To maintain a profitable business model and maximize productivity, producers need to be highly efficient with available resources. In today’s dairy industry, high genetic potential in heifers is one of the most rewarding resource investments.

However, the capability to take advantage of replacement heifer potential is not always straightforward. It is an area where critical aspects may not be considered. This leads to many producers taking two steps forward and one step backward.

Where to start

One might think puberty is the starting point for focusing on reproductive success. Unfortunately, puberty is actually too late. Many research studies have proven that raising productive and profitable replacements begins at birth.

Low-quality colostrum and poor health as calves cause many of the problems seen in unthrifty heifers later in life. Calves that do not have a strong start in life will have slowed growth rates and delayed age at first calving.

Furthermore, when compared to contemporary herdmates that received optimum colostrum quality, they will have lower production performance. These disadvantaged heifers will have lower production performance for their 305-day ME and fat content during the first lactation.



The onset of puberty can be interpreted in two ways. The first is to define puberty as age at first estrus and ovulation. The second is the age at which a female is able to support a pregnancy without deleterious health effects for the dam or calf.

The first explanation is inaccurate because the first ovulation is often not accompanied with estrus. This common occurrence is known as a silent estrus. In addition, the first estrus might not reflect the individual’s full reproductive capacity. If this is the case, puberty precedes physical maturity.

This makes the second definition more applicable from a management perspective in food-producing animals. It is the ability of hypothalamic GnRH hormones to produce high frequency and high amplitude of GnRH pulses. In general, this will be around seven to nine months. Evidence exists that metabolic signals affect the development of hypothalamic neurons and, thus, their production of GnRH.

Although the energy requirements for follicular development, ovulation and embryo transport are low, the metabolic costs of pregnancy and lactation at an early age are quite high. Thus, the female must cross a “metabolic threshold” before puberty occurs.

In addition to age and breed, several other external factors may influence the onset of puberty. These might include nutritional status (puberty age is closely related to bodyweight), presence of a male (or androgenized females) and health status (affects growth rate).

Entering the breeding group

Initiation of a breeding program based on sexual maturity should consider the following parameters: size, bodyweight and/or age. If a scale is accessible, bodyweight is the best assessment. The ideal for Holsteins is 750 to 850 pounds.

However, this is not always the most practical measurement under field conditions. Gauging body size – 47 to 48 inches wither or 51 to 52 inches hip height for Holsteins – is also difficult to calculate in some settings. A means of using this method could be to set a horizontal wood stick with fresh paint or a marker 51 inches above the ground over a gate. Any heifers marked with color on their hip after going through the gate would be breeding candidates.

The easiest assessment is to select for age. Holstein heifers have their first ovulation between 9 and 11 months old; however, breeding them prior to 11 months old may affect growth and future production performance. The recommended breeding age is 13 to 14 months old.

This should be combined with physical consideration of size or weight. Common practice is to first select for age, then elimination based on size. Lastly, have a veterinarian inspect reproductive health to discard free martins, identify reproductive tract abnormalities or to determine cyclicity.

Calving age and bodyweight

Calve replacement heifers at an age that will be most profitable. Milk yields and lifetime profits per individual are maximized when heifers calve between 23 and 25 months old. Calving less than 22 months old can have reduced milk yield or increased dystocia.

Optimum first-lactation yields will occur with first calving at about 24 months old and 1,250 pounds post-calving. A post-calving weight below 1,100 pounds or above 1,300 pounds drastically reduces milk production. Research has proven that body size or bodyweight has a greater impact on production performance.

To reach parameters of approximately 24 months at first calving and 1,250 pounds post-calving without increasing extra days on feed, young heifers should have an average daily gain (ADG) of 1.7 or 1.8 lbs per day. After conception, an ADG of 1.7 to 2.1 lbs per day is acceptable until the last two months of pregnancy.

Follow a nutritionist’s recommendations for ADG to reach size and age goals for breeding and calving. Remember no reproduction or nutritional program will work without good health, nutrition and management plans.

Heavier heifers at calving typically milk more because of less nutritional demands to finish growing, higher nutrient availability and more competitiveness for feed when grouped with mature cows.

However, overconditioned heifers with an ADG greater than 2.0 lbs per day and that weigh more than 1,300 pounds post-calving are highly susceptible to metabolic complications such as fatty liver and ketosis. They will also have reduced intake and lower milk production.

Likewise, higher ADG in young heifers (greater than 2.0 lbs per day) results in an expensive rearing program and leads to fat deposition in the mammary tissue. During the critical age for mammary development, before 9 months old, this will reduce the volume of secretory tissue and affect production for the first and later lactations.

Breeding program

Cows will often be bred to the best recommended A.I. sire mating match. Often, heifers are not as lucky. Regardless of genetic quality compared to herdmates, many heifers are bred to natural-service sires. Some producers justify that their heifers will be bred to proven sires in future lactations. However, this is giving up the opportunity to increase genetic quality.

Producers should invest in elite genetics for the future of their herd. Heifers need to be bred to A.I.-proven sires. If the time and labor for heat detection and insemination is a concern, this service can be contracted through a semen provider.

New technology for heat detection aids and estrus synchronization programs are also an option for increased convenience of a heifer A.I. breeding program. Choosing to incorporate A.I. into a heifer reproduction program will accelerate a herd’s overall genetic progress. PD

Humberto Riverais a reproduction field specialist with Accelerated Genetics. Email Humberto Rivera.