As the optimum cow size debate rages on, I have become interested in whether cow size really makes that much difference from an economic perspective. Common sense and general knowledge of biology would suggest that you don't need to have a PhD to figure out that a smaller cow should eat less than a larger cow and therefore should have a lower annual feed cost. I don't think there is really much debate on this point either amongst academia or industry.

Similarly, but less obvious, is the question of whether a larger cow will produce more pounds of weaned calf than a smaller cow. There are a few research papers out there that show it both ways. In some reports, smaller cows produced less total pounds of weaned calf than larger cows and in other reports they weaned about the same number of pounds.

Unfortunately, many of these reports don't address the economic side of the issue. They simply report comparative weaning weights vs. cow size and proclaim that one is more profitable than the other (whether one is really more profitable can't really be determined from the data presented).

And finally, the question that many have attempted to answer subjectively is whether cow size affects overall profitability and if it does, by how much? So the questions becomes not so much whether small cow A produces more or less pounds than larger cow B, but rather does smaller cow A convert dollars of feed fed into pounds of weaned calf more efficiently than larger cow B.

Inevitably, I got tired of wondering about this so I began amassing a data set of cow weights and weaning weights. I have pulled data from reported literature, producers around the state, and at the SDSU Cow Camp Experiment Station. All-in-all I have about 1,750 pairs in the data set.

So let's look at the first question, Do larger cows wean more pounds of calf than smaller cows?

To look at this, what I did was take the data set and sort by cow weight and then grouped into the lightest 25%, the second lightest 25%, the heaviest 25% and the second heaviest 25%. So out of 1,685 pairs in this set, there are approximately 420 pairs in each weight class. I then the average cow weight in each weight class and divided it by the average weaning weight for that same class, giving a weight production ratio.

Most academians don't like these weight ratios, and I tend to agree with them. The problem is that when you look at these weight ratios, you tend to make all kinds of assumptions about them with no real data to confirm or deny. So, when you look at a cow weight to weaning weight ratio, it is just that, a ratio of weight produced by weight maintained.

The first table shows that as cow weight increased, calf weaning weight also increased. So it seems that larger cows do tend to produce slightly larger calves on average. There is 114 lb. difference between the weaning weights of the lightest and heaviest groups of cows.

What is interesting though is to look at the ratio of cow weight to weaning weight. This ratio does not tell us anything about efficiency per se, but it does indicate that weaning weight does not necessarily increase at the same rate as cow size.

If you look at data from the SDSU Cow Camp you see the same trend:

The actual weaning weights did not increase as linearly as cows got larger as they did in the other data set, but the weight ratio shows approximately the same trend.

Well, that's fine if larger cows produce slightly larger calves on average, but does it really make that much of a difference to the bottom line of a cow outfit.

What I did to test this was to model the economic inputs and outputs of these pairs based on weight class

This model shows that in fact it does make quite a bit of economic difference between the smaller and larger cows as the smallest cows returned 50% more on a per head basis than did the largest cows even though the smaller cows weaned 19% less weight.

The principle reasons for this is that smaller cows eat less and therefore have a lower annual feed cost. It's that simple. The larger cows weaned more pounds and grossed more on a per head basis, but the cost differentiation of getting there compared to a smaller cow with a lighter calf was larger. Therefore, the efficiency of feed fed to production was greater for the smaller cows.

We see the same pattern in the cow herd out at the SDSU Cow Camp.

The big thing here to understand is that from this data we cannot say that Rancher A has smaller cows than rancher B therefore rancher A must be more profitable.

This is simply a misnomer that has no factual basis. In fact, this is so dependent on feed cost, that rancher B, with bigger cows could easily be more profitable than rancher A with smaller cows, simply because rancher B can produce cheaper feed.

What we can say about this data set is that if rancher A currently has 1500 lb. cows and works towards a more moderate cow size through breeding and culling management, rancher A can improve profitability pretty dramatically.

Let's look at a model of this concept:

This model illustrates that if we fix land area and stocking rate, all other things equal, as would be the case with an individual cow outfit that changes cow size, how profitability changes.

In this model I fixed land area at 5,000 acres with a fixed stocking rate of 0.75 AUM's/acre.

By changing from a cow size of 1541lbs. to an average cow size of about 1000 lbs., profitability increases by about 68%. The big difference here is that on a fixed amount of land, you can run 36% more 1000 lb. cows at the same fixed stocking rate than you can 1541 lb. cows.

Again, you see the same pattern using the data collected from the SDSU Cow Camp:

Here, we are running on less acres and a higher stocking rate, but it is fixed across all weight groups.

What is really striking is that the difference in profitability between the largest group and the smallest group is about the same as the other, totally unrelated data set at 68%!!

I was very surprised by the results of this, as I stated earlier, I figured that small cows ate less than large cows and large cows wean more weight, but I never realized there was such a large efficiency difference.