Because one goal of proper heifer development is to improve second parity pregnancy percentage, a beef producer may ask "what
is the impact of higher pregnancy percentages during the second breeding season on costs and income?" Table 1 displays the
effect of changing pregnancy percentage for first-calf heifers in 5-percentage point increments on the percent of the herd
that must be replaced each year and the average age of the herd. In general, given the assumptions in the table, for every
5-percentage point improvement in first-calf heifer pregnancy percentage, the number of replacements needed for the herd decreases
by about 1 percentage point and average cow age increases by .01 years.
Table 1. Effect of 2nd parity pregnancy percentage on herd replacement percentage and average cow age at the start of the breeding season.
Heifer Breeding Model: Critical Control Points in Heifer Development
A critical control point (CCP) can be defined as a point in the production process where a value or values can be measured
that are a direct result of previous management and that impact the success of the remaining production process. A number
of CCPs are possible in heifer development (Table 6). Not all of these points are important as control points for every farm.
How well a farm can manage earlier or later CCPs in the production process impacts the necessity of measuring each of the
following potential control points. The following CCPs are based on a production system where replacement heifers are moved
into the replacement pool at weaning, estrous synchronization and artificial insemination (AI) are utilized for the first
breeding opportunity at approximately 14 months of age, followed by exposure to bulls for the remainder of a 60-day breeding
season, and use of natural service only for the second breeding season at approximately 27 months of age. The following potential
heifer development CCPs start with the closest to the end of heifer development as I have defined it (bred for second calf)
and work backwards to selection of heifers for the replacement pool.
Table 6. Calendar for Heifer Development Critical Control Points
I. Weight per day of age at weaning, sire EPDs for milk production, growth, and calving ease/birthweight, and structural soundness (feet, legs, genetic defects)
(fall for spring-calving herds)
Successful heifer development starts with selection of candidates for the replacement pool that are likely to reach puberty
prior to the start of the breeding season, become pregnant early in the breeding season, have little calving difficulty, and
re-breed early in the second breeding season. Because puberty is age and weight dependent, only heifers whose age and weight
at weaning are compatible with being old enough and heavy enough prior to the start of breeding to reach puberty should be
selected. Expected progeny differences for the sires and dams (when available) of individual heifers should be examined to
find those heifers that are predicted to meet herd goals for mature size, growth rate, milking ability, and calving ease.
Heifers with undesirable structural confirmation of feet and legs should not be included in the replacement pool, as well
as heifers with a familiar history of genetic defects such as vaginal prolapse.
If subsequent CCPs have high failure rates, re-examining the selection criteria may be necessary to identify those traits
most likely to be correlated with failure later in the heifer development system.
II. Vaccination protocol to enhance herd immunity to pregnancy-wasting diseases
For most beef herds, the potential list of diseases in a vaccination program would include: brucellosis, Infectious Bovine
Rhinotracheitis (IBR), Bovine Viral Diarrhea (BVD), vibriosis (Campylobacteriosis), and leptospirosis. Other diseases for
which vaccines are available include: Hemophilus somnus and trichomoniasis.
III. Body weight gain (ADG) during weaning to breeding period
(winter for spring-calving herds)
Because weight is a primary factor determining the onset of puberty, ensuring that the nutritional program is meeting average
daily gain requirements for the period from weaning to breeding is critical for a successful heifer development program. If
weight gain is not as projected, the energy content of the diet can be increased so that target weight will be met. In addition,
the use of ionophores, progestogens, and anthelmintics will help ensure that heifers reach target weights and puberty prior
to the start of the breeding season.