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Virginia Cooperative Extension -
 Knowledge for the CommonWealth

The Cow-Calf Manager

Livestock Update, March 2004

John B. Hall, Extension Animal Scientist, Beef, Virginia Tech

Why Do Only Some Heifers Breed?

Over the last several months, we have focused on the reproductive biology of the beef cow. What about the most expensive member of the herd, the replacement heifer? Understanding how heifers develop will allow producers to do a better job of managing and developing replacement heifers.

Ready to go, but everything's on hold.
Heifers are born with all the eggs (oocytes) they will ever have several hundred thousand eggs. Unlike bulls that make new sperm daily after puberty, heifers do not make new eggs. However, these eggs will start to develop and grow in waves as puberty approaches and continue for the rest of her life. These waves grow and regress on approximately a weekly basis, but a mature fertilizable egg is only released every 18-23 days just after the heifer is in heat.

Experiments have demonstrated that heifers as young as 2 months of age can be induced to ovulate a fertile egg if they are given the proper hormone treatments. In fact, some dairy embryo transfer companies are using hormone treatments to produce embryos from young heifers. What keeps heifers on hold is the absence of these reproductive hormones (LH and FSH) being released from the brain to cause egg development. It is the beginning of release of LH and FSH and the response of the ovary and eggs to these hormones that indicates the onset of puberty.

So what are the brakes on the system?
Considerable research, including some of mine, has gone into understanding the factors controlling the onset of puberty. Simply, there are three major controls breed, age and nutrition. Those three factors can affect the age that heifers attain puberty by several months. All other factors have minor impacts on age at puberty usually changing age at puberty by a month or less. This month we will talk about the major effectors of puberty, and their management. Next month, we'll visit about those minor factors and managing them to produce better heifers.

Quite simply, the normal timing of puberty is dependent on the central nervous system being mature enough to begin the release of LH and FSH. All the mechanisms behind control sexual maturation of the brain are not known. The primary factor is genetics of the animal which predetermines when heifers will reach puberty. However, nutrition and prepuberal exposure to reproductive hormones can speed up or delay the timing of puberty. By use of management strategies, producers can reduce age at puberty by 1 or 2 months, but that is the limit.

Breeds have a dramatic effect on age at puberty in beef heifers. The range in age at puberty based on breed can be from 6 months to 18 or 20 months of age. With the goal to have heifers calve at 24 months, heifers need to be bred by 14 or 15 months of age. Furthermore, research indicates that heifers that have 2 or 3 cycles before the breeding season have increased pregnancy rates to the first service. Of the major breeds in Virginia, Gelbvieh and Tarentaise are the earliest to reach puberty; Angus, Hereford and Simmental intermediate; Charolais late, and Brangus very late (Table 1). Even within a breed, there is considerable variation in age at puberty in different lines.

Table 1. Summary of the relative age at puberty of cattle breeds.
Age at puberty Breeds
Very Early (< 9 mo.) Jersey
Early (9-12 mo.) Red Poll, South Devon, Tarentaise, Pinzgauer, Brown Swiss, Gelbvieh, Holstein
Moderate (12-14 mo.) Hereford-Angus, Devon, Simmental, Maine-Anjou
Late (14-16 mo.) Limousin, Charolais, Chianina, Brangus, Santa Gertrudis
Very Late (>16 mo) Brahman, Sahiwal
Adapted from Maren et al., 1992

Crossbred heifers are younger at puberty than purebred heifers. Reproductive traits are lowly heritable, but there is considerable heterosis in reproductive traits. To improve reproductive efficiency in commercial herds, producers should have a crossbred cowherd based on breeds that reach puberty by 12 to 14 months of age.

Proper feeding of replacement heifers is essential to successful reproduction. While positive nutritional manipulations can reduce age at puberty by a month or more, poor nutrition may delay puberty almost indefinitely. Heifers that are not heavy enough by the beginning of the breeding season will fail to breed (Table 2). Under nutrition blocks the release of LH and FSH by a different mechanism than age or genetics. Therefore, if heifers are old enough to breed but too light, rapid growth by increasing nutrition can eventually overcome poor nutrition.

Heifers should be 65% to 70% of their projected mature weight by breeding - often called Target Weight. Typical heifers should weigh 750 to 850 lbs by the beginning of the breeding season. This weight will ensure that nutrition is not the limiting factor for reproduction. From weaning until breeding, heifers need to gain 1.5 lbs. to 1.75 lbs. per day. Note that the heifers in Table 2 were 1970 model heifers. That's why the weights at puberty are much lighter than what is needed today.

For heifers in Virginia, energy is the nutrient that limits age at puberty. Diets for replacement heifers will need to include energy supplements in addition to high quality forage or grazing. Supplements should be fed at the rate of 1/2 to 1% of body weight. Feeds such as soy hulls, corn gluten feed or wheat mids that contain highly digestible fiber are better choices than corn.

Protein availability can also influence age at puberty onset. By the start of the breeding season, only 40% of the heifers receiving a low protein (9% crude protein, CP) diet had reached puberty compared to 90% of heifers receiving adequate nutrition. In addition, extra energy (i.e. corn) could not overcome the protein deficiency. Similarly, additional protein could not overcome a lack of available energy. Protein requirements for developing heifers are between 11 and 12% CP.

Table 2. Effect of feed level on reproductive performance in beef heifers
  Low Medium High
Gain lb./day 0.5 1.0 1.5
Age at first estrus 434 412 388
Weight at first estrus 523 545 563
Conception rate first 20 days of breeding season 30% 62% 60%
Overall conception rate 50% 86% 87%
Adapted from Short and Bellows, 1971

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 an increase in age at puberty. Iron, which is found in high levels in soils, is antagonistic to copper uptake. It may be wise to test forages for high iron levels if copper deficiencies or reproductive problems are noted.

There are approximately 60 days before the breeding season for most spring born heifers. Producers should weigh replacement heifers to make sure heifers are on track to meet their target weight. Heifers that are not growing as expected this winter can be put on a higher plane of nutrition until breeding season. The results will be more heifers ready by the beginning of the breeding season.

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