by: Stephen B. Blezinger

Development of replacement heifers is a critical part of every commercial cow calf operation. While not every breeder raises the replacements that go back into the herd, those replacements have to be developed somewhere. A proper development plan focuses closely on health and nutrition with nutrition comprising a significant portion of the cost incurred in growing that heifer from weaning through breeding, calving and rebreeding.

Nutrition is the management tool that can have the greatest impact on the age at which heifers reach puberty. Heifers of a similar breed composition can reach puberty several months apart when fed different diets, particularly different amounts of energy. In addition, as referenced above, feed cost accounts for 60 to 70 percent of the costs of raising replacement heifers. Therefore, the financial impact of puberty onset is dictated by age at puberty, as well as the feed costs associated with reducing this age when the heifer can be bred. In other words, cost of feeding heifers to reach breeding age earlier should be considered against the income gained by increased conception rates and heavier weaning weights.

It has been long known that energy is the primary limiting factor in most replacement heifer diets. In an experiment reported by Montana researchers (Short and Bellows, 1971), heifers were fed for low (.5 lb/day), medium (1.0 lb/day) or high (1.5 lb/day) gain from weaning until breeding. The high-gain heifers reached puberty earlier than those in the other two groups. In addition, 60 percent of the heifers in the medium and high gain groups conceived in the first 20 days of the breeding season, and overall conception rates were greater for the medium- and high-gain heifers compared to those for the low-gain heifers (Table 1).

Results of this study and others have led to the recommendation that heifers should gain between 1.25 and 1.75 pounds per day between weaning and breeding. Depending on the size of the heifer, the desired rate of gain, and feed intake, diets will need to contain between 62 and 70 percent TDN.

Protein availability can also influence age at puberty onset. By the start of the breeding season, only 40 percent of the heifers fed a low protein (nine percent crude protein, CP) diet had reached puberty compared to 90 percent of heifers fed adequate protein levels (11.5 to 13 percent). Added dietary energy (i.e., feeding increased levels of corn) could not overcome the protein deficiency. Similarly, additional protein could not overcome a lack of available energy.

Minerals and vitamins are also essential for proper reproductive health. Symptoms of deficiencies in Vitamin A, copper, zinc and phosphorous often appear as decreased conception rates or delayed age at puberty. Iron, sulfur and molybdenum may be found in high levels in certain soils, are antagonistic to copper uptake or absorption. It is always wise to test forages for high levels of these minerals if copper deficiencies or reproductive problems are observed. Forages and other feed ingredients should be tested before formulating a mineral mix. Use of a mineral and vitamin supplement specific to the forage base has been shown to improve reproductive responses in virgin heifers.

Feeding management can also affect both age at puberty and feed costs. Separating developing replacement heifers into heavy and light weaning weight groups for better feeding management can result in a 20 to 30 percent increase in cycling and conception rates in lightweight heifers.

Additionally, timing of supplementation of grazing cattle will influence feed costs. Cattle supplemented in morning or late afternoon will stop grazing to eat supplement, while cattle that are supplemented during their normal rest period (around noon) require less supplement for similar gains or performance.

Nutritional Tools – Ionophores, Direct-Fed Microbials and Implants

As mentioned, age at puberty can be reduced by increased growth rates as a result of improved nutrition. Subsequently, producers are often interested in the effects of nutritional management tools such as growth promoting implants and feed additives. Although results may be similar (increased growth rate), ionophores (Rumensin®, Bovatec®) and growth implants work quite differently, in different areas of the animal's physiology. This said, the effect on reproduction can be different.

Ionophores act by altering microbes in the rumen, thereby enhancing digestion and growth rate.

Addition of ionophores to replacement heifer diets can reduce age at puberty by 15 to 30 days while increasing growth rate (average daily gain). Although some of the effect may be due to ionophore action in the rumen, which causes a shift in microbial fermentation patterns, more recent research suggests that there may be systemic actions as well. The response to feeding an ionophore appears to be less dramatic in lightweight or poorly fed heifers.

Other additives such as direct-fed microbials including yeasts, microbial fermentation product or blends of these can also provide positive growth responses. The all-natural feed ingredients work in the rumen and in the intestine to improve fiber and other nutrient digestion as well as stabilize rumen activity. This aids in creating a healthier, more efficient rumen as well as a more functional intestinal tract exhibiting increased absorption of critical nutrients. These products are generally a source of enzymes such as cellulase, α-amylase and protease which serve to increase fiber and feed digestibility.

Most growth implants are steroids (estrogens or androgens) or have steroid-like activity. Steroids increase the release of growth hormones to improve muscle and bone growth, but they also affect the reproductive system. Therefore, if given at the wrong time or in improper doses, growth implants can have a detrimental effect on reproduction. Many different studies have investigated implants and reproduction and results are varied. Effects on replacement heifers can be summarized as follows:

1. Implanting before two months of age dramatically decreased fertility in heifers.

2. One implant during the heifer's lifetime appears to improve growth rate without hurting reproduction.

3. One implant after weaning appears to be less risky than an implant before weaning.

It is strongly recommended to research the use of an implant program in heifers to be used for replacements carefully before implementing this practice. In general, however, avoiding implanting heifers to be used for replacement purposes makes the most sense.

Management of Nutrition

Heifers should be fed in at least two separate weight groups (heavy and light) from weaning until breeding, if at all possible. Diets should be formulated to contain 12 to 13.5 percent Crude Protein and sufficient energy to achieve gains of 1.50 to 1.75 pounds per day, depending on weaning weight and expected mature weight of the animal. Diets should also include a balanced mineral and vitamin supplement that contains trace minerals, vitamins A, D and E, an ionophore and a direct-fed microbial product.

It is important to avoid overfeeding protein. In current markets it is expensive and some researchers believe high levels of protein in breeding animals have negative effects on conception. Overfeeding energy should also be avoided because fat heifers tend to be less fertile and have more calving difficulty. This is a particular problem for heifers that have been feed for and shown extensively.

The goal is to have heifers reach 60 to 65 percent of their projected mature weight 30 to 45 days before the target breeding season. This is known as the Target Weight concept. Target weight for exotic breed (Simmental, Charolais, Limousin, etc.) heifers is usually 65 to 70 percent of their projected mature weight, a bit larger than English breeds. By using a target weight, producers can calculate the rate of gain heifers need to achieve before the breeding season. Diets can then be formulated based on desired gains, and heifers monitored by periodic weighing. Let's consider an example of the target weight system.

Let's say we are feeding a group of Angus X Hereford heifers averaging 500 pounds at weaning in a herd where the mature females average 1,200 pounds with a body condition score 5 or 6. The heifers are weaned on October 1 and we want to begin breeding on May 1. We have decided that the target for this herd will be 65 percent of mature weight by May 1 (approximately 62 percent if April 1).

Target Weight Calculation Example:

• Target Weight by May 1 = 1200 lb x .65 = 780 lb

• Total weight gain needed = 780 - 500 = 280 lb Days to May 1 = 211

• Average daily gain needed to reach Target Weight = 280 lb¸ 211 days = 1.32 lb/day

The related feeding program may be structured as follows:

• A winter diet is fed from October 1 to March 15 (166 days) that gives a daily gain of 1.25 pounds resulting in cattle weighing approximately 707 pounds on March 15.

• The spring diet will be fed from March 16 to May 1 (45 days). The heifers need to gain an additional 73 pounds to reach the target weight for breeding. The spring diet will need to be formulated to provide an ADG of 1.61 pounds (73 lb¸ 45 days).

• Ideally, heifers should be weighed and rations adjusted at least four times between weaning and breeding dates.

Adapted from Hall, J. B., VA Polytech University.

Ten Tips for Better Replacement Heifers

Finally, some basic guidelines:

1. Weigh and condition score heifers at weaning, mid-winter, pre-breeding and breeding to keep growth rate on track.

2. Feed heifers to gain 1.5 to 1.75 pounds per day from weaning until breeding. Remember to include a complete mineral and vitamin mix.

3. Sort heifers into light and heavy weight groups at weaning.

4. Use the target weight concept, i.e. 65 to 70 percent of mature weight by breeding.

5. Include ionophores and direct-fed microbials in diets, but avoid growth implants.

6. Measure and use pelvic areas and reproductive tract scores to cull heifers prior to breeding.

7. Feed MGA and/or use hormone programs to synchronize estrus in heifers. Or use other synchronization programs to tighten breeding and calving periods.

8. Breed heifers to low birth weight bulls.

9. Feed pregnant heifers to calve in body condition score 6.

10. Monitor heifer calvings and provide assistance early if necessary.


Developing heifers can require a higher degree of management than other classes of cattle. But since these animals are the future of the herd, this time and attention is well spent. A good heifer development and feeding program helps insure long term herd productivity and efficiency.

Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, TX. He can be reached at or at (903) 352-3475. For more information please visit us on at www.facebook/reveillelivestockconcepts.

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