The bell curve, named for its shape, is a way to represent data that is normally distributed. Even though the whole topic of statistics can incite fear in some people, bell curves are important in a lot of things including dairy heifer reproduction. For example, the age of animals at first calving will typically look like a bell curve in most herds. The heifer pregnancy rate will determine what the bell curve of age at first calving looks like. A high pregnancy rate will create a taller and more narrow bell than a low rate. Taller and more narrow means more animals calving at or around the same age, and fewer very young or very old heifers. 
Can we change the bell curve? Absolutely. For one, we can move it. For example, there has been a long-term trend toward calving heifers younger. However, some producers have been trying to calve heifers at heavier weights because of some research showing heifers calving at 85% or more of adult weight give more milk during first, and every subsequent, lactation. In both cases, farmers may be trying to move the bell curve of age at first calving to the left or the right by changing the voluntary waiting period. Is this a good idea? Well, maybe.
Take two herds; the first has a low (14%) heifer pregnancy rate, and the second has a high (44%) rate. The first herd calves 46% of heifers within a range of two months of age. The second calves 75% within a range of two months of age. The bell curve of the first herd is really fat, and it has a long tail to the right, with some heifers calving in at well over 2.5 years of age. In fact, 60% calve at 27 months or more of age. The bell curve of the second herd is much narrower, and exactly zero animals calved at or over 27 months of age. The left side of the bell curve of the first herd looks like the left side of a bell, while the left side of the bell curve for the second herd is much steeper, skewing the shape of the bell. Perhaps both producers read the study suggesting larger first calving weights, and both decided to just move their bell curves by changing the voluntary waiting period from 390 to 405 days. Is this the right decision? For the first herd, probably not. The biggest bottleneck in this herd is just a fat bell curve. Moving it to the right will increase the percentage of animals calving in really old. It will reduce the number calving too young, but that is a pretty small group now anyway. For the second herd, moving the bell curve may indeed be the right idea. Their bell curve is already svelte, and moving it to the right will move the vast majority of the animals to an age and weight at calving just where they want them. Their bottleneck may indeed be the position of the bell curve rather than the shape. 
We can change the shape of a bell curve by interventions. For example, using 100% timed artificial insemination at the first breeding will skew the curve, or change its shape. Assuming good conception rates, that bell curve will look more like a ski jump: very steep, or even straight up, on the left and then quickly dropping to the right. A simple change of giving prostaglandin to every heifer at the end of the voluntary waiting period will accomplish something similar, though not quite as dramatic, assuming heat detection is very good. Simply adhering to the voluntary wait period often changes the shape. This is because there are plenty of farms where the voluntary wait period seems to be a range rather than a day. Often this happens because people do not want to waste a natural heat, so they breed some animals too early, or they want to wait for a natural heat for too long of a period beyond the voluntary waiting period, so they breed some animals way too late. Some believe, erroneously, that a heat following a prostaglandin injection is less fertile than a “natural” one. Practices like these create a very long ramp on either or both sides of the bell curve. 
Why is this important? Because it costs more now to raise a heifer than it ever has. Studies suggest it costs around $2,500 to raise a heifer to calving, or about $3.42 per day. Herd replacement costs can easily be 15%-20% of the total cost of production on a dairy. Getting heifers bred sooner can save a lot of money. A study by Lauber, et. al, in the December 2021 issue of the Journal of Dairy Science showed a savings of nearly $17 per pregnancy by using a five-day CIDR synch program versus giving prostaglandin and using heat detection, for example. They did not compare the cost of the TAI program versus heat detection alone with no prostaglandin, but the savings most likely would have been much greater had they done so. (One injection of prostaglandin costs less than the rearing cost for just one day.) Thus, the key is not to breed too early, but when the VWP is reached, breed really aggressively. Doing this will get you a very skinny bell curve that is skewed with a really steep, almost cliff-like ramp on the left and heifers calving almost all exactly at the same age, and hopefully with very similar weights. This means fewer animals calving too young and never reaching their full potential, and fewer animals calving in way too old after eating the herd’s profit for lunch. Remember, too, that old heifers are often fat and have more difficulty calving, and a greater prevalence of post-partum disease. So yes, that darn bell curve really matters. 
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.