Changes in Average Number of Lactations

Dr. Allen Young
USU Extension Dairy Specialist

I was sitting in the annual meeting put on by DHI-Provo and started to look at some of the numbers they had summarized. I started to mentally calculate the number of lactations cows were currently completing, and after determining the number decided to go back and take a closer look.

There has been much talk and concern about the decrease in the number of lactations cows are completing. Using data generated for our Rocky Mountain DHIA annual summary, we can calculate an approximation of this number. The standard calculation is to take the average age of all cows at their last calving, subtract the average age at first calving and divide that number by the calving interval (CI). I did something different and added the average days in milk (DIM) for the current lactation. This would skew the value upward to include how far they were into the current lactation. I calculated this number for the past 10 years. The results of those calculations are found in Figure 1.

LactFig1.gif - 3394 Bytes

The two messages are that none of the values reaches two lactations, a great concern, and that the results seem to be cyclic. These numbers include all breeds. I calculated the differences between breeds for 2005 only. The average number of lactations for Holstein herds was 1.85 and for Jersey herds was 2.22, a difference of 0.372 lactations. At the current price of heifers, that is significant.

I was further interested in why the pattern was cyclic. As you can see from Figure 2, the cyclic changes probably were not due to the average DIM or CI. Both increased and then leveled off during this period. Age at first calving also decreased similar to the changes seen in Figure 2, and offset the increase in CI.

LactFig2.gif - 4009 Bytes

Figure 3 shows the changes in average age for all cows in the herd. You can see in this figure that the average age of the herd was cyclic, and for most of the 10-year period is in phase with the cull rate. Even though I don�t have the numbers for the percent of the herd that was first lactation cows, these two lines together suggest that herds were in expansion mode at two different times in the past 10 years and probably were buying significant numbers of heifers. Decrease in cull rates many times coincides with a desire to retain marginal cows so that herd inventories can be built. The decrease in average age of the herd also suggests that younger animals, probably heifers, were being bought to aid the expansion.

LactFig3.gif - 4098 Bytes

Even though the data supports my supposition that more cows are being added for expansion, the troublesome thing for me is that, at some point, the average number of lactations should rise as these new animals move through successive lactations and I don�t see that happening. Maybe expansion has not leveled off or we can only suppose that even the new animals coming into the herd are still leaving at an accelerated rate. As I see it, this means that we need another mind-set in managing cows, because I don�t see how dairy farmers can continue to pay high prices for heifers that won�t, on average, stay around long enough to pay for themselves. Any comments can be directed to Allen Young at (435) 797-3765 or alleny@ext.usu.edu. ©