This Calf Note was originally prepared as a video Note, based on a presentation prepared and presented to DCHA in September, 2022. The video has been adapted to Tech Seminars at Calf Notes Academy.
Click here to see the video at Tech Seminars
Introduction
The 2021 update to the NASEM (National Academies of Sciences, Engineering, and Medicine) Nutrient Requirements for Dairy Cattle represents a significant step forward in how we evaluate and feed dairy calves. While earlier recommendations provided general guidance, the new system incorporates a more dynamic, model-based approach to estimating nutrient requirements and predicting calf performance. This provides producers and advisors with improved tools to design feeding programs that meet specific growth goals.
Predicting Intake and Growth
One of the most important advances in the new NASEM system is the inclusion of equations to predict dry feed intake in calves. Dry feed intake is influenced by factors such as body weight, intake of energy from liquid feed, environmental temperature, and timing of starter introduction. These relationships highlight a key concept: higher milk intake delays the onset and rate of dry feed consumption, which has important implications for rumen development and weaning management.
The model also emphasizes that energy intake is the primary driver of growth, while protein must be supplied in appropriate proportion to support that growth. The balance between metabolizable energy (ME) and metabolizable protein (MP) is critical to ensure efficient lean growth without excess fat deposition.
Understanding Energy Requirements
Energy requirements in the NASEM model are based on maintenance plus growth. Maintenance requirements depend on body weight and environmental conditions, including both cold and heat stress. Growth requirements are determined by the energy content of tissue gain and the efficiency with which metabolizable energy is used.
Importantly, the system allows users to evaluate how different feeding programs—varying in milk allowance and starter intake—affect growth. Feeding more liquid increases growth rate early in life but reduces dry feed intake. Conversely, lower milk feeding encourages earlier starter intake but may limit early growth. These tradeoffs must be considered when designing feeding programs.
Protein Requirements and Supply
The new NASEM system adopts a metabolizable protein (MP) approach, replacing earlier methods based on apparent digestibility. Protein requirements are calculated based on maintenance needs and the level of growth supported by energy intake.
As calves develop, the source of protein changes. Initially, all protein is derived from milk. As rumen development progresses and starter intake increases, microbial protein production becomes a significant contributor. This shift affects both the quantity and quality of protein available to the calf and is incorporated into the model’s estimation of requirements.
Applying the Model in Practice
A key limitation of the NASEM approach is that it represents a “snapshot in time,” evaluating requirements at a specific body weight and feeding level. In practice, calf growth is dynamic and influenced by previous nutrition, health status, and management conditions.
To address this, modeling approaches that simulate growth over time can be used to evaluate feeding programs from birth through weaning and beyond. These models allow producers to estimate growth, feed intake, efficiency, and cost under different feeding strategies. They also help identify when calves are ready to wean based on starter intake and nutrient consumption.
Take-Home Message
The new NASEM requirements provide a powerful framework for feeding dairy calves, emphasizing the central role of energy in driving growth and the importance of balancing protein accordingly. While the system improves our ability to predict intake and requirements, successful implementation still depends on good management, including colostrum programs, health, housing, and environmental conditions.
Ultimately, the goal is not simply to meet nutrient requirements, but to design feeding programs that achieve desired growth rates in a cost-effective and practical manner. By combining the NASEM model with on-farm evaluation and experience, producers can develop feeding strategies that optimize both calf performance and long-term productivity.
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