Heifers are an expensive part of the dairy business. By most accounts, heifer rearing costs are the second or third largest contributor to the total cost of milk production, behind feed for the milking cows and labor.
Studies show that the age of the heifer at calving has a profound impact on what it costs to raise that heifer. In addition, we know that body weight at calving and rate of growth before puberty impact the first-lactation production potential of that heifer. Heifers seem to get more attention now than they did several years ago, partly because of these factors and partly because we are doing a better job of controlling heifer diets and management. We recommend heifers calve at 22 to 24 months of age.
Since 2002, we have periodically obtained records from DRMS for all first-lactation Holsteins in Pennsylvania to observe trends in age at calving over time. We can also access DRMS Dairy Metrics for the average age at first calving by herd, which is reported monthly and annually. The graphs presented here give us a closer look at what is happening to age at first calving in Pennsylvania.
In Pennsylvania Holstein herds, average age at calving had been hovering around 25.5 months for several years, but since 2015 it has been trending lower. Data from DRMS Dairy Metrics indicate that herd-average age at calving was 25.66 months in 2007, 25.51 months in 2009, 25.48 months in 2011, 25.68 months in 2013, and 25.51 months in 2015. In 2017 the average age at calving was 25 months, and Figure 1 shows the current trend from June of 2017 through June of 2018. The herd average for age of first lactation cows dropped below 25 months in May of 2017 and has stayed there for 12 additional months.
Figure 1. Age of first lactation Holsteins (months) in Pennsylvania from June of 2017 through June of 2018. Data are herd averages from Dairy Metrics, DRMS, Raleigh, NC; July 2018.
Figure 2 shows the number of Holstein heifers who calved at various ages during 2017. This graph represents 112,318 heifers with age at calving ranging from 18 to 30 months (average 23.9 months, standard deviation 2.25 months; records collected by DRMS). In 2017, there were more PA heifers calving at 24 months than any other age, which we have seen for a number of years now; however, almost as many heifers calved at 23 months. The aspect of this graph that has changed a lot over the last few years is the number of heifers calving at less than 24 months. As you can see, in 2017 more heifers calved at 23 months than at 25 months, and the number calving at 22 months was more than the number calving at 26 months. Generally, calving at less than 21 months of age is not recommended. Heifers need to achieve adequate frame size and body weight to calve without increased risk of dystocia or low milk production.
Figure 2. Distribution of age at calving of 112,318 first-lactation Holstein heifers in Pennsylvania in 2017. Data from DRMS, Raleigh, NC.
While there are a few heifers that calve very late--30 months and beyond (not shown here), they are a small number in comparison. In fact, of these Holstein heifers from 2017, 66% calved at 24 months or less and 78% calved at 25 months or less. The other noticeable aspect of this graph is that most of the heifers (66%) are clustered between 22 and 25 months, meaning they are getting bred in a relatively short interval. This may be enhanced by improved reproduction management and grouping or more aggressive culling of problem breeders. In addition, over half of the heifers calved in the recommended window of 22 to 24 months.
Figure 3 shows the actual 305-day lactation records from the heifers in Figure 2. This data set is based on a calendar year, so many heifers had not yet completed their first lactation. The actual average days in milk was 281, with a range of 45 to 305 days. Heifers produced similar amounts of milk on average, regardless of calving age, and there was no noticeable drop in production for heifers calving at less than 24 months of age. Average milk production (again, including incomplete records) was 19,203 pounds for the 2017 heifer crop.
Figure 3. Average actual 305-day milk production by age at calving of first-lactation Holstein heifers in 2017 in Pennsylvania. Data from DRMS, Raleigh, NC.
Another way of looking at the milk production data is presented in Figure 4. In this graph, each X represents a single heifer. Again, it is clear that heifers calving before 24 months of age can achieve high yields. However, a close look at the solid section near the top of each column shows that the number of heifers milking at the highest levels increases for each age category from 20 months through 23 months and then decreases through 28 months.
Figure 4. Actual 305-day milk production and age at calving of first-lactation Holstein heifers in 2017 in Pennsylvania. Each X represents an individual heifer. Data from DRMS, Raleigh, NC.
For some perspective on how much age at calving has changed in recent years, Figure 5 presents the heifers calving at various ages (as a percentage of the total number) for 2002 compared to 2017. In 2002, less than a third (31.1%) of heifers were 21 to 24 months of age at calving. In 2017, nearly two-thirds (63.4%) calved at that age.
Figure 5. Distribution of age at calving (as a percentage of all heifers) for first-lactation Holstein heifers in Pennsylvania in 2002 (light blue) compared to 2017 (dark blue). Data from DRMS, Raleigh, NC.
Figure 6 compares milk production data for 2010 to 2017. The improvement in milk production for heifers calving before 24 months of age is obvious and impressive. We suspect this reflects increased attention to heifer nutrition before and after calving and management of fresh heifers separate from older cows.
Figure 6. Average actual 305-day milk production by age at calving of first-lactation Holstein heifers in Pennsylvania in 2010 (blue bars) and 2017 (teal bars). Data from DRMS, Raleigh, NC.
Reducing age at calving without sacrificing milk production is possible, and Pennsylvania dairy farms are making great progress in getting heifers into the milking string earlier. These data do not allow us to evaluate the number of heifers that calve but do not complete their first lactation, but past research has shown that heifers calving too early have a higher risk of being culled in the first lactation. Average days in milk by age at calving was lower for heifers that calved at 18, 19, or 20 months of age (271, 272, and 279 days, respectively) compared to the average of 281 days for all heifers. While we cannot definitively conclude that these heifers died or were culled, as opposed to calving near the end of the calendar year, experience and previous research suggest it is likely that many of these heifers did not complete a full lactation. Calving at 22 to 24 months of age is recommended to balance rearing cost control with health and production performance in the first lactation. Herds that want to raise the bar for their heifer goals can target the lower end of this range.
Take a look at your herd's age at calving. Studies show that for every month beyond 24 months, total rearing costs increase by 5% per month. Focus on breeding by 13 to 15 months and consider culling those that fail to breed within that window. Define strategies to identify heifers that are ready for breeding, make sure they are bred in a timely fashion, and provide adequate nutrition to support heifer growth throughout the rearing period.
This is an updated and expanded version of an article first published in Hoard's Dairyman, April 25, 2013.