It is often stated that it is easier to get first lactation animals pregnant than older cows. They do not milk as much, and because it is also often thought that high milk production reduces conception rates, we might think that older cows are less fertile because they produce more milk. A scientific paper published way back in 1986 by Gwazdayskas listed average conception rates for virgin heifers, lactation one, lactation two, lactation three, lactation four and lactation five of 54%, 44%, 41%, 40%, 27% and 18%, respectively. Until recently, it was assumed that overall dairy cow fertility was declining, too, in part because of declining conception rates. For example a scientific paper by Lucy, in 2001, was titled “Reproductive loss in high producing dairy cattle, when will it end?” A paper by Royal, in 2000, stated that “first service conception rates are now below 40%.” Thatcher, in a scientific paper in 2006, said, “The high producing dairy cow of the 21st century is sub fertile during lactation.”
    In reality, none of the above assumptions are true. Case in point: Recently, I typed the command “Bredsum by lact” into DairyComp 305 on a high-producing, high-fertility farm. The conception rates for lactation one, two, three, four, five, six and seven were, respectively, 53%, 56%, 58%, 53%, 57% 50% and 68%. The overall conception rate for all breedings on all cows was 55%. By the way, this dairy consistently produces between 105 and 110 pounds of energy corrected milk per cow per day. Their annual pregnancy rate is 41%. Are older cows less fertile in this herd? It sure does not look like it. Furthermore, if one looks at a lot of data, high-producing herds, with high-producing cows, have higher not lower conception rates than low-producing herds. Go figure. I decided to look at another herd. Here are the conception rates by lactation for lactations one through five and overall: 48%, 45%, 53%, 36%, 52% and 48%. This herd produces around 110 pounds of energy corrected milk per cow per day, while the pregnancy rate is 37%. To be fair, first service conception rates may be slightly lower than they might be because both herds use some sexed semen in lactation one. Nevertheless, in both herds, older cows conceive as well or better than lactation one, and overall conception rates are great.
    Why would it be normal for a second lactation cow to have a lower conception rate than a first lactation cow, or for that matter, a first lactation cow to have a lower rate than a heifer? Do we accept this as normal for any other species? Sure, more calvings are more opportunities for infections and reproductive damage, but these are not all that common anymore. So it must be milk production that changes the reproductive efficiency, right? But remember, high-producing herds have better conception rates, so it is not production. The answer is most likely negative energy balance. Higher-producing cows do not necessarily have more negative energy balance than low-producing cows, but they need to eat a lot more, so anything that interferes with intake can be relatively more important in a high-producing versus a low-producing cow. So maybe the reason these herds have such great reproductive performance and why their old cows do so well is that they are owned by great dairymen and dairywomen. We also know herds have gotten better in recent years, not worse. Some of this improvement may be genetic selection for reproduction. In our practice, the average conception rate for all breedings in 2004 was 33.9%, and in 2020, it was 46%. That is a relative increase of 40% with a straight trend line going up. So perhaps the new headline should be, “Reproductive performance gains in high producing dairy cattle, when it will end?”
    The new normal for high-producing herds is probably around 50% conception rates for all lactations. That is impressive. It is important, too, not just because reproduction is important but because just about every dairy farmer has been told repeatedly to raise fewer heifers, calve fewer heifers, cull fewer cows and increase the proportion of the herd that is second lactation and up. Farmers do this because it is expensive to raise heifers and old cows produce a whole lot more milk than heifers. Thus, if one wants to keep older cows in a herd, one has to get them pregnant. If it is significantly harder to get them pregnant than lactation one animals, it will be hard to keep them in the herd. The good news is we now know how to get older cows pregnant, and we know how to get high-producing cows pregnant. Take a look at conception rates by lactation in your herd. Are your older cows underperforming? Older cows are not inherently significantly less fertile nor are cows giving a lot of milk. What we once thought was normal is no longer so. Keep up the great work.
    Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at bennettnvac@gmail.com with comments or questions.