Balancing diets for physically effective fibre and ruminally degradable starch: A key to lower the risk of sub-acute rumen acidosis and improve productivity of dairy cattle

Finding an optimal balance between the amount of physically effective fibre and rumen fermentable carbohydrates in the diets of high-producing dairy cows is difficult, but critical to prevent sub-acute ruminal acidosis (SARA), optimize digestion, nutrient utilization, and improve productivity. The main aim of the present review was to provide a summary of recent achievements regarding the role that interactions between dietary factors such as grain fermentability and physically effective neutral detergent fibre (peNDF) have on rumen fermentation and SARA as well as on production performance in high-producing dairy cows. The review first considers factors related to grain fermentability and finds that the latter variable depends mainly on the source of starch as well as in processing and conservation method; however, dairy cow's response to grain fermentability should be considered in relation with the total amount of starch and/or concentration of peNDF in the diet. Forages and current methods used in the evaluation of their physical characteristics were also reviewed suggesting the use of Penn State Particle Separator as an adequate approach to determine the distribution of particle fractions as well as the content of peNDF in both forages and total mixed rations. Furthermore, results reviewed in this study suggest that a content of peNDF>₁.₁₈ (expressed inclusive of particles-dry matter >1.18mm) in the diet of about 30-32% is sufficient in maintaining a daily mean of ruminal pH of 6.2, lowering the risk of SARA, and preventing milk fat depression without exerting any negative effects on daily feed intake and milk production. The review concludes with a practical recommendation in terms of balancing diets with peNDF>₁.₁₈ and ruminally degradable starch, suggesting that a ratio between the two variables lower than 1.45 should be avoided when formulating diets for lactating dairy cows.