30 August 2010

15 minute read

Energy balance or plane of nutrition influences reproductive performance in heifers and cows (Butler and Smith, 1989; Randel, 1990; Robinson, 1990; Short and Adams, 1988; Swanson, 1989).

Numerous studies have reported inverse correlations between postweaning growth rate and age at puberty (Arije and Wiltbank, 1971; Ferrell, 1982; Short and Bellows, 1971; Wiltbank et al., 1966, 1969, 1985) and pregnancy rates in heifers were shown to be dependent upon the number displaying estrus prior to or early in the breeding season (Byerley et al., 1987; Short and Bellows, 1971).

Thus, rate of postweaning growth was determined to be an important factor affecting age of puberty, which influenced pregnancy rates. This and other research conducted during the late 1960s through the early 1980s indicated puberty occurs at a genetically predetermined size, and only when heifers reach their target weight can high pregnancy rates be obtained (reviewed by Patterson et al., 1992). Guidelines were established indicating replacement heifers should achieve 60 to 65 per cent of their expected mature body weight by breeding.

Traditional approaches for postweaning development of replacement heifers used during the last several decades have primarily focused on feeding heifers to achieve or exceed an appropriate target weight, and thereby maximise heifer pregnancy rates. Substantial changes in cattle genetics and the economy have occurred over this time, indicating traditional approaches should be re-evaluated. Intensive heifer development systems may maximise pregnancy rates, but not necessarily optimise profit or sustainability.

These systems require significant use of fossil fuels, cereal grains, and high capital investment in equipment and facilities. Cereal grains, often used as an energy source in heifer diets, detract from the system’s sustainability due to growing demand for human food and ethanol production. Furthermore, almost all studies on heifer development conducted over the last half century have focused on production to first calving with little information concerning effects of heifer development systems on lifetime productivity.

Since inception of target weight guidelines, subsequent research demonstrated the pattern of growth heifers experience prior to achieving a critical target weight could be varied. Altering rate and timing of gain can result in periods of compensatory growth and/or allow producers to limit supplementation to critical periods of heifer development thereby providing an opportunity to decrease feed costs (Clanton et al., 1983; Freetly et al., 2001; Lynch et al., 1997).

For example, delaying heifer gain until 47 or 56 d prior to the breeding season did not negatively influence reproductive performance, but reduced the amount of feed needed (Lynch et al., 1997). In one year of this study, puberty was delayed in heifers fed to achieve lower early gains, but first-service conception rate tended to be improved in these same heifers.

Similarly, Freetly et al. (2001) found delaying gain until the later part of the postweaning period reduced total energy intake, but calving rate, age at calving, postpartum interval, and second year pregnancy rate were not impacted. These studies indicate that total energy intake, and possibly heifer development costs, may be reduced by limiting heifer gain early in the postweaning period followed by accelerated gains before the breeding season.

Review of Target Weight

As indicated previously, substantial research contributed to the guidelines of developing heifers to 60 to 65 per cent of mature body weight at time of breeding. Studies evaluating different postweaning rates of gain or target weights have used either different amounts of feed, or different types of feeds varying in energy and/or protein content to obtain differences in rates of growth.

A review of these studies conducted over the last several decades along with new research discussed later, indicates the association among BW, puberty and heifer pregnancy rate appear to have changed over time. Research reports published through the late 1980s have shown much greater negative effects of limited postweaning growth on age of puberty and subsequent pregnancy (Patterson et al., 1989; Short and Bellow, 1971; Wiltbank et al., 1985), where as more recent studies indicate less of a negative impact of delayed puberty on pregnancy response (Buskirk et al., 1995; Freetly and Cundiff, 1997; Lynch et al., 1997).

Several factors likely contribute to this change over time. Initial research corresponds to the industry shift from calving heifers at 3 years of age to calving at 2 years of age. Thus, selection pressure for age of puberty was probably minimal in animals in the early studies. While selection intensity would have increased with the reduction in calving age of heifers, genetic progress would take time due to the long generation interval in cattle. In 1978, researchers identified the association between scrotal circumference in bulls and age of puberty in their daughters (Brinks et al., 1978).

Since then, scrotal circumference has been used as an indicator trait for puberty. Breed association web sites show substantial increases in scrotal circumference occurring from 1985 to the present, indicating great progress has been made through selection for this trait; a similar response in age of puberty would be expected. Indeed, the inability of heifers to attain puberty prior to breeding may not be as problematic as heifers reaching puberty before weaning (Gasser et al., 2006a and 2006b).

The association between timing of puberty and subsequent pregnancy rate also seems to have changed over time. Early research indicated heifers should experience two or three estrous cycles before the onset of the breeding season because fertility of the first estrus is lower than subsequent estrous cycles (Byerley et al., 1987). Thus, delayed onset of puberty was expected to be associated with lower pregnancy rates. However, several studies have not shown strong associations between nutritionally related changes in age of puberty and final pregnancy rates (Buskirk et al., 1995; Ferrell, 1982; Freetly and Cundiff, 1997; Lynch et al., 1997).

Evidence for a genetic basis for these differences is provided by Freetly and Cundiff (1997), who reported pregnancy rates were greater in heifers AI sired by bulls born after 1988 than bulls born between 1982 and 1984, but age and weight at puberty were not. These changes, combined with the continued increase in cost of harvested feedstuffs indicate the need for alternative development systems, which allow heifers the opportunity to conceive early as yearlings at reduced cost.

Current Research

Feeding replacement heifers to traditional target weights increased development costs relative to more extensive heifer development systems where heifers were developed to lower target weights ranging from 51 to 57 per cent (Funston and Deutscher, 2004; Larson et al., 2009; Martin et al., 2007; Roberts et al., 2007 and 2009b).

Feeding to pre-breeding weights as low as 51 per cent of mature weight was shown to be more cost effective than development to 57 per cent of mature weight, even though lighter heifers were allowed a 15 d longer (45 vs. 60 d) breeding season (Martin et al., 2007). Extending the breeding season by 15 d for lighter heifers resulted in similar conception rates between systems, but pregnancy rates for the first 45 days of the breeding season, were 89.8 and 77.9 per cent for heifers fed to 57 per cent or 51 per cent of mature weight, respectively. Further characterisation of non-pregnant heifers within each system revealed 78.9 per cent of open heifers developed to 51 per cent of mature weight but only 45 per cent of open heifers developed to 57 per cent of mature weight were pre-pubertal prior to start of the breeding season.

This lends support to the hypothesis that one of the major determinants to a heifer’s ability to conceive during her first breeding season is the age she reaches puberty, especially in relation to the start of the breeding season. Heifers calving early during their first calving season have greater lifetime calf production than those calving late and are more likely to become pregnant sooner at two years of age (Lesmeister et al., 1973).

However, there was no difference in second-calf conception rates between cows developed to 51 or 57 per cent of mature weight prior to breeding as yearlings (Martin et al., 2007). This indicates lighter heifers that became pregnant during the 15 d extension during the first breeding season rebred with similar efficiency as those pregnant within the initial 45 days. Therefore, proportion of heifers retained as pregnant 2-yr olds was similar between systems. Thus, heifers may be developed to lighter than traditional target weights without negative effects on profitability or future productivity.

Research at Fort Keogh evaluating lifetime productivity of heifers developed with either unlimited or restricted (27 per cent less feed) feed during the postweaning period supports the potential to reduce target weights and costs during heifer development (Roberts et al., 2007 and 2009b).

The association of age at onset of breeding and cumulative pregnancy rate was similar for heifers developed on the two protocols. However, restricted heifers were lighter at a given cumulative pregnancy rate. Thus, age at the beginning of the breeding season was more critical than body weight. Furthermore, rate of growth from birth to weaning accounted for more variation in puberty and AI pregnancy rate than did ADG during the postweaning period. Neither age nor ADG prior to postweaning period influenced final pregnancy rate. Thus, age and early growth rate (up to ~ 8 mo. of age) influenced time of puberty and conception, but did not alter overall pregnancy rate in a 48 to 60 d breeding season.

When summarised over the last 7 years, heifer pregnancy rate was 3.5 per cent less in heifers developed under restricted feeding at Fort Keogh. Restricted feeding during the 140-d postweaning period reduced harvested feed inputs by 22 per cent and increased efficiency of gain. After restriction, restricted heifers remained lighter but had greater ADG. Restricted feeding improved biological and economical efficiency during and after the feeding period.

Pregnant heifers resulting from the two postweaning treatments were also fed at different levels throughout each subsequent winter. Heifers developed without restriction were provided adequate levels of harvested feed from early December through calving while heifers developed on restricted feeding were fed 20 to 45 per cent less harvested feed. Restriction resulted in lower bodyweights throughout 5 yr of age (Roberts et al., 2009a) which may equate to lower maintenance requirements.

Heifer offspring from the two management groups were randomly assigned to restricted or non-restricted protocols resulting in 4 treatments: restricted cows from restricted dams, restricted cows from control dams, control cows from restricted dams and control cows from control dams. Interestingly, cows from restricted dams were 35 to 50 lbs heavier than cows from non-restricted dams at 3 to 5 yr of age, due in part to differences in BCS (Roberts et al., 2009a).

Thus, method of developing and maintaining replacement heifers may influence offspring growth and development. Differences in weight and BCS may also impact longevity. Current data indicate that retention to the 5th breeding season was influenced by dam and cow treatments. Retention was lowest for restricted cows from non-restricted dams (39 per cent), intermediate for non-restricted cows from either restricted (50 per cent) or non-restricted (51 per cent) dams, and greatest for restricted cows from restricted dams (66 per cent).

Preliminary evaluation of the performance of the third generation of calves found that calves from restricted cows out of restricted dams were lighter at birth and weaning by 3 and 13 lbs, respectively. Thus, restricted cows from restricted dams may have a lower level of production and greater fleshing ability resulting in greater retention. Current data indicate that the small decrease in calf output may be more than compensated by increased longevity.

Several similarities exist between the heifer development studies conducted at the University of Nebraska (Funston and Deutscher, 2004) and Fort Keogh. Both locations used similar types of cattle (composites with ~½ Red Angus and ½ continental breeding) and the treatments resulted in development to similar target weights at breeding (53 vs. 58 and 55 vs. 58 per cent of expected mature weight).

Growth rates during the development period were similar between locations for the two treatments imposed and both locations observed approximately a 10 per cent reduction in proportion of heifers pubertal at breeding in the lower input groups. Magnitude of savings achieved by lower target weights was also similar (~22-24$/pregnant heifer). In contrast to the Nebraska research, a slight decrease in pregnancy rate (3-5 per cent) has been observed in heifers under restricted feeding at Fort Keogh (Roberts et al., 2009b). Methods used for restricting rate of development differed between Nebraska (lower quality diet) and Fort Keogh (lower quantity fed) which may contribute to differences in pregnancy. These studies indicate an opportunity to improve efficiency and decrease production costs by decreasing amount and/or quality of harvested feeds used for heifer development.

The challenge then, is adapting these theories to common productions systems in widely varied environments. In the Midwest, including Nebraska, South Dakota, Iowa, and perhaps southern Minnesota, the opportunity to graze corn stalks is an attractive option to dry lot feeding.

We (Larson et al. 2008, 2009) have conducted experiments grazing heifers on corn stalks or winter range as an alternative to dry lot feeding. In each study, heifers grazed on corn stalks gained approximately 0.5 lb/day less than their more traditional fed counterparts, whether that be winter grass or a dry lot. It is important to note that grazing heifers were only supplemented with the equivalent of 0.30 lb of protein per day and gained between 0.5-1.0 lb/day of ADG during winter grazing.

However, once placed on higher quality spring pasture, the heifers had the ability to gain 2.5-3.0 lb/day prior to and after breeding. Regardless of these compensatory gains, heifers developed grazing corn stalks weighed 5 to 7 per cent less prior to breeding and achieved 55-60 per cent of their mature weight by that point. They were also lighter prior to first calving.

Maximal gain should not be the major goal in heifer development programs. Producers should strive for a sound, functional, low-cost, and pregnant heifer. Previous research (Patterson et al. 1992) supported the concept that a heifer must reach 65 per cent of her mature body weight for maximal reproductive efficiency. More recent data (Larson et al., 2009) provides evidence that a lower body weight is sufficient for attainment of puberty and pregnancy success. However, puberty was delayed by low prebreeding gain, but pregnancy rates were similar (85-90 per cent) between development systems. Previous research (Byerly et al. 1987) indicated heifers that do not reach puberty before breeding may become pregnant later than pubertal heifers.

This is borne out by conception rates to artificial insemination and may partially explain recent results from Larson et al. (2009). Heifers developed in the dry lot had approximately a 10 per cent greater AI pregnancy rate than corn stalk developed heifers even though overall pregnancy rates were similar. As a result, calving date was pushed back on the corn stalk developed heifers, indicating that non-pubertal heifers were inseminated on the second or third cycle during breeding and became pregnant.

All of these data provide evidence that heifers can be successfully developed into productive cows using low quality feedstuffs. Heifer development costs can be reduced by limiting forage quality or quantity without compromising productivity. Regardless of source, low quality feedstuffs exist in every environment and can be fed to beef cattle depending on stage of production. Moving heifer development out of the drylot, in favor of cornstalks or winter range reduced development cost by $45/pregnant heifer. The take home message should be to find a strategy to reduce forage quality to improve profitability.

Meeting Nutrient Requirements

A young, growing heifer at approximately 600 lb will require about 10.5-11.0 per cent crude protein and 0.45 Mcal/lb of energy for gain. These numbers may seem high unless one accounts for increased maintenance due to cold temperatures. In the upper Midwest, many heifers, due to snow cover, are fed in the dry lot using harvested forages. This typically includes some type of dry hay, a grain source, and perhaps an ensiled feedstuff. Often, the dry hay is at least 12 per cent crude protein and even moderate provision of a grain source will meet energy demands.

In contrast, if an ensiled feed is used, energy needs will be easily met by the silage. This will produce a gain of 1.5 lb/day, which is sufficient to produce a heifer of adequate size for breeding by spring. This is especially true when one considers compensatory gain. Compensatory gain is the type of gain experienced by a previously restricted animal when exposed to a more nutrient dense environment, such as spring/early summer pasture.

Perhaps of greater concern in areas of the country where weather necessitates extended dry lot feeding is creating heifers with excessive body condition. Older data in dairy type heifers indicate that heifers weighing 30-35 per cent more than lighter counterparts at calving produced 700 lb less milk during their first lactation (Swanson, 1960). This was likely due in part to inhibited mammary development in fat heifers.

This observation is supported by Ferrell et al. (1976). Ferrell offered pregnant heifers a high or low level of nutrient intake. Heifers were slaughtered and those on the high level of intake had greater udder weight and a larger proportion of that weight was fat. In addition, heifers developed to weigh 700 versus 630 lb at breeding required more services per conception, which may suggest an increase in embryonic mortality for heavier heifers (Short and Bellows, 1971). Arnett et al. (1971) studied a more substantial difference in prebreeding weight compared to Short and Bellows (1971).

Sets of twin heifers were developed at a rate of 0.75 lb/day or at a rate designed to induce “a high degree of body fatness” (Arnett et al., 1971). The overdeveloped twins were 30 per cent heavier than more appropriately developed twins at first mating and essentially maintained this difference through their third calving season.

Developing heifers to become obese increased the number of service required for a conception by 7 per cent compared to leaner heifers. This difference persisted through the third breeding season. More striking was the difference in calving difficulty, where obese heifers required 86 per cent more assistance for calving than lean heifers and this difference also persisted through the third calving season. Across three lactation periods, heifers developed to become obese produced less milk than lean counterparts did and as a result, their calves were lighter at weaning. The difference in milk production is further supported by data from Ferrell (1982).

Heifers developed at 1.75 lb/day produced 1.20 lb/day less milk than heifers developed at 1.30 lb/day. Furthermore, the heavier developed heifers weaned 14 kg lighter calves. Patterson et al. (1992) cited numerous other studies that provide additional support for the concept that developing heifers to become obese negatively affects reproduction and calf production. However, more importantly, developing heifers to excessive weights is expensive and requires additional dollars per each heifer developed.

Summary

Postweaning management of heifers to achieve traditional target weights, particularly by feeding high-energy diets, is not supported by current research. Heifers developed on forage, however, generally require additional protein supplementation to achieve even modest gains. One reason reproductive performance has not been drastically impaired by feeding to lower target weights may relate to genetic changes in age of puberty. While the systems presented herein are not directly applicable to the upper Midwest, the principles of low input development are if used correctly. Regardless of the system, minimising expensive feedstuffs will reduce development cost, which is a major determinant in lifetime cow profitability.

Further Reading

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