Breeds and breed types vary with respect to age at puberty. In addition, sire selection within a breed also plays a role in determining age and weight at puberty. Age and weight at puberty are moderately to highly heritable traits. This means that producers can use selection to improve these traits within a given herd.
An easy method of selection for age at puberty in replacement heifers is to select daughters of bulls with large scrotal circumference. An interesting correlation, in general, is that bulls with larger scrotal circumferences have daughters that reach puberty earlier.
Target Breeding Weights
Heifers that breed and calve early their first year have been shown to have an advantage in lifetime production. This is in addition to a reduction in overall production costs to the initial calving. For early breeding to occur, heifers must be cycling at the start of the breeding season. Furthermore, conception is greatly improved by breeding after several heat cycles compared to the first estrus. Therefore, heifers should be cycling 60 days prior to breeding or by about 12 months of age.
The level of nutrition the heifer receives the first winter following weaning will influence her rate of development, weight gain, and the age and weight at which she reaches puberty. Heifers fed for a higher rate of gain will be heavier and younger at puberty. Low rates of gain will delay puberty, but heifers will reach puberty at a lower weight. The fact that weight has such an important impact on sexual development allows use of a simple nutritional management concept known as target weight. A heifer's target weight is the minimum weight she should achieve by the time she is exposed for breeding.
Current target weight recommendations call for heifers to weigh 65 percent of their estimated mature weight at the time of breeding. Mature weight of heifers can be estimated from frame scores determined by measuring height at the hip or from weights of similar cows in the herd.
Feeding management is especially important at this time. In addition to greater feed costs, overfeeding heifers may also contribute to decreased productivity. The period from about three to nine months of age is critical to mammary growth in heifers. Both inadequate nutrition and overfeeding in this period have been shown to result in reduced milk production.
Target gains will vary depending on weaning weights, frame size, breed type, and length of the backgrounding feeding period. Typical gain targets from weaning to breeding are 1.25 to 1.5 lbs per day for British breed type heifers and 1.5 to 1.75 lbs per day for Continental breed types. Research suggests that the rate of gain in the development period does not need to be constant as long as the target weight is reached. In fact, some research identifies advantages to developing heifers in stages of reduced energy and gain followed by periods of compensatory growth. A slight reduction in feed expenses has been shown for heifers developed at fairly slow rates of gain early followed by a period of accelerated growth just prior to breeding.
Feeding and Nutrition
It is relatively easy to feed heifers from weaning to breeding to accomplish targeted moderate rates of gain with fairly simple rations. Replacement heifers have nutrient requirements which differ from the rest of the cow herd; consequently, they should be fed and managed separately.
Heifers are commonly developed most economically on high forage rations supplemented with grains and grain by-products, protein concentrates, and minerals as needed to meet their needs and gain target. Modest levels of gain can be achieved solely on high quality roughage fed on a free-choice basis.
In addition, it is important to understand the composition and quality of feeds to be fed. Forages, in particular, vary considerably in level of protein and energy and should be analyzed in order to accurately balance rations. High quality hays are those with over 12 percent crude protein and 58 percent TDN. Hays with crude protein values between 8 and 11 percent and TDN in the mid 50s would be considered average quality hays. Hay with less than 8 percent crude protein and 52 percent TDN would be considered low quality forage.
Insufficient energy intake which results in poor growth can have negative effects on breeding performance of heifers as yearlings and on their subsequent performance in the cowherd.
If large groups of heifers will be developed, producers should consider splitting the heifers into two or more feeding groups (based on weight). This will allow more precise feeding of each group based on necessary target breeding weights and daily gains.
Proper feeding and nutrition of developing heifers is absolutely important. The costs of mismanagement are considerable and include:
1) Increased age at puberty
2) Lower conception rates
3) Greater degree of calving difficulty
4) Increased calf morbidity and mortality
5) Calves born later in the calving season
6) Lighter weaning weights
7) First calf heifers with poor reproductive performance during rebreeding
8) Later rebreeding of first calf heifers
9) Reductions in lifetime productivity
10) Increased rate of culling
Conclusions
Building a quality cow herd requires significant investments of time, money and above all, dedication. It is up to the producer to determine which methods he wishes to follow and implement the appropriate strategies to achieve the desired goals which is a cow herd that is profitable today and tomorrow.
Dr. Steve Blezinger is a nutritional and management consultant with an office in Sulphur Springs, TX. He can be reached at 667 CR 4711 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at sblez@verizon.net. For more information please visit www.blnconsult.com.
