Evaluating Feed Efficiency with Body Reserves in Mind

Introduction

Feed efficiency is an important concept for dairy farmers. With feed being the largest cost on most farms, it makes sense to track how efficiently cows convert feed into milk. One common way to measure this is energy-corrected milk divided by dry matter intake (ECM/DMI). While this ratio provides a basic understanding of efficiency, it doesn’t capture the full picture. 

For one, ECM/DMI does not directly reflect economic efficiency, which depends on both feed costs and milk value.  Additionally, it fails to account for energy mobilized from or stored in body reserves. This article explores how thinking in terms of energy, rather than just pounds of milk and feed, can lead to a more accurate picture of feed efficiency.

Rethinking Efficiency, It’s About Energy

The core question when evaluating feed efficiency is:

“How effectively is the cow using the feed energy I provide her?”

ECM/DMI doesn’t answer this directly. It tells you how many pounds of ECM a cow produces per pound of feed consumed. But it doesn’t account for differences in feed energy density, or the energy cows may pull from or store in their bodies. For example, if two cows have the same ECM and DMI but one eats feed with more energy packed into each pound, she’s actually using more total energy to produce the same amount of milk and is therefore less efficient. 

To truly evaluate efficiency, we need to think in terms of:

• Energy In (feed energy) 
• Energy From Within (body reserves used or stored) 
• Energy Out (primarily milk energy) 

Feed Energy Efficiency, Breaking it Down

Energy In: Feed Energy

This is the energy cows consume through feed.  It depends on how much feed they eat, the digestibility of that feed, and how energy-dense the diet is. 

Energy from Within: Body Reserves

Cows store energy primarily in body fat, and they can draw on these reserves when feed intake doesn’t meet their needs such as in early lactation.  This can be viewed as the “borrowed energy” effect.  The “borrowed energy” effect can make a cow appear more efficient (higher ECM/DMI) simply because she’s using her body’s stored energy to support milk production.   

Later in lactation, or in growing cows, energy may be stored from feed into body reserves.  This can lower ECM/DMI but does not mean the cow is inefficient.  In some cases, gaining body condition or supporting growth is exactly what we want.  This can be viewed as the “stored energy” effect. Understanding these shifts is essential to interpreting ECM/DMI accurately. 

Tracking changes in body reserves at the individual cow level can be difficult, especially in large herds.  However, monitoring average body condition scores (BCS) and BCS change over time at the pen level can provide valuable context not just for feed efficiency, but also for evaluating cow health, performance, and management. 

Some BCS loss is typical for dairy cattle, especially after calving.  However, cows that lose excessive BCS are at greater risk for poor fertility, health events, reduced milk production, and culling (1, 2).  At the same time, excessive BCS gain can reduce feed efficiency, milk yield, and increase the risk of metabolic disorders such as milk fever and ketosis (1). 

Understanding how cows are using internal energy reserves is important not only for evaluating feed efficiency, but also for making management decisions that support animal health and productivity.

Energy Out: Milk Energy

Most of the energy we’re interested in leaves the cow as “milk energy”.  This represents the energy in milk fat, protein, and lactose.  For example, one pound of milk with 4.3% fat and 3.25% protein contains about 0.36 Mcals (3).  

Some feed energy is used to maintain normal body function or lost as heat, gas, feces, or urine. For this discussion, we’re focused on how much energy is captured in milk or body reserves instead of these other functions.

What is Captured Energy?

When using ECM/DMI, we’re asking: 

“For every pound of feed I provide, how many pounds of milk do I get?” 

But in the following scenarios, we ask: 

“How much of the feed energy the cow consumes ends up as milk or body reserves, versus how much is lost or used for maintenance?” 

Captured Energy refers to the portion of the energy a cow consumes that is actually used for producing milk or adding to her body reserves. It excludes energy lost as heat, gas, urine, feces, or simply used to keep the cow alive (maintenance). 

Think of it as the “productive energy” you get from the feed, the energy that contributes directly to production or growth. By focusing on captured energy, we can better understand true feed efficiency beyond just milk output per pound of feed. 

In the following scenarios, assume all the cows consume the same amount of feed with the same energy density and produce milk with 4.3% fat, 3.25% protein, and 5.2% lactose.  All energy calculations utilized equations from NASEM 2021.

Scenario 1: True Improvement in Efficiency

This scenario represents the ideal situation: a cow producing more milk with the same feed intake and stable body condition, reflecting genuine efficiency gains.  Cow B produces more milk than Cow A while consuming the same amount of feed, and neither cow is gaining or losing body condition.  In this situation, Cow B is capturing more feed energy and using it for milk production.  While this scenario shows true efficiency, real-world situations often involve shifts in body condition that affect feed efficiency metrics.

Scenario 2: The Borrowed Energy Effect

Scenario 2 highlights how two cows can appear equally efficient based on ECM/DMI yet differ once body energy mobilization is taken into account. Cows A and B have the same ECM and DMI, but Cow B is mobilizing 1 kg (2.2lbs) of body reserves daily. ECM/DMI is identical, but Cow B is actually less efficient when you consider the extra internal energy she’s mobilizing for use. Cow B is actually only capturing 21.2% of the energy she consumes whereas Cow A captures 25.4%.

Cow C produces more ECM than Cow A but is also mobilizing body reserves. Despite her higher ECM/DMI, her actual captured energy efficiency is still lower.

Scenario 3: The Stored Energy Effect

Scenario 3 illustrates how cows with similar or even lower ECM can appear less efficient based on ECM/DMI, but actually can be more efficient when accounting for energy stored as body reserves.  In this scenario, cows are putting energy into body reserves. Cow B produces the same amount of milk as Cow A but is gaining 1 kg (2.2 lbs) of body reserves daily.  This means she is capturing more energy. 

Cow C produces less milk but is also gaining condition. Her ECM/DMI is lower, but her overall feed energy capture is higher than Cow A’s.  If we want these cows to be gaining condition, cow B is the most efficient.   

However, if the cows are already at a desirable BCS and we do not want them to gain additional weight, then the extra energy being captured by Cow B and Cow C may not actually be desirable.  This highlights the importance of evaluating metrics within the context of your specific management goals and animal needs. 

Table 3

Table 3. Comparing three cows and how when a cow stores energy in her body reserves, feed efficiency calculated as ECM/DMI may not reflect captured energy efficiency.

	Cow A	Cow B	Cow C
ECM (lb)	104.6	104.6	99.8
DMI (lb)	65	65	65
ECM/DMI	1.61	1.61	1.54
Feed Energy (Mcal)	132.75	132.75	132.75
Energy Mobilized from Body Reserves (Mcal)	0	0	0
Energy Sent to Storage in Body Reserves (Mcal)	0	5.6	5.6
Milk Energy (Mcal)	33.7	33.7	32.1
Captured Energy	25.4%	29.6%	28.4%

Final Thoughts

Feed efficiency is more than a simple pounds-in, pounds-out equation. While ECM/DMI is easy to calculate on-farm and can provide valuable insights, it doesn’t always tell the full story, especially when cows are gaining or losing body condition. 

A more nuanced, energy-based view can reveal when cows are truly efficient or when apparent efficiency is coming at the cost of body reserves. 

It’s important to note that evaluating feed efficiency with the depth described here is complex and not practical for everyday use on most dairy farms. This article is intended to provide background on why farmers should be cautious when interpreting ECM/DMI values and keep these energy dynamics in mind when using this metric.   

In addition, feed efficiency should always be assessed in the context of the farm’s goals for animal performance and economic outcomes.  For example, are cows expected to gain or lose body condition?  The answer to that question can significantly influence how we interpret feed efficiency and captured energy.  What may appear efficient on paper might not actually align with the farm’s management needs and goals. 

Recognizing these limitations can lead to a deeper understanding of cow performance and better management decisions.  

For a more complete picture of feed efficiency and how to apply it on your farm, explore our series covering the basics of ECM/DMI, factors that influence it, practical management strategies, creating a data action plan, and how to combine it with Income Over Feed Costs (IOFC). 

Note: All energy calculations utilized equations from NASEM 2021. Values were originally calculated using kilograms to align with NASEM equations, then converted to pounds and rounded for readability. As a result, some metrics (e.g., ECM/DMI) may differ slightly if recalculated using the displayed, rounded figures.

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