Key Challenges to Dairy Cattle Production in the Midwest Environment: Forage Quality

The objective of this presentation is to discuss challenges to optimizing forage utilization by dairy cattle in the Midwest. Corn silage, alfalfa, grasses, co-product feeds and crop residues are widely used in Midwest dairy cattle diets. These materials provide fiber (NDF) and energy to dairy cattle diets. In diets for high producing dairy cows about 20 to 25% of the energy for milk production comes from digested fiber. The digestibility of NDF is more variable than the digestibility of any other feed component and can profoundly affect intake and milk production. Variation in total tract fiber digestion can account for enough digestible energy to support as much as 4 to 5 liters of potential milk yield. Fiber digestion is affected both by characteristics of the forage and by the animal consuming the fiber. To accurately predict how fiber will be utilized, laboratory measures that predict the rate of fiber digestion and the proportion of total fiber that is potentially digestible are needed. The rate and potential extent of NDF digestion are heavily influenced by the genetics and growing environment of the forage. Fiber digestion is also affected by the rate of passage of the potentially digestible fiber through the animal’s rumen and hindgut. University of Wisconsin researchers have developed an in vitro assay and model for predicting the total tract digestibility coefficient for NDF (TTNDFD). The TTNDFD value is benchmarked to fiber digestibility values that have been obtained from feeding studies where NDF digestion has been directly measured. Total tract fiber digestibility is reported because this value can be used not only to predict in vivo fiber utilization but also to predict forage DE, NE or TDN values. The in vitro method has been calibrated to Near Infrared Spectroscopy (NIR) so that kd and iNDF fractions can be predicted quickly and with little additional cost. Several feeding studies have been conducted with various forages to test the model and to validate that the in vitro estimates of digestion and passage are consistent with what is measured in cattle fed diets containing the test forages. Optimizing use of forages and fiber in dairy diets requires accurate, precise measures of the amount of NDF and NDF digestibility. The TTNDFD assay and model provide information that can used to evaluate forage quality and optimize forage utilization by dairy cattle.