of lactation. Higher starch digestibility and lower fiber digestibility were obtained in SF diets, mainly at low fiber level. In a 72 d trial, four cannulated cows were allotted to four treatments containing 49% or 39% alfalfa hay, with 32 or 40% SF or DR sorghum. Milk yield and intake were increased at the lowest NDF/Ruminally degradable starch (RDS) ratio. Ruminal pH and ammonia were decreased in SF diets mainly at low forage level. Ruminal fermentation was stimulated in SF diets with low forage level, resulting in a faster passage rate of the solid phase, however with reduced passage rate of the liquid phase. In a third trial, four cannulated cows were used in a 4 x 4 Latin square design to determine the effects of RDS and NDF levels on in situ degradation of nutrients. Alfalfa hay, sorghum grain (DR or SF) and diets were incubated on experimental diets ranging from 2.3 to 1.3 NDF/RDS ratio. Low ratios stimulated starch degradation and decreased fiber degradation. SF diets improved by 4.9% DM and by 7.5% starch degradation when compared with DR. Maximum degradation rates of diets were obtained at intermediary NDF/RDS ratios due to the associative effects.