The success of your heifer breeding program next spring is largely dependent on your actions over the next seven months. Many ranchers do not initiate a heifer development program early enough to reach target weight prior to spring breeding.
Age, weight and breed of cattle influences puberty. Of the three, inadequate body weight is usually the cause of non-cycling heifers at breeding (See sidebar "A Case Study").
Determining Target Weight As a rule of thumb, "65 percent of adult weight" is used to determine the correct target breeding weight for replacement heifers. This is how it works. Determine the weight of the mature cow herd in average flesh and multiply the average mature cow weight by 0.65 or 65%. For example, if the average mature cow weight on a ranch is 1,000 lbs., then the minimum target weight for replacement heifers at breeding is 650 lbs. (1,000 x 0.65).
The next step is to determine the average daily gain (ADG) required for heifers to reach target weight. One way of doing this is to test weigh replacement heifers at the start of the development phase. Subtract the target weight from the starting weight and divide that figure by the days in the developing period.
For example, if heifers weigh 500 lbs. on Dec. 1 and the desired April 15 target weight is 650 lbs., then a 150-lb. gain is needed to reach target weight. The developing period is 136 days (Dec. 1 to April 15).
By dividing 150 lbs. by 136 days we determine it will take 1.10 lbs./day gain to reach the target weight on April 15. The longer the development phase, the easier it is to reach target weight. Waiting until a few weeks (or months) before breeding to add several hundred pounds can be nearly impossible.
Research at Montana State University indicates that conception rates are higher on the second and third heat cycles of heifers compared to the first pubertal heat. This suggests that getting heifers to their desired target weight at least one month prior to breeding may increase first conception rates.
Developing A Target Weight Ration There are many combinations of feedstuffs that can be successfully used during the development phase. A popular ration fed in northeastern Nevada has been free choice high-quality grass hay and 3 lbs. of wheat middlings. Wheat middlings are approximately 14% protein and 78% energy.
This combination compliments grass hay very well. When feeding this ration, gains of 1 to 1.25 lbs./day are obtainable. This ration requires the development phase to start early enough so target weights are reached with a lower ADG (more days on feed).
It is often more cost effective to feed a cheaper ration for a longer period of time. It is important that the diet be balanced. One that is short in energy or protein will not produce the desired gain.
When the grass hay in the ration is substituted with alfalfa, wheat middlings are economically replaced with corn. The protein content of alfalfa (16%) and energy content of corn (90%) compliment each other well. Gains of 1.25 to 1.5 lbs./day have been realized when feeding this ration, so fewer days on feed are required to reach target weight.
Producers often wait until early winter to initiate a heifer development program. By doing so, the days on feed are shortened thereby increasing the ADG to reach target weight.
Parasite control at the start of the development phase along with the addition of an ionophore to the ration will also increase ADG thereby reducing required days on feed to reach target weight.
There are many computer ration balancing programs available to help develop a least-cost ration utilizing the feedstuffs available on your ranch and in your area. It is also important to have feedstuffs tested for nutrient content to properly balance diets.
Your local Extension agent or nutritionist can help you in this area. The critical point to keep in mind when developing a ration is that heifers must reach target weight prior to breeding.
It is suggested that ranchers weigh heifers throughout the developing period to ensure the ration is adequate and not excessive. Adjustment in the ration may be required. Too much weight as fat is costly and is as detrimental to fertility as not enough weight.
A two-year study at the University of Nevada Gund Research and Demonstration Ranch shows the importance of winter feeding replacement heifers to reach target weight prior to breeding.
Sixty weaned English bred heifer calves were selected for replacements in the fall of 1995 and again in 1996. Heifers were synchronized for estrus for ease of heat detection and artificial insemination in both the 1996 and 1997 groups. Heat detection involved the use of "Heat Watch," an electronic detection system manufactured by DDx Inc.
Winter nutrition from weaning to breeding included alfalfa hay and 3 lbs. of energy supplement. Hay quality was poor in the 1996-97 heifer group which resulted in fewer heifers reaching target weight at breeding. The two years of data was pooled for analysis (see Table 1).
Results of the University of Nevada study are supported by similar trials conducted by Texas A&M University in 1979. The Texas study showed estrus as high as 90% in heifers exceeding target weight. Those heifers not reaching target weight showed no estrus, while marginally developed heifers showed a 50% cycle rate. The Texas data suggests it is better to error on the high side of target weight rather than the low side.
The heifers classified as marginally developed in the 1996 Nevada heifer group did cycle and conceive within 45 days of the April 15 heat detection date. Not ready heifers did not conceive within the 45-day breeding season. This was evident by the 45-day calving period in 1997. This suggests that the nutritionally high values of green grass play an important role in bringing marginal heifers to target weight and estrus.
Conclusion When early conception of first-calf yearling replacement heifers is the goal, feeding heifers to reach target breeding weight (65% of mature weight) is the best management practice to follow. Starting the development phase early in the year allows ranchers to feed heifers to reach target weight with a less expensive ration that results in a lower ADG (more days on feed).
Test weighing heifers in the middle of the development phase and adjusting the ration accordingly is recommended. For best results only heifers that have attained target weight should be included in an artificial insemination and heat synchronization program.