Few dairy farmers complain about having too many female calves. It is now possible to purchase semen that will produce about 85 percent female calves. This technology has changed both the dairy and the dairy industry. Like most new technologies, there are advantages and disadvantages, multiple applications and different approaches as we gain experience.

In addition to the other factors that have affected milk prices, an increased number of replacement heifers will increase milk supply by a combination of a greater culling of low-producing cows or a greater number of milking cows.

Economics say that if milk prices are depressed, replacement heifer prices will fall. This will lead to a situation where it is more expensive to raise the additional heifers than to purchase them.

Thus, widespread use of sexed semen as a tool to increase the replacement rate might not be economically advisable for many farms. More heifer calves require more resources in housing, labor and feed. These resources are needed long before the income from the increased milk production arrives, so there can be significant cash-flow constraints.

The second cost is less obvious. Most dairies have experienced a decreased conception risk from sexed semen use. The typical drop is about 25 percent, when compared to traditional semen. That is, if the average conception risk in heifers is 60 percent, the conception with sexed semen might be 45 percent. Dairy Comp 305 users can easily compare the conception risk of sire by semen code in heifers, because sexed semen is coded by adding a 5 as a prefix to the NAAB code. For example, the sexed semen straw of bull 11H9798 is 511H9798. This allows us to compare the conception of the 11 bulls with the conception of the 511 bulls.

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Evaluating heifer data
Great care must be taken when evaluating conception risk, as many breedings are needed to gain precision. But there are other, less obvious issues that can confound the analysis. If first-service conception is normally higher than later services, incorrect conclusions will be drawn if sexed semen is used primarily on early breedings, and traditional semen is used on later breedings. It will be difficult to separate the effect of sexed semen from breeding number.

A similar issue arises when there is a nonuniform use by season. The fertility of U.S. cows and heifers tends to be lower in the summer. If the adoption of sexed semen was started in May, it is difficult to separate the effect of season from the effect of sexed semen.

Dairy Comp 305 users have tools within BREDSUM to attempt to adjust for all these factors simultaneously, but estimating the effect of more factors requires substantially more breedings. Bottom line: Use extreme care when attempting to evaluate the difference in conception between sexed and unsexed semen.

Even assuming that you could document a 25-percent decrease in fertility, it is also difficult to model the economic impact of this. It depends on how many breedings are subjected to sexed semen, and also how many months a farm will allow heifers to remain open before they are sold as beef instead of springers. There is a hidden cost to this, because it is income not realized, rather than an actual expense.

Does all this mean dairies should not use sexed semen? Absolutely not! In fact, the opposite is probably true. It is likely that almost every dairy should be breeding at least some of their heifers to high-quality sexed-semen sires. But the benefit is better heifers, rather than more heifers.

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Making greater genetic improvement
Because of the need for replacements and cows to refreshen, most dairies breed all their cows and heifers. Thus, the genetic improvement in the dairy industry is nearly all due to the intense selection of sires. Sexed semen provides an opportunity to change this if top genetic heifers can be identified. A sample solution might be to breed the top 33 percent of your heifers with sexed semen, breed the bottom 25 percent of the heifers with beef semen, and the middle with conventional semen. The beef crosses and males will be sold at birth. But the genetics of the top dams will be replacing the genetics of the bottom dams.

According to the model developed by Dr. John Fetrow at the University of Minnesota, the level of sexed semen used should increase with the accuracy of identification of top heifers.

For herds that have accurate sire and dam identification, and routinely test and have their data processed, the USDA and processing centers can calculate various indexes that can help rank the heifers. There are still decisions to be made regarding selection of milk versus cheese versus other traits, and these will vary by market.

For dairies without traditional testing data, there are some methods to rank heifers based on the Predicted Transmitting Ability (PTA) of the sire and maternal grandsire. Once a goal is selected (Milk, Cheese, Net Merit, etc.), the index can be estimated by using 0.5 of the sire PTA + 0.25 of the maternal grandsire PTA. This requires accurate knowledge of the sire, and, ideally, the maternal grandsire, and a recent data set containing all the sire PTA information. The SIRES command in Dairy Comp 305 can create this breeding value index.

Once inexpensive genomic testing becomes available, the accuracy of ranking heifers will be substantially improved, and indexes should be based on genomics instead of just the parent PTAs.

Genetic improvement is permanent, assuming a dairy farm keeps their offspring. Using sexed semen in the highest index heifers will accelerate genetic improvement.

Beyond sexed semen
As discussed, sexed semen is not the only factor that can impact the heifer program. Our goal is to freshen healthy, properly sized, high genetic potential heifers. This requires proper nutrition and feeding so heifers are at an appropriate stature when breeding starts. After they become eligible to be bred, they should become pregnant as fast as possible.

These are two related, but separate processes, and need to be monitored separately:

  1. How soon are heifers reaching proper stature? (Age at eligibility)
  2. How quickly are they becoming pregnant after they become eligible? (Heifer pregnancy risk)

Most heifer growers measure height and weight to monitor growth. Measuring growth is far more proactive than measuring age at eligibility (the age when they are moved to an A.I. pen).

Measuring heifer pregnancy risk is inaccurate unless the date that the heifer is first moved to the A.I. pen is recorded and then used as the start of the eligibility period.

Traditionally, we expect heifer pregnancy risk to be substantially better than milk cow pregnancy risk. However, this is not always the case. Routine monitoring of heifer growth and heifer pregnancy risk can identify opportunities to focus on your heifers. Superior heifer reproduction increases the value of sexed semen and genetic improvement of the herd. PD

Gene Boomer