A common discussion between dairy producers and their consultants continues to be about the optimum age of calving for dairy heifers. Today, we focus on reproductive efficiency to get heifers into the milking string on time.

Historically, dairy heifer research focused on feeding heifers higher energy diets to increase average daily gain (ADG) to help reduce the calving age below recommended levels (22 to 24 months). The thought was to shorten heifer-raising period and correspondingly reduce feed cost. Although this strategy has the potential to lead to an earlier return on investment, decreasing the calving age frequently resulted in a decrease in first lactation performance.

Some loss in first lactation milk yield with early calving ages (less than 22 months) is well-documented but questions regarding lifetime milk yield often follow. Some research has suggested lifetime milk yield is improved when heifers calve earlier (less than 22 months), yet other research demonstrated little difference in lifetime milk yields. In short, research has not identified early calving of dairy heifers (less than 22 months) as a high priority to maximize lifetime milk yield of dairy cows. As a result, the general guideline is to calve dairy heifers at between 22 to 24 months of age.

While this calving age guideline of 22 to 24 months exists, there continues to be a dairy industry emphasis on management practices to reduce the age of first calving further to 19 to 21 months.

It just seems logical to increase ADG of pre-breeding heifers to get them to breeding weight earlier, calve earlier and save rearing cost. The questions and approach is logical, but it is unfortunately filled with many assumptions making application of them challenging.

Listed below are general assumptions about the age of first calving in a given dairy herd or in the dairy industry.

1) The age of first calving in the United States is not changing. Dairy producers are not adopting management practices to reduce the age of first calving.

This assumption is false.

Considerable progress has been made in reducing the age of first calving. Scientists from the USDA reported that in 1980 only 26 percent of Holstein heifers calved between 22 and 25 months of age. In 2004, over 55 percent of Holstein dairy heifers calved between 22 and 25 months of age.

2) Many herds have first calving ages less than 22 months.

This assumption is true and false.

Many individual heifers in a herd may calve at 22 months of age, but the herd’s average age (DHIA) at first calving will always be greater. This means individual heifers that calve at 22 months of age likely come from herds with a herd average calving age of 24 months.

The reason for this is relatively simple. Calving age distributions in a dairy herd or in the dairy industry as a whole are not evenly distributed. The normal inefficiencies of breeding and re-breeding, string pregnancy and calving age into a righttailed distribution.

3) Improving ADG is the most important management practice to reduce herd breeding age and herd average calving age.

This assumption is mostly false.

The herd average age at first calving is not solely a function of ADG. Age at first calving is a function of the following factors: voluntary age waiting period, pregnancy rate, variance of pre-breeding ADG, mean pre-breeding ADG and rebreeding cutoff criteria.

For example, a dairy producer with prebreeding Holstein heifers growing at 1.8 pounds per day sets a voluntary age waiting period of 13 months to avoid calving ages less than 22 months. The producer then sets a second breeding weight criteria of 875 pounds (for Holsteins) to assure adequate size. If prebreeding heifer ADG is highly variable (SD 0.20 pounds per day), heifers actually reach the breeding weight criteria from 13.0 to16.0 months. If the pregnancy rate is only 50 percent, this increases re-breeding rates.

Under this scenario, the first heifers calve at 22 months and the last heifers calve at 29 months. The difference in the number of days on feed is greater than 200 days, making grouping, diet formulation, control of body condition and weight at calving very challenging. Are you getting your heifers into the milking herd on time?

In contrast, by decreasing the variance of heifer growth 0.10 pounds per day, through the use of good housing, bedding, and nutrition, and increasing the pregnancy rate from 50 to 60 percent, a herd’s average age at first calving decreases from 25.6 months to 23.8 months without any appreciable management change in the voluntary age waiting period or ADG.

In dairy herds with herd average calving ages of less than 23 months, common denominators include excellent breeding efficiency and very low variance in heifer growth. These two measures define heifers reaching the breeding weight criteria at very similar ages, and heifers become pregnant at that age with a high degree of efficiency. It is important to understand that pre-breeding ADG is not the sole influence on age at first calving.

In conclusion, the secret to managing age at first calving is to intensively manage the pieces of the puzzle that determine a herd’s calving age objectives, and worry less about research defined optimum age, or optimum lifetime milk yields.

Source: P.C. Hoffman, professor, Department of Dairy Science, University of Wisconsin-Madison, Accelerated Genetics