Cow Size: A Piece of the Cow-calf Efficiency Puzzle

What answers do you choose to the following questions regarding beef cow-calf production?

Question 1
"An efficient beef cow will wean a calf that weighs at least 50% of her body weight"

True checkbox
False checkbox

Question 2
Which type of beef cows are most efficient?

  1. small cows are most efficient checkbox
  2. medium sized cows are most efficient checkbox
  3. big cows are most efficient checkbox
  4. cow size is not related to efficiency checkbox

Many factors can influence cow efficiency, such as reproductive rate, milk yield, backfat level, mothering ability, and the bull's genetic input into the calf, as well as cow size. However, in this review we will focus only the effects of cow size, and assume that all other factors are held constant so we can do a meaningful comparison. Likewise, when considering economic efficiency, we will assume that price differentials for breed types and the like are constant, allowing us to evaluate the impact of changing cow size without confounding the issue.

Productivity or efficiency?

The study of biologic efficiency in beef cow-calf production is both important and intriguing. Improved biologic efficiency is desirable as it indicates that fewer resources are used in the production of a unit of product. When economic values are applied to biologic and other inputs, as well as the product, producers are empowered to make well informed decisions regarding their production systems.

Cow puzzle
Figure 1. Cow puzzle

Biologic productivity in cow-calf operations can be defined as the weight of weaned calf produced per cow per year. This measure includes reproductive rate and calf survival as well as weaning weight. It is commonly recommended as a beef herd evaluator. However, while it is an excellent productivity measure, it does not take into account the inputs (such as feed) required to achieve the production. If we want to evaluate biologic efficiency, we have to account for feed, the major input required by the animals.

In a simplified analysis, beef cows can be considered to be biological factories. They convert feed into weaned calves. We can estimate cow biologic efficiency as the ratio of feed energy consumed to the weight of weaned calf produced. This ratio of input / output can be calculated on a per cow basis as:

kcals of feed energy consumed
lbs of calf weaned

Cows which require less feed to produce a pound of weaned calf are more efficient, and all other things being equal, will be more profitable.

How size affects volume and surface area

As any 3 dimensional solid object gets bigger, two things change: the object's volume increases, and its total surface area also increases. Since an object's volume is directly related to its weight, we can say that its weight is increasing at the same rate as its volume. A critical aspect of increasing size is how the rates of increase for volume (weight) and surface area compare with each other.

Here's an example based on a very simple shape, the cube. The principles hold true for other shapes such as spheres, cylinders, rectangular boxes, cows, etc.

Figure 2. Cube diagram

For a cube, Volume = Length x Width x Depth, or V = L x W x D

and Surface Area = (Length of a side x Width of a side ) x 6, or SA = (L x W) x 6

Let's consider two cubes of different sizes: Small, which has sides of 2 inches each, and Large, which has sides of 4 inches each.

Small cube Volume = 2 x 2 x 2 = 8 cubic inches

Small cube Surface Area = (2 x 2) x 6 = 24 square inches

Large cube Volume = 4 x 4 x 4 = 64 cubic inches

Large cube Surface area = (4 x 4) x 6 = 96 square inches

The results are summarized in Table 1. As would be expected, both volume and surface area increase when we go from the small cube to the larger cube. However, the ratio of surface area to volume is greater for the smaller cube compared with the larger cube. The small cube has 3 in2 of surface area for every 1 in3 of volume, while the big cube has only 1.5 in2 of surface area for every 1 in3 of volume.

Table 1. Surface Area and Volume of Two Sizes of Cubes
Size of Cube
Surface Area
Ratio of
Surface Area : Volume
(2 in. sides)


(4 in. sides)
1.5: 1

How body size affects cow efficiency

Now back to cows … we can apply the results of the cube comparison to beef cows. Larger sized cows have a lower ratio of surface area to weight than smaller sized cows (remember that volume is directly related to weight).

Why is this an important finding when it comes to feeding beef cows? Cows are like all other mammals in that they are "warm blooded". This means that they need to keep their core body temperature within a very narrow range, around 38.5 C. Most of the time, cattle in temperate zones are in an environment which is much cooler than their core temperature, so they are continually losing heat to their environment. The rate at which an animal loses heat is directly related to its surface area. However, the amount of heat produced by an animal is directly related to the amount of living tissue it has (ie its weight). Since small animals have a high ratio of surface area to bodyweight, they tend to lose heat to the environment at a fast rate, relative to their body weight. By comparison, large animals have a low ratio of surface area to body mass and therefore tend to lose heat at a slow rate, relative to their body weight.

One of the most important outcomes of these relationships is that smaller mammals have faster metabolic rates than larger mammals. They need to produce more heat from each gram of tissue in order to compensate for their faster rate of heat loss to the environment. This means that smaller animals have a faster rate of base metabolism, and they need to eat more food per unit of body weight to provide the extra fuel needed to power it.

These same principles apply to cows of different mature body sizes. In Table 2, the energy requirements of beef cows from 1000 to 1800 lbs are shown. While total feed energy required increases as cows get bigger, the amount of energy needed per lb of body weight decreases. As cows get bigger, they become more efficient at using feed because they need a smaller proportion of it to provide body heat. When compared on a per lb of body weight basis, an 1800 lb cow is 13.5% more efficient at utilizing feed to maintain itself than a 1000 lb cow.

Table 2. Energy requirements of beef cows over different body weights.
Cow wt
Cow ME*
Mcals /day**
Cow ME,
kcals /lb of
cow wt
Energy required per lb of body wt, relative to 1000 lb cow (%)

* metabolizable energy in the feed consumed
** NRC for beef cattle

How does this impact the efficiency of weaned calf production? It means that in terms of cow energetics, large cows have an inherent advantage in calf production over small cows. Table 3 helps to place these differences in perspective. If we start with a basic assumption that a 1000 lb cow should be able to wean a 500 lb calf (50% of her body weight), we can calculate the weight of calf required from larger cows to be as feed efficient. For example, a 1400 lb cow needs to wean a 644 lb calf, only 46% of her body weight, to be as efficient in feed utilization as the 1000 lb cow. An 1800 lb cow needs to bring in a 779 lb calf, only 43.3% of her body weight, to be as efficient in converting feed to weaned calf as the 1000 lb cow. (It is important to remember that this assumes that other factors such as milk yield, backfat etc. are similar across cow weights.)

Table 3. Calf weights required for equivalent energetic efficiency amongst cows of different weights.*
Cow wt. (lbs.)
calf wt. (lbs)
Calf wt. as a % of Cow wt.

* assumes other factors such as milk yield and back fat are common across cow weight range

Back to our definition of biologic efficiency for cow-calf production:
Biologic efficiency = kcals of feed energy consumed
lbs of calf weaned

Calf weight as a percent of cow weight doesn't work as a good evaluator of cow efficiency. To fairly compare cows of different body sizes we need to use a sliding scale to create targets for calf weight as a percent of cow weight. This is due to the changing ratio of surface area to body weight as cow size increases, which affects the loss of body heat.

The bottom line

All other things being equal, big cows tend to have an inherent feed efficiency advantage over small cows, because big cows need less energy per lb of body weight to maintain their body temperature. This is reflected in the weight of weaned calf which must be produced by different sized cows for equivalent feed efficiency. As cow size increases, the ratio of weaned calf weight to cow weight required for a set level of efficiency decreases. For example, a 1600 lb cow weaning a calf which is 44.5% of its weight would be as efficient as a 1000 lb cow weaning a calf which is 50% of its weight.

While these results are important, they are only one piece of the overall cow efficiency puzzle. They must be used in conjunction with other strategies, such as crossbreeding to exploit hybrid vigour in cows and calves, using terminal sires to maximize calf output per cow, and matching breeds/types to market demand.

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