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Heifers managed postweaning to achieve a target BW at breeding [approximately 65% of mature BW (MBW)] will attain puberty sooner, have higher conception rates, and have more viable and heavier calves at weaning (Corah et al., 1975; Lemenager et al., 1980; Wiltbank et al., 1985). The benefits from increasing ADG from weaning to breeding in heifers is realized through improved calving ease and decreased postpartum interval in the 2-yr-old cow, rather than through increased yearling pregnancy rates.

There is a relationship between attaining adequate body condition score (BCS) as young cows and their subsequent reproductive performance. Restriction of dietary energy during the pre-partum period can reduce subsequent cow and calf performance (Patterson et al., 1991). Numerous data sets from across the country indicate 2-yr-olds typically have lesser BW and reduced BCS when compared with older cows. Arango et al. (2002) reported decreased BW and numerically lesser BSC in 2-yr-old cows compared with 3-, 4-, 5-, 6-, 7-, and 8-yr-old Hereford-, Angus-, Red Poll-, Braunvieh-, Maine Anjou-, Chianina-, and Gelbvieh-sired cows. Production data collected from the Cow Herd Appraisal Performance program indicate that young cows typically weigh less and have a lesser BCS than mature cows (Ringwall, 200Ob). In contrast, data collected on Brahman crossbred cows in Oklahoma indicated that 2-yr olds weigh less than 3- or 4-yr olds, but BCS was not different (McCarter et al., 1991). Becasue heifer development is an investment in future production of the animal, it is important that input cost be balanced against expected future income potential. Although it may be possible to feed heifers at a level to increase their likelihood of retention in the herd, if the expected future income does not off-set this expenditure, plus interest and a profit margin, the development investment would not be economically viable. Thus, the producer is faced with the need to balance inputs and outputs.

Review and Discussion

Considerations in Development of 2-Yr-Old Beef Cows.

Cost of Heifer Development. The cost of heifer development will vary across production environments and is generally correlated to annual cow costs within a production system. Producers will incur this cost, regardless of having a pregnant 2- or 3-yr-old cow. Although there will be a level of salvage value for non-pregnant young cows, this value will generally be less than the development costs incurred. Therefore, it is imperative to recover these costs of production by improving second parity reproductive performance.

Heifer development periods differ in length and cost (Table 1). Cost is generally higher during periods of dormant forage and when nutrient requirements for physiological processes (., late gestation and lactation) increase.

Applied Interactions of Nutrition and Reproduction. Dr. Duane Zimmerman at the University of Nebraska (personal communication, 1983) summarized the existing literature at that time. The following were described as key principles in heifer development.

* The level of prepartum nutrition (as evidenced by body condition at calving) generally affects the time required to return to estrus after calving.

* The level of postpartum nutrition (as evidenced by body condition at breeding) generally affects the fertility of the estrous cycle during the breeding season.

These observations are supported by previous and current reports (Corah et al., 1975; Bellows and Short, 1978; Dunn and Kaltenbach, 1980; Whittier et al., 1988; Ciccioli et al., 2003).

Within animal biological type, BW provides a relatively easy method and check point to plan and monitor the nutritional development of heifers. Some general guidelines that can be used to provide benchmarks or targets to plan for the development system are listed in Table 2. A challenge to this is accurately estimating the projected MBW. However, using dam BW and other historical data from within the herd (cull cow BW) and adjusting them to a BCS 5 (Richards et al., 1986, where 1 = thin to 9 = obese) basis will provide a logical starting point. The NRC (1996) uses this approach for targeting needed gain to reach puberty. The NRC (1996) used 60% of MBW approximately 30 d before breeding as a target. Recently, Funston and Deutscher (2004) reported that spring-born heifers developed to either 53 or 58% of MBW had similar reproduction and first-calf production traits between the two groups. Furthermore, these researchers reported that calving difficulty with the second calf was greater for heifers developed at 58% of MBW at breeding. Subsequent second-calf weaning weight and ADG were decreased for heifers developed to 58% of MBW at breeding. Feed costs were $22 per heifer less for heifers developed to 53% of MBW. It is important to understand that attainment of target BW is necessary for each animal, not the herd average. Also, it is important to recognize that heifers do not reach MBW until beyond their second calving. If the needed care and development of heifers are not accomplished, it can lead to early fall-out in 3- and 4-yr-old cows (Meek et al., 1999). More specifically, Patterson et al. (1991) studied the development of two genotypes of heifers to two target BW at the start of the first breeding season. Heifers were developed to either 55 or 65% of MBW. Subsequent production was monitored. These data clearly showed improvement in reproduction (decreased dystocia and shorter postpartum intervals) in heifers developed to 65% of MBW by first breeding-even though heifers were managed the same after breeding.

The benefits of increasing ADG from weaning to breeding in developing heifers is realized through additional calving ease and decreased postpartum interval in the 2-yr-old cow, rather than through increased yearling pregnancy rates.

There are three important points concerning nutritional management of replacement females.

* Early management carries over into later production (Patterson et al, 1991).

* Flexibility exists in the timing of BW gain, as long as the target is met (Clanton et al., 1983; Lynch et al., 1997).

* Early-calving heifers become early-calving cows and increase the likelihood of retention in the herd (Lesmeister et al., 1973).

Attaining Target BW. Erectly et al. (2001) investigated timing of growth as it relates to heifer productivity. Erectly et al. (2001) further suggested that as long as heifers were growing and met target BW consistent with recommended practices, there was little difference in the ability of the heifer to conceive or the ability of the calf to grow. However, limit feeding heifers did result in a decrease in calf survival. Lynch et al. (1997) and Clanton et al. (1983) also reported timing of BW gain (steady gain throughout the development period or low rates of gain early, followed by relatively high rates of gain late in the development period) did not affect heifer performance and subsequent reproduction as long as heifers achieved similar BW at the start of the breeding season.

BCS Target. The BCS is another tool that can be used to monitor and project development targets. Momont and Pruitt (1988) suggested that cows should have a BCS of 5 (1 = thin to 9 = obese; Richards et al., 1986) at calving to have a high potential for conception during a restricted breeding season. Whitman (1975) concluded that as BCS at calving increased, the likelihood that a range beef cow would return to estrus by 60 through 90 d after calving increased. Anderson and Lewis (1991) recommended feeding first-calf heifers to achieve a BCS of 6 or greater at calving. This recommendation is based on the fact that young cows are still growing and require an added level of body condition to achieve desired reproductive performance compared with mature cows.

There are many variables that can be incorporated into development of heifers; however, the most impact is generally achieved by making certain that BCS is optimum at the critical times of calving and rebreeding. If management is effective in having young cows in adequate body condition at critical times, the retention of 2-yr-old cows will generally be optimized.

Opportunities for Nutritional Manipulation.

Fat Supplementation. Lammoglia et al. (2000) tested the effects of increased levels of fat supplementation in Hereford-, Limousin-, and Piedmontese-sired heifers. Limousin- and Piedmontese-sired heifers fed diets containing % fat had greater percentages of heifers that attained puberty at the start of the breeding season. Those researchers speculated that there may be breed differences with respect to response to high-fat supplements. Lammoglia et al. (1999) also reported increased cold tolerance in calves born to heifers fed high-fat supplements during gestation. More research is necessary to determine whether this practice is viable under field conditions and to determine the factors that interact to enhance or negate the beneficial effects observed in these studies. Bottger et al. (2002) investigated the effects of high-oleate or high-linoleate safflower seed supplementation compared with a corn and soybean meal-based control supplement for lactating primiparous cows. No differences in overall productivity caused by fat supplement were reported.

Funston (2004) reviewed literature on supplemental fat in the diets of beef females and reported that feeding supplemental fat has resulted in varied and inconsistent effects on reproductive function. Elucidating how supplemental fat can influence reproductive function has been a difficult process. The complexity of the reproductive system and makeup of fat supplements are often confounded by management conditions and forage quality, both in research and commercial feeding situations.

Metabolizable Protein. Patterson et al. (2003b) evaluated the effects of meeting metabolizable protein (MP) requirements for primiparous beef heifers on large commercial ranches in western Nebraska. Pregnancy rate was improved by five percentage units in the heifers fed to meet their MP needs. The value of each pregnant heifer was also improved by over $13 per head by formulating supplements to meet the MP needs of the heifer rather than the CP needs. Patterson et al. (2003a) reported small differences in BW change with MP vs CP supplementation programs. However, Patterson et al. (2003b) also reported reductions in DMI as heifers advanced in gestation, suggesting that energy intake may become limiting as parturition approaches. Kane et al. (2004) suggested that differing levels of undegradable intake protein (UIP) may alter ovarian and pituitary function in heifers and influence reproductive performance in beef heifers. Wiley et al. (1991) reported a higher percentage of heifers pregnant during the first estrous cycle for heifers that received UIP supplementation compared with degradable intake protein supplementation. However, Rusche et al. (1993) reported no differences in reproductive performance of postpartum, primiparous cows fed degradable intake proteinor UIF-containing supplements.

Mineral Nutrition. Mineral nutrition may also play a role in the proper development of 2-yr-old cows (Manspeaker et al., 1987; Stanton et al., 2000; Ahola et al., 2004). However, Muhlenbein et al. (2001) were unable to demonstrate significant proauction responses in 2-yr olds fed organic or inorganic mineral supplements 45 d before and 60 d following parturition. Furthermore, Oison et al. (1999) reported reductions in pregnancy rate with elevated levels of Cu, Co, Mn, and Zn in postpartum 2-yr-old cows. Additional research in this area seems warranted.

Other Management Practices.

Animal Behavior and Nutritional Supplementation. Behavior may also play a role in supplementation programs. Wagnon (1965) reported that 2-yr-old cows were driven away from the supplement tank by older cows. Bowman and Sowell (1997) reported that variation in individual supplement intake exists regardless of the supplement form or method of delivery. High levels of competition for supplement, as in hand-feeding, generally increase the proportion of animals not consuming supplement, whereas low levels of competition, as with self-fed supplements, generally increase variation in supplement intake (Bowman and Sowell, 1997). Sowell et al. (2003) reported 2- and 3-year-old cows spent less time at a liquid supplement feeder and had a lesser number of daily feeding bouts than 6-yr-old cows in yr 1 of a 2-yr study. Producers may wish to consider behavioral differences as they design supplementation programs for young cows.

Heterosis. Heterosis is a major factor in reproductive success of cattle (Cundiff et al., 1992; Martin et al., 1992). Cundiff et al. (1992) reported improved reproductive performance in crossbred females compared with straightbreds. In addition, those researchers also noted that 2-yr olds (whether straightbred or crossbred) had lesser actual weaning BW and weaning BW per cow exposed in their progeny compared with older cows. Riley et al. (2001) noted Nellore and Gir-crossbred cows had higher productivity and reproductive performance compared with more commonly used Brahman crossbreds in central Texas. Furthermore, Nunez-Dominguez et al. (1992) noted reduced cost of production with the use of crossbred females.

Calving Date. The decision of when to calve beef females is critical to production efficiency and profitability in a cow and calf enterprise. This subject was reviewed by Sprott et al. (2001), who reported that calf production and associated costs are affected by calving season because environmental conditions, stage of production, and season of the year interact to affect nutritional status and reproductive performance. Selection of calving date as it relates to the individual ranch is also a critical means for matching the animal with the production system. Adams et al. (2001) compared spring and summer calving systems in western Nebraska. They reported similar weaning rates but lesser weaning BW for the summer calving system. They also reported a reduction in labor inputs and the amount of hay fed for the summer calving system. Carriker et al. (2001) reported lesser total costs and increased returns with the summer calving system. Additional research may be necessary to determine proper methods of managing the 2-yr-old cow in these calving systems because of their greater nutrient requirements.

Avoiding Dystocla. The decision of sire to mate with the young cow has a major impact on likelihood of an unassisted birth. Using genetic prediction information (., EPD) has been shown to decrease the incidence of dystocia (Colburn et al., 1997; Lykins et al., 2000). As dystocia increases, fertility for rebreeding decreases. Management to avoid dystocia is imperative to successfully retain heifers. Bellows et al. (1994) investigated the effects of forced exercise 90 d before parturition and reported no differences in dystocia score or incidence. However, exercise increased feed requirement by 31% and also resulted in a % improvement in subsequent pregnancy rates. Those researchers attributed this response to a feed-endocrine response related to BW gain, body composition, metabolic changes, or subsequently greater feed intake.

Determining Cost and Benefit. An important question that needs to be answered in any beef production environment is, "Should costly inputs be used during the first or second year of a heifer's development?" Data from Meek et al. (1999) indicated that the breakeven for increasing 2-yr-old pregnancy by 1% was $ per head in the pregnant heifer analysis, but only $ per head in the yearling analysis. In other words, ranchers could spend over double the amount on the pregnant heifer as on the yearling to effect the same change. Meek et al. (1999) stated that, assuming yearling pregnancy goals can be met, it is advantageous to complete the development of heifers in their second year rather than to overdevelop yearling heifers as insurance that they will breed back the following year. Costly interventions to retain heifers that simply do not fit the production environment because they are of the wrong biological type has led to a high fall-out rate from the herd and, hence, increased cost of production.

In many cases when European breeds were introduced into US production systems in the late 1960s and early 1970s, the industry used feed as a modifier to make them fit existing cow production systems. This was costly, and the heifers often failed when the high level of feed inputs were taken out of the system. In general, producers have recognized this and have made improvements in the practice of matching biological type and production environment.

Replacement Rates. Mackay et al. (2004) analyzed data from a Nebraska cattle ranch to compare three replacement strategies for cows to determine optimal replacement and marketing strategies of females for maximizing the long-term profit potential of that operation. The optimal replacement strategy involved marketing weaned heifer calves in lesser-priced years and marketing pregnant yearlings in higher-priced years, which resulted in an older herd during lesser-priced years and a younger herd during higher-priced years. This strategy is impacted by changes in relative price between price categories and by price variations throughout a cattle cycle. Changing the cost structure or calf crop percentages also alters the replacement strategy, suggesting that the optimal strategy may differ from operation to operation depending on the cattle enterprise configuration. The majority of female sales occurred in the fall. Because the replacement strategy varies with different circumstances, this decision should be repeatedly addressed to ensure the future success of an operation.

Implications

The 2-yr-old beef female represents a significant investment of management and capital to ranching operations. Because this female has requirements for growth in addition to maintenance and lactation, additional resources must be invested to ensure successful reproduction. Determining the balance between when to invest and how much to invest must be matched against the potential return to the investment. Taking steps, both from a nutritional and management perspective, before calving will generally reduce the need for crisis management later. Management of the 2-yr-old replacement cow must be approached in the context of a system, rather than focused on one or two aspects of the problem. Additional research that focuses on integration of systems approaches to 2-yr-old cow management is needed to determine cost-effective approaches for the various ranching environments found in North America.

1 Presented at the Extension Symposium at the Midwest American Society of Animal Science sectional meetings in Des Moines, IA, on March 16, 2004. The symposium was sponsored by NCR-87, Committee on Cow-Calf Nutrition and Management.

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J. C. WHITTIER*,2, PAS, G. P. LARDY[dagger]; and C. R. JOHNSON*, PAS

* Department of Animal Science, Colorado State University, Ft. Collins 80523-11 71 and [dagger] Department of Animal and Range Sciences, North Dakota State University, Fargo 58105

2 To whom correspondence should be addressed: