Introduction
The What’s Happening? video series continues with Episode #10. This episode highlights a number that appears in many places in history and culture — 15. From the Ides of March, to Quinceañeras, to Kentucky becoming the 15th state of the United States, the number 15 shows up in many unexpected ways.
But when it comes to calf nutrition and management, the number 15 has a very important meaning. In calf feeding programs, 15 kilograms of cumulative non-fiber carbohydrate (NFC) intake represents an important milestone in rumen development and helps determine when calves are ready to be weaned.
Understanding Non-Fiber Carbohydrates (NFC)
To understand the significance of the 15-kg target, we first need to understand the types of carbohydrates found in calf starter.
Carbohydrates in calf starter are generally divided into two categories:
- Fibrous carbohydrates (Neutral Detergent Fiber, or NDF)
- Non-fiber carbohydrates (NFC)
The NFC fraction includes:
- Starch
- Sugars
- Pectins
- Organic acids
In a typical pelleted calf starter, nutrients might look approximately like this:
- Water: ~12%
- Protein: ~18%
- Fat: ~4%
- Ash: ~8%
- Carbohydrates: ~58%
Most of these carbohydrates fall into either the fiber (NDF) or non-fiber carbohydrate (NFC) categories.
Why NFC Is Critical for Rumen Development
When calves are born, their rumen is not yet fully functional. Instead, the abomasum functions as the primary digestive organ while calves consume milk or milk replacer.
As calves begin to nibble on calf starter and drink water, bacteria present in the rumen start fermenting carbohydrates — especially NFC. This fermentation produces volatile fatty acids (VFA), primarily:
- Acetate
- Propionate
- Butyrate
These VFAs are extremely important because they stimulate:
- Growth of the rumen epithelium (rumen wall)
- Development of rumen microbial populations
- Maturation of metabolic processes needed for ruminant digestion
As calves consume more starter and more NFC is fermented, the rumen becomes increasingly functional. Eventually, the calf becomes capable of extracting energy from dry feed in a manner similar to a mature ruminant.
The 15 kg Threshold
Research and modeling indicate that when calves consume approximately 15 kilograms of cumulative NFC – that is, the total NFC from birth – the rumen has developed enough to function like that of a mature ruminant.
At this point:
- The rumen can effectively ferment feed.
- Nutrient digestion resembles that of adult cattle.
- The calf can obtain sufficient nutrients from starter feed.
In other words, the calf is ready to be weaned.
Unlike simple recommendations based only on age or starter intake, this approach focuses directly on the biological development of the rumen, making it one of the most scientifically grounded ways to determine readiness for weaning.
When Do Calves Reach 15 kg of NFC?
The age at which calves reach the 15-kg NFC milestone varies depending on several factors:
Milk intake
Calves receiving larger amounts of milk generally:
- Start eating starter later
- Consume less starter initially
As a result, they may reach the 15-kg NFC threshold at an older age.
Starter composition
Starter feeds differ widely in carbohydrate composition.
- High-starch starters (such as textured feeds) often contain more NFC.
- High-fiber starters contain less NFC.
Calves fed high-starch starters may reach the 15-kg threshold sooner, allowing earlier weaning. High-fiber starters generally delay the point at which calves reach 15 kg of NFC.
Environmental conditions
Factors such as ambient temperature also influence starter intake and rumen development.
Age and growth
As calves grow older, their capacity to consume dry feed increases, accelerating NFC intake.
Based on many modeling scenarios, few calves are truly ready for weaning before about 9 weeks of age, particularly when milk feeding rates are high.
Tools to Estimate the 15-kg Target
Several tools are available to estimate when calves will reach the 15-kg NFC milestone.
One option is Calf Sim, developed by Dr. João Costa and colleagues at the University of Vermont. This online model allows users to enter feeding and management parameters and estimate when calves reach 15 kg of cumulative NFC intake. This link will take you to the Calf Sim model.
Another option is the spreadsheet linked in Calf Note #230 – Further Musings on Dry Feed Intake. This Excel tool provides a simple way to estimate when calves will reach the 15-kg threshold. Find the Note here.
Finally, the NASCalf program, available at Calf Notes Tools, also provides calculation of cumulative NFC intake. This link will take you to Tools.
These tools allow producers and advisors to evaluate their feeding programs and better understand how milk allowance, starter composition, and environmental conditions affect rumen development.
The Take-Home Message
Many recommendations for weaning calves rely on age or daily starter intake. While these methods can work, they do not always reflect the biological readiness of the calf.
A more scientifically grounded approach focuses on cumulative NFC intake.
When calves have consumed about 15 kg of non-fiber carbohydrates, their rumen is typically developed enough for successful weaning.
Keep in mind:
- High-starch starters help calves reach 15 kg sooner.
- High-fiber starters may delay rumen development.
- Higher milk feeding programs usually delay starter intake and therefore delay weaning readiness.
The key message from this episode is simple:
Remember — 15 is the magic number.