Building beef cow nutritional programs with the 1996 NRC beef cattle requirements model

ABSTRACT Designing a sound cow-calf nutritional program requires knowledge of nutrient requirements, diet quality, and intake. Effectively using the NRC (1996) beef cattle requirements model (1996NRC) also requires knowledge of dietary degradable intake protein (DIP) and microbial efficiency. Objectives of this paper are to 1) describe a framework in which 1996NRC-applicable data can be generated, 2) describe seasonal changes in nutrients on native range, 3) use the 1996NRC to predict nutrient balance for cattle grazing these forages, and 4) make recommendations for using the 1996NRC for forage-fed cattle. Extrusa samples were collected over 2 yr on native upland range and subirrigated meadow in the Nebraska Sandhills. Samples were analyzed for CP, in vitro OM digestibility (IVOMD), and DIP. Regression equations to predict nutrients were developed from these data. The 1996NRC was used to predict nutrient balances based on the dietary nutrient analyses. Recommendations for model users were also developed. On subirrigated meadow, CP and IVOMD increased rapidly during March and April. On native range, CP and IVOMD increased from April through June but decreased rapidly from August through September. Degradable intake protein (DM basis) followed trends similar to CP for both native range and subirrigated meadow. Predicted nutrient balances for spring- and summer-calving cows agreed with reported values in the literature, provided that IVOMD values were converted to DE before use in the model (1.07 × IVOMD – 8.13). When the IVOMD-to-DE conversion was not used, the model gave unrealistically high NE_m balances. To effectively use the 1996NRC to estimate protein requirements, users should focus on three key estimates: DIP, microbial efficiency, and TDN intake. Consequently, efforts should be focused on adequately describing seasonal changes in forage nutrient content. In order to increase use of the 1996NRC, research is needed in the following areas: 1) cost-effective and accurate commercial laboratory procedures to estimate DIP, 2) reliable estimates or indicators of microbial efficiency for various forage types and qualities, 3) improved estimates of dietary TDN for forage-based diets, 4) validation work to improve estimates of DIP and MP requirements, and 5) incorporation of nitrogen recycling estimates.

Key Words: Cattle, Degradable, Forage, Grazing, Metabolizable, Protein

© 2004, by the American Society of Animal Science. All rights reserved.

J. Anim. Sci. 2004. 82(E. Suppl.):E83-E92

Implications

The computer model in the 1996 NRC Nutrient Requirements of Beef Cattle publication is a useful tool for predicting supplement needs when reliable information regarding protein degradabilities and diet digestibilities are available. In our data, subirrigated meadow increased in nutrients rapidly in the spring and remained higher in quality during the fall, compared with native range. Native range samples were highest in quality during the warmer summer months. Undegradable intake protein, for both subirrigated meadow and native range, was highest during periods of active growth. Beef cows are maintained on a wide variety of forages throughout the United States. Future research efforts should focus on generating additional data for protein degradabilities and microbial efficiency of grazed and harvested forages. In addition, extension programming efforts for extension educators should be focused on increasing knowledge of the metabolizable protein system and its application to the grazing beef cow.