Selection and Development of Replacement Heifers

Cooperative Extension Service

Kansas State University

Manhattan Selection and Development of Replacement Heifers

Since 10 to 20 percent of the cow herd is typically ated the effect of implants during replaced by heifers each year, heifer selection and devel- the suckling period on subsequent reproductive performance of opment decisions significantly affect an operation’s pro- replacement heifers. Basically, the ductivity and profitability. This productivity and prof- summary of the recommendations itability is largely dependent upon reproductive perfor- would be as follows: mance as research has shown that heifers calving early a. With any of the current im- plants cleared, replacement in their first calving season continued to calve early and heifers should not be wean heavier calves throughout their lifetime than later implanted at birth or within calving heifers (Lesmeister et al., 1973). In other words, the first month of birth. the most productive cows over a lifetime, as measured b. Implanting at 2 to 3 months of age appears to have by actual calf weaning weights, are those that conceive very little adverse effect early when bred as yearlings. on subsequent reproductive The replacement heifer is a mixed blessing for most performance. c. The multiple use of implants, cow-calf operators. On the one hand, she represents the such as at 2 to 3 months of future profitability and genetic improvement of the cow age and again at weaning, herd. On the other hand, the replacement heifer is an generally tends to have some inconvenience, at best. Her smaller size and higher negative impact on subse- quent reproduction in the nutritional requirements dictate that she be raised and . managed separately from the rest of the herd. The fact d. In instances where heifers that she is essentially non-productive for the first 2 years have been kept for later pro- of life makes her easy prey for mismanagement. How- duction, there does not appear to be any impaired ever, the growth and development of the replacement impact on subsequent level of female from birth until she weans her first calf is of criti- milk production. cal importance, in order for her to become a highly pro- National figures indicate that 16 to 18 percent of the cow herd is ductive part of the cow herd. Thus, her selection and replaced annually by heifers. Since development is of paramount importance to the contin- half the calf crop is heifers, that ued success of the operation. means 32 to 36 percent of the heifers serve as replacements, but 64-68 percent are sold as stocker heifers. Not implanting these is a The development of replace- placement heifers can be based 600- to 700-pound loss in weaning ment heifers can be divided into upon objective records of birth weight per 100 cows, which is four phases: dates and weaning weights from enough extra weaning weight to 1. Preweaning consistently early calving, high equal a calf and a half. 2. Weaning to Breeding producing cows. Since producers typically don’t 3. Breeding to Calving The replacement heifer should know which heifers will be kept 4. Calving to Rebreeding weigh at least 450 to 600 pounds at for replacements at implanting weaning, depending upon , time, an easy way to avoid possi- Preweaning frame size and feed supply. How- ble problems is to implant only ever, it is important that this weight those heifer calves born during the During the preweaning phase, be the result of true skeletal and last half of the calving season and producers largely depend on the muscle growth without a substan- selecting replacements from the dam to nurture and care for the re- tial amount of fat. older group of non-implanted placement heifers until weaning. A question that often arises dur- earlier born heifers. However, the influence of a few ing this period is: “What effect management practices should be would implanting or creep feeding Creep feeding. The concept of mentioned. Producers are encour- have on future productivity?” creep feeding is basically one of aged to individually identify by ear providing extra energy by feeding tag, number brand, etc., both cows Implanting. Some recent excel- concentrates or providing a forage and calves, so that selection of re- lent research studies have evalu- creep for suckling calves to

Replacement Heifers—1 supplement their mother’s milk. estrus and artificially inseminate 4. Heifers from heifers may The key in deciding whether to (AI) their yearlings to high accu- have slower growth rates creep feed or not is whether the racy, progeny-proven calving ease, during the suckling phase inclusion of creep diet will comple- moderate growth, optimal milk with less udder fat deposition ment the dam’s milk production bulls, “keeping heifers from resulting in less risk of and whether calf and grain prices heifers” may have tremendous reduced subsequent milk are such that it will be economi- merit for the following potential production particularly in cally advantageous to creep. reasons: the English . 1. If the yearlings were bred Puberty consideration. Heifers Does creep feeding have a nega- prior to the mature cow herd, cannot calve early unless they con- tive impact on subsequent produc- heifers from heifers are born ceive early and heifers cannot con- tivity of replacement heifers? earlier and therefore, more ceive early unless they reach pu- Typically, the recommendation apt to reach puberty by the berty by 12 to 14 months of age, or has been to not use creep rations beginning of their yearling at least early in their first breeding with heifers that will be kept as breeding season. season. To calve at 22 to 24 months replacements. That is particularly 2. If sired by calving ease, mod- of age, a heifer must reach puberty true in situations where the cows erate growth, optimal milk by 12 to 15 months of age. Heifer are already good milkers and situa- bulls, heifers from heifers puberty is largely a function of tions in which a full fed creep will should represent the most age, weight, and genotype (Table 1) be used where from 3 to 6 pounds “balanced trait” in based on Meat Animal Research of grain is being consumed by the herd. As mature cows, Center (MARC) data (Cundiff, et al., future replacement heifers. they can have the ability to 1985), which shows tremendous It appears that moderate framed, restrict calf birth weight even variation in age and weight at pu- English breed replacement heifers when bred to terminal growth berty due to breeds of sire (Table 1). consuming high energy diets from type sires. Sire selection, within a breed, also 6 to 8 months of age do suffer 3. Heifers from heifers maybe plays a role in determining the age altered udder development, caus- more moderate in frame size and weight at puberty. Heritability ing lipid-deposition which can sev- and easier fleshing represent- estimates for age and weight at pu- erely reduce later milk production. ing less cow maintenance berty are moderate to high which In contrast, an extensive summary cost. of the American Simmental Asso- ciation data by Montana State showed that creep fed Simmental heifers had no reduction in subse- TABLE 1. Breed Group Averages for Age and Weight at Puberty quent productivity when compared to non-creep fed heifers. Breed Weight at Age at Group Puberty (Ibs) Puberty (days) Weaning to Breeding * Low-Moderate Growth, Milk: A rigorous selection program Jersey-x 518 308 should be followed at weaning time Hereford-Angus-x 622 357 for prospective replacements, based Red Poll-x 580 337 on available records and visual South Devon-x 639 350 appraisal. It is highly advantageous Tarentaise-x 622 349 to select the early born, growthy Pinzgauer-x 611 334 heifers, because these females are * Brahman Influence: more likely to be cycling at breed- Saihwal-x 642 414 ing time as yearlings. This repre- Brahman-x 712 429 sents indirect selection for fertility *High Growth, Milk: and milk production since larger Brown Swiss-x 615 332 heifers tend to be out of the earlier Gelbvieh-x 626 326 calving, heavier milking cows. Simmental-x 666 358 Additional selection criteria may Maine-Anjou-x 672 357 include structural soundness, dis- *High Growth: position, fleshing ability muscle Limousin-x 679 384 expression and frame size. Charolais-x 703 384 Traditionally, cattlemen have Chianina-x 699 384 kept few heifers weaned from first calf heifers. However, for those From Beef Research Report No. 2: Roman L. Hruska U.S. Meat Animal cattle producers who synchronize Research Center and University of Nebraska College of Agriculture.

Replacement Heifers—2 means that these traits should re- TABLE 2. Bull Testicle Size Versus Heifer Age at Puberty1 spond to selection. One of the most practical meth- ods to decrease age to puberty in Scrotal Circumference Heifer Age at heifers both between and within Breed of Yearling Bulls (cm) Puberty (days) breed is to select daughters of bulls Gelbvieh 34.25 + .5 341 + 9 with large scrotal circumference. Brown Swiss 33.75 + .5 347 + 8 (MARC) data displayed in Table 2 Red Poll 33.00 + .5 352 + 8 shows breed average differences in Angus 32.25 + .5 375 + 10 yearling scrotal circumference and, Simmental 32.25 + .75 372 + 6 of greater importance, reduced age 30.2 + 5.5 390 + 13 to puberty in half-sibs and/or Hereford 30.00 + .75 398 + 7 daughters. Genotype difference in Charolais 29.75 + .5 398 + 6 scrotal circumference has practical Limousin application to the selection of 1MARC Data breeds and individual sires within breed for rotational and terminal crossbreeding systems. Management practices can also influence the onset of puberty. TABLE 3. Effect of Bull Exposure on Puberty of Yearling Heifers Exposing heifers (from weaning through breeding) to gomerized Age at Puberty Pregnant at bulls will reduce age of puberty. In a 2-year Nebraska study (Rober- Group Days 21 Days (%) 50 Days (%) son et al., 1991), 80 heifers were ex- Exposed 399 51 86 posed to 4 gomerized bulls and Non-exposed 439 17 81 80 heifers were non-exposed. The Difference 40 34 5 bull-exposed heifers reached pu- berty 40 days earlier with 51 per- cent pregnant the first 21 days of the breeding season compared to 17 percent of the non-exposed heifers (Table 3). TABLE 4. Effect of Rumensin on Puberty and Conception Rates Another management practice that affects puberty is the feeding Group No. of Heifers % Cycling % Conception of ionophores. Research (Moseley et al., 1977) has shown that iono- Rumensin 24 92 55 47 phores, such as Rumensin and Control 26 58 Bovatec, will hasten onset of puberty in heifers (Table 4). Age to puberty decreased and percent cyclicity at the beginning of the breeding season increased in 1. Induce cyclicity in some pre- Adequate nutrition. Nutrition Rumensin fed heifers, resulting puberal heifers that are close; from weaning to breeding dictates in increased conception rate dur- however, have not quite early heifer conception and subse- ing a short breeding season. reached sexual maturation quent reproductive performance. possibly due to insufficient Table 5 shows the major nutrient Use of progestin compounds weight and/or age. requirements of replacement will induce puberty. Melengestrol 2. Keep cyclic heifers out of heifers from weaning to first acetate (MGA), an orally active heat until desired as part of breeding. This information serves progestin, has been used by the an estrous synchronization only as a general guide. Weight feedlot industry to keep feedlot program. and body condition should be heifers out of heat resulting in re- Another progestin with the evaluated periodically to ensure duced riding and improved aver- same effect is the synchromate B that the heifers are developing age daily gains and feed efficiency. implant. sufficiently. This same MGA (.5 mg/head/ day) can be mixed and fed with supplemental grain to replacement heifers to:

Replacement Heifers—3 TABLE 5. Major Nutritional Requirements of Replacement Heifersa

Crude NEm NEg Calcium Phos. Body Daily Protein Mcal/ Mcal/ TDNC Grams/ Grams/ Vit. A Weight b Gain Lb/Day Day Day Lb/Day Day Day lU/Day Replacement Heifers:

400 1.5 1.2 4.10 2.06 6.9 20 11 10,000 500 1,5 1.3 4.84 2.44 8.2 19 12 12,000 600 1.2 1.3 5.55 2.79 8.8 19 13 14,000 700 1.2 1.4 6.24 3.13 9.9 19 14 16,000 a Adapted from National Research Council, “Nutrient Requirement of Beef Cattle, “ 1984. b Average body weight during feeding period. c The energy (TDN) levels reported are sufficient in relatively mild climates, As a general rule, the amount of TDN should be increased by 1% for each 1°F decrease in the windchill temperature below 30°F for cattle with dry, winter hair or below 55°F for wet or summer hair coats.

The effect of first winter nutrition level on heifer development, repro- TABLE 6. Effect of First Winter Nutrition on Subsequent ductive performance and calf pro- Performance of Replacement Heifers duction is shown in Table 6 (Lemen- ager et al., 1980). Lbs of Grain/Hd/Day Insufficient postweaning energy (plus low quality fescue hay) supplementation reduced the per- Item 0 2.7 5.4 centage of heifers breeding as year- lings. Postweaning energy levels No. heifers 112 113 112 affected the weaning weight of the Initial wt. (Ibs) 496 502 493 heifers’ first calf and reduced the ADG-150 days (Ibs) 0.07 0.50 0.80 percentage of heifers conceiving Breeding wt (Ibs) 506 577 613 for their second calf. % bred as yearlings 69.2 73.9 83.5 It should be emphasized that (60 days) replacement heifers need to be fed separately from the rest of the Subsequent Production herd. Because of their size and age, as well as higher nutritional % rebred after 1st calf 67.3 75.4 87.1 demands, they simply cannot com- Weaning wt 405 433 443 pete with the rest of the mature 1st calf (Ibs) cow herd, nor can they be expected to efficiently utilize poorer quality Purdue, 1980 (3-year summary). forages and still breed as yearlings.

Target weight concept. In gen- eral, heifers should be targeted to gain 1 to 1.5 pounds per day from Example: weaning to breeding, depending upon weaning weight, target pu- Projected mature cow size 1200# beral weight and length of feeding Target yearling breeding weight 780# (65% of 1200#) period prior to breeding. Specific Current weight 500# target gains can be calculated Current date October 1 according to the “target weight” Start of yearling breeding season May 1 concept and shooting for 65 per- Feeding period 210 days (7 months) cent of mature cow size at the Total gain needed 280# beginning of the yearling breeding ADG needed 1.3 ADG season.

Replacement Heifers—4 Specific rations can be formu- TABLE 7. Effect of Heifer Nutritional Development on Subsequent lated for target gains of 1.3# ADG Performance and periodic weighing along with ration changes, if needed, will keep the heifers on target. % of Mature Calf Kansas State Research (Patterson Weight at Pre- Calf Death et al., 1987) revealed that if heifers Breeding as Breeding Calving Birth Calving Loss are fed to only 55 percent of target Yearling Number Weight Wt Wt Difficulty % weight, calving difficulty and calf death loss increase and rebreeding 55 60 600 834 70.9 52.3 6.2 for the second calf decreases (Table 7). 65 61 683 897 73.3 28.8 4.5 In short, if spring-born heifers are nutritionally mismanaged dur- ing their first winter, penalties include: TABLE 8. Weight Changes and Reproductive Performance for Light ■ increased age at puberty and Heavy Heifers When Fed Separate or as a Group ■ lower conception rates ■ increased calving difficulty ■ increased calf morbidity and Fed Together Fed Separate mortality Light Heavy Light ■ later born calves Heavy ■ lighter weaning weights No heifers 10 10 19 20 ■ decreased percent rebreeding Weaning weight 376 475 374 464 of first calf heifers ADG 1.3 1.5 1.8 1.2 ■ later rebreeding of first calf Breeding weight heifers Target 715 715 715 715 ■ reduced lifetime productivity Actual 620 720 670 720 Spread 100# 50# Feeding of weight groups. If the Puberty Age (days) 423 404 405 389 producer has a large number of % Cyclic (start of breeding) 60 90 79 90 heifers, work done by Varner et al., % pregnant in 45 day season 60 80 80 90 1977 indicates that reproductive per- formance can be enhanced by sorting heifers into light and heavy weight groups and fed to reach the same tar- get breeding weight, however, at sep- still growing and lactating. This fertile replacement females that con- arate target gains (Table 8). postpartum interval can be even ceive promptly. Late calving first- Heifers fed separately were more longer if the heifers experience calf heifers can be a reproductive similar in weight at breeding than calving difficulty. Therefore, year- liability for the rest of their life. heifers fed together. The smaller ling heifers should be bred starting Don’t worry about a few open year- heifers did not have to compete with 3 to 4 weeks prior to the mature lings. This represents a form of the heavier heifers for feed, and feed cow herd. This allows first-calf “retaining ownership” with addi- dollars were allocated to where they heifers extra time to start cycling tional marketing opportunities. did the most good. The light-weight and stay on schedule with the ma- ture cow herd. Calving labor and heifers fed separate had a 19 percent Breeding Until Calving advantage in cyclicity at the start of facilities can be committed to the the breeding season, and heifers fed heifers and the extra age increases The next step in the profitable separately had a 15 percent advan- weaning weights and makes for a management of the replacement tage in pregnancy rate over heifers more uniform calf crop. heifer is to ensure her adequate fed together. growth and development from Short breeding season. Produc- breeding until she calves as a Lead breeding. In most cow ers are encouraged to retain more 2-year-old at about 85 percent of herds, the most difficult reproduc- heifers at weaning than are needed her mature weight. During this time, tive group of females to get preg- as replacements to allow further the bred heifer should gain about nant is the 2-year-old females nurs- at yearling and after breed- ¾ to 1 lb per day or about 250 to ing their first calf. First-calf heifers ing. The yearling heifers’ breeding 300 pounds. Thus, British bred typically have a 2 to 3 week longer season should be limited to 45 days heifers and crossbred heifers of postpartum interval than older fe- with open heifers culled, another se- British breeding should go into males, due to the fact that they are lection criteria. This selects for re- the calving season weighing 900 to productive efficiency resulting in 1050 pounds and the large-framed

Replacement Heifers—5 breeds and crosses should weigh gestation might be responsible for the dam’s energy level pre-calving 1000 to 1150 pounds. It is prefer- dystocia problems, more recent did not affect calf vigor but able to have the heifer growing research has not substantiated this reduced the calf’s daily heat pro- continuously throughout this theory. Therefore, producers duction by 11 percent. phase. For spring calving herds, should be warned not to underfeed Part of this dietary energy effect summer pasture is usually ade- protein to the gestating heifer in an on calf vigor and its susceptibility quate for the first half of this effort to reduce calving difficulty. to subsequent diseases relates to period. However, it is important to Several studies showed that low the decreased calf vigor that results recognize that most of the fetal protein feeding during gestation in a calf that is slower to suckle fol- growth occurs during the last resulted in decreased calf vigor, lowing parturition. It is important 50-60 days prior to calving. Thus, delayed uterine involution, increased to remember that the way that the adequate nutrition, especially interval to estrus and decreased con- calf acquires passive immunity is energy (fed apart from the mature ception rates following calving. through its initial consumption of cows), is essential for proper These problems appear to be com- colostrum or the first milk pre- development of the fetus and to pounded when energy is also defi- sented by the heifer or cow. prepare the heifer for calving and cient, illustrating the need for a To acquire this immunity the lactation. properly balanced diet. calf must nurse shortly after birth Research at several stations has Restriction of the dam’s dietary for two reasons: 1) the content of consistently shown that roughing energy pre-calving will not only the immunoglobulins in the milk the heifer the last few months have an impact on birth weight, decreases rapidly; 2) the calf’s abil- prior to calving results in lighter, but will have an impact on the calf ity to transfer the immunoglobu- weaker calves at birth without any vigor and the calf’s ability to sur- lins across the epitheral cells of the decrease in calving difficulty, vive and the subsequent growth calf’s small intestine decreases greater calf sickness and mortality rate. At the time of calving, the each hour after the calf is born. lower milk production, slower re- newborn calf moves from a very Dietary energy of the dam prior to turn to estrus and poorer overall protected uterine environment to calving will have a major impact reproductive performance. Thus, what can be a pretty harsh extra on how quickly the calf suckles. “shorting” the bred heifer nutri- uterine life very quickly. Table 9 shows the major nutri- tionally prior to calving is an invi- Since many heifers calve before tional requirements of replacement tation to disaster! the cowherd calving, this may heifers from breeding through Recent research has studied the occur during cold weather, result- calving. This information can be effect of protein supplementation ing in a calf that must have the used as a guide to feeding these fe- on calving difficulty, calf vigor and ability to produce heat (thermoge- males. Body condition and weight calf survival. Although researchers nesis) very rapidly after birth. should be used to modify the feed- in the late 70s thought excess pro- Recent work by Colorado State ing program. Consult KSU Animal tein supplementation during late University showed that restricting Science Bulletin No. C-842, Feeding

TABLE 9. Major Nutritional Requirements of Bred Yearlings1 — Crude NEm NEg Calcium Phos. Body Daily Protein Mcal/ Mcal/ TDN4 Grams/ Grams/ Vit. A Weight 2 Gain 3 Lb/Day Day Day Lb/Day Day Day lU/Day Bred Yearling Heifers—Last Third of Pregnancy: 700 1.0 1.3 7.95 .3 8.5 19 14 19,000 800 1.0 1.4 8.56 .3 9.2 21 15 21,000 900 1.0 1.5 9.15 .3 9.9 22 17 23,000 Trace mineralized salt should be provided either free choice in a mineral supplement, or mixed into the ration at 3% of the dry matter to all cattle.

1 Adapted from National Research Council, “Nutrient Requirement of Beef Cattle,” 1984. 2 Average body weight during feeding period. 3 About 0.8 lb gain/day during the last third of pregnancy is made up of fetal growth. 4 The energy (TDN) levels reported are sufficient in relatively mild climates. AS a general rule, the amount of TDN should be increased by 1% for each 1°F decrease in the windchill temperature below 30°F for cattle with dry, winter hair or below 55°F for wet or summer hair coats.

Replacement Heifers—6 TABLE 10. Nutrient Requirements of First Calf Heifers1.2

5 Crude NEm NEg Calcium Phos. 4 Body Daily Protein Mcal/ Mcal/ TDN Grams/ Grams/ Vit. A Weight 3 Gain Lb/Day Day Day Lb/Day Day Day IU/Day 750 .5 1.8 9.51 .92 10.8 26 18 30,000 850 .5 1.9 10.11 1.01 11.6 27 19 33,000 950 .5 2.0 10.69 1.09 12.5 28 21 35,000 1 Adapted from National Research Council, “Nutrient Requirements of Beef Cattle.” 1984. 2 Average milk production (10 lb/day). 3 Average body weight during feeding period. 4 The energy (TDN) levels reported are sufficient in relatively mild climates. As a general rule, the amount of TEN should be increased by 1% for each 1°F decrease in the windchill temperature below 30°F for cattle with dry winter hair or below 55°F for wet or summer hair coats. 5 Trace mineralized salt should be provided free choice in a mineral supplement, or mixed into the ration at 3% of the dry matter.

Your Cows by Body Condition, for cows, must lactate, continue to Ideally, first calf heifers should more detailed information on ra- grow and prepare her reproductive graze separate pastures from ma- tion formulation according to body tract for rebreeding, hopefully ture cows. Running these pairs condition and heifer requirements. early so that she can calve early with yearling heifers may be an again next year. Feed will be used option in situations with a limited Calving To Rebreeding for maintenance, lactation and number of pastures, especially if growth before reproduction. After two-year-old heifers are to be Calving is a critical time for calving underfed heifers will have mated to the same calving ease 2-year-old heifers since most calv- longer postpartum intervals and sires as yearlings. Health and vac- ing difficulty occurs at this age. may even fail to rebreed for their cination programs apply to first- Separate the heifers from the ma- second calf. Nutrient requirements calf heifers as well. If possible, a ture cows during the calving sea- for first calf heifers nursing calves further selection of replacement son and check them every 3 to are listed in Table 10. heifers after they have weaned 4 hours for needed assistance. If a Again, this is only a general their first calf yields the most con- heifer has not delivered a live calf guide. Genetic potential for milk fidence in future productivity. within 2 hours after the water sac production greater than 10 lb/day appears, she should be examined and environment can dramatically to determine the need for assis- increase the heifer’s needs. Again, Summary tance or possibly veterinary exper- body condition and weight should Heifers developed according to tise. Prolonged deliveries cause be used to modify the feeding pro- these management techniques undue heifer stress, increased calf gram. The “eye of the master” still have an excellent chance of: losses, increased postpartum inter- plays an important role in feeding 1. Becoming pregnant early in nal and lower rebreeding rate. the cow herd. the first breeding season as Within a few hours, newborn In spring calving programs, yearlings. calves should be dried off and nurs- grass is usually adequate several 2. Giving birth to a live calf with ing colostrum for protection from weeks before the mature cow minimal difficulty. infections and scours. Calves which breeding season begins. In situa- 3. Raising their calf to an accept- fail to nurse within a few hours after tions of high range/pasture grass able weaning weight. birth should be given colostrum moisture content, lactating heifers 4. Breeding back early for their stripped from the mother or frozen may not be able to consume enough second calf. previously from a mature cow. Pur- dry matter from grass alone to meet 5. Continuing to reproduce and chased colostral products can also be their needs. Insufficient dry matter wean calves every year for used. intake may result in weight loss, 10 to 12 years. Nutrient requirements of heifers preventing early recycling and after calving greatly increase be- rebreeding. Supplementing lactat- cause the heifer, unlike mature ing heifers on grass maybe neces- sary early in the grazing season.

Replacement Heifers—7 Literature Cited of first calving in beef cows and fed to prebreeding target subsequent calf production. J. weights. K.S. Agri. Expt. Station Cundiff, L.V., K.E. Gregory and Anim. Sci. 36:1. Report of Progress 514:60. R.M. Koch. 1985. Beef Research Progress Report No. 2, Roman Moseley W.M., M.M. McCartor L. Hruska U.S. Meat Animal Roberson, M., M. Wolfe, T. and R.D. Randel. 1977. Effects of Research Center and University Stumpf, L. Weth, A. Cupp, N. rumensin on growth and repro- Kojima, R. Kittok and J. Kinder. of Nebraska College of ductive performance of beef 1991. Influence of growth rate Agriculture. heifers. J. Anim. Sci. 45:961. and bull exposure on age at pu- berty in beef heifers. Nebr. Beef Lemenger, R. P., W.H. Smith, NRC. 1984. Nutrient Require- T.G. Martin, W.L. Singleton and Cattle Report. MP56. ments of Beef Cattle (6th Ed.). J.R. Hodges. 1980. Effects of National Academy Press. Varner, L., R.A. Bellows and winter and summer energy lev- Washington, DC. D.S. Christensen. 1977. A man- els on heifers’ growth and re- agement system for wintering productive performance. J. Patterson, D.J., L.R. Corah, J.R. Anim. Sci. 51:837. replacement heifers. J. Anim. Brethour and W.R. Negus. 1987. Sci. 44:165. Calving and reproductive per- Lesmeister, J. L., P.J. Burfening formance of Angus x Hereford and R.L. Blackwell. 1973. Date and Brahman x Hereford heifers

Replacement Heifers—8 Ron Bolze Extension Specialist Livestock Production, Northwest

Larry R. Corah Extension State Leader Animal Sciences and Industry

COOPERATIVE EXTENSION SERVICE, MANHATTAN, KANSAS C-841 May 1993 Issued in furtherance of Cooperative Extension Work, acts of May 8 and June 30, 1914, as amended. Kansas State University, County Extension Councils, and United States Department of Agriculture Cooperating, Richard D. Wootton, Associate Director. All educational programs and materials available without discrimination on the basis of race, color, national origin, sex, age, or disability. File Code: Animal Science: 2-1,2,3,4 5-93—5M