What Age Are Beef Cattle Bred
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The Cow-Calf Manager
Livestock Update, March 2004
John B. Hall, Extension Animal Scientist, Beef, Virginia Tech
Why Practice Just Some Heifers Brood?
Over the last several months, nosotros take focused on the reproductive biology of the beef cow. What about the most expensive fellow member of the herd, the replacement heifer? Understanding how heifers develop will allow producers to practise a better task of managing and developing replacement heifers.
Ready to go, only everything's on hold.
Heifers are born with all the eggs (oocytes) they will ever take � several hundred k eggs. Unlike bulls that make new sperm daily later on puberty, heifers do not make new eggs. Yet, these eggs will start to develop and grow in waves every bit puberty approaches and go on for the remainder of her life. These waves grow and regress on approximately a weekly ground, but a mature fertilizable egg is but released every 18-23 days merely after the heifer is in heat.
Experiments have demonstrated that heifers as young as 2 months of age can exist 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 concur is the absence of these reproductive hormones (LH and FSH) being released from the brain to cause egg development. It is the start 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 arrangement?
Considerable research, including some of mine, has gone into agreement the factors controlling the onset of puberty. Simply, there are 3 major controls � breed, historic period and nutrition. Those three factors tin touch the age that heifers attain puberty by several months. All other factors have minor impacts on historic period at puberty usually irresolute age at puberty past a month or less. This month nosotros will talk nigh the major effectors of puberty, and their management. Adjacent month, we'll visit near those small-scale factors and managing them to produce ameliorate heifers.
Age
Quite simply, the normal timing of puberty is dependent on the key 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 volition accomplish puberty. Withal, nutrition and prepuberal exposure to reproductive hormones can speed up or delay the timing of puberty. Past use of management strategies, producers can reduce age at puberty past i or 2 months, only that is the limit.
Breeds
Breeds have a dramatic effect on age at puberty in beef heifers. The range in age at puberty based on breed tin can exist from half dozen months to xviii or twenty months of historic period. With the goal to have heifers calve at 24 months, heifers need to be bred by 14 or fifteen months of age. Furthermore, research indicates that heifers that have 2 or 3 cycles before the breeding flavor accept increased pregnancy rates to the beginning service. Of the major breeds in Virginia, Gelbvieh and Tarentaise are the earliest to attain puberty; Angus, Hereford and Simmental intermediate; Charolais late, and Brangus very late (Tabular array 1). Even within a breed, there is considerable variation in historic period at puberty in different lines.
Table one. Summary of the relative age at puberty of cattle breeds.
| Historic period at puberty | Breeds |
| Very Early (< 9 mo.) | Jersey |
| Early on (9-12 mo.) | Ruddy Poll, South Devon, Tarentaise, Pinzgauer, Brown Swiss, Gelbvieh, Holstein |
| Moderate (12-xiv mo.) | Hereford-Angus, Devon, Simmental, Maine-Anjou |
| Belatedly (fourteen-16 mo.) | Limousin, Charolais, Chianina, Brangus, Santa Gertrudis |
| Very Late (>16 mo) | Brahman, Sahiwal |
Crossbred heifers are younger at puberty than purebred heifers. Reproductive traits are lowly heritable, merely in that location 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 past 12 to 14 months of age.
Nutrition
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 diet may delay puberty almost indefinitely. Heifers that are not heavy enough by the offset of the convenance season will neglect to brood (Table 2). Under nutrition blocks the release of LH and FSH by a different mechanism than age or genetics. Therefore, if heifers are erstwhile enough to breed but as well light, rapid growth past increasing nutrition can eventually overcome poor nutrition.
Heifers should be 65% to 70% of their projected mature weight by breeding - ofttimes called Target Weight. Typical heifers should weigh 750 to 850 lbs by the kickoff of the convenance flavour. This weight will ensure that nutrition is non the limiting factor for reproduction. From weaning until breeding, heifers need to gain 1.5 lbs. to 1.75 lbs. per 24-hour interval. Note that the heifers in Tabular array 2 were 1970 model heifers. That'due south why the weights at puberty are much lighter than what is needed today.
For heifers in Virginia, free energy is the nutrient that limits age at puberty. Diets for replacement heifers will need to include energy supplements in addition to loftier quality forage or grazing. Supplements should be fed at the rate of one/2 to 1% of body weight. Feeds such every bit soy hulls, corn gluten feed or wheat mids that comprise highly digestible fiber are better choices than corn.
Poly peptide availability can too influence age at puberty onset. By the start of the convenance flavor, only 40% of the heifers receiving a depression 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 poly peptide deficiency. Similarly, additional protein could non overcome a lack of available free energy. Protein requirements for developing heifers are between 11 and 12% CP.
Tabular array 2. Issue of feed level on reproductive performance in beef heifers
| Depression | Medium | Loftier | |
| Gain lb./twenty-four hour period | 0.5 | one.0 | 1.five |
| Historic period at offset estrus | 434 | 412 | 388 |
| Weight at first estrus | 523 | 545 | 563 |
| Conception rate first 20 days of breeding flavor | 30% | 62% | 60% |
| Overall formulation charge per unit | 50% | 86% | 87% |
Minerals and Vitamins are too essential for proper reproductive wellness. Symptoms of deficiencies in Vitamin A, copper, zinc and phosphorous often announced as decreased formulation rates or an increase in historic period at puberty. Iron, which is plant in high levels in soils, is combative to copper uptake. It may be wise to test forages for high iron levels if copper deficiencies or reproductive problems are noted.
At that place are approximately threescore days before the breeding season for about spring born heifers. Producers should weigh replacement heifers to make sure heifers are on track to meet their target weight. Heifers that are non growing as expected this winter can be put on a higher aeroplane of nutrition until convenance season. The results will exist more heifers ready by the commencement of the convenance season.
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