Final Report for GNC06-058
Project Information
Four studies concluded that developing heifers to less than 65% of mature weight prior to breeding utilizing dormant, standing forage or crop residue does not reduce pregnancy rates in virgin beef heifers. In addition, neither corn residue nor dormant winter range development post-weaning negatively impact subsequent calf production or second season rebreeding. Short-duration supplementation around breeding did not improve pregnancy rates in these experiments. However, further research is needed to explore the effect of breeding supplementation in more severely restricted animals.
Introduction:
Traditional heifer development programs involve placing weaned heifers into a feedlot or drylot situation and then providing them with a ration formulated so the heifers weigh 65% or more of projected mature weight at the time of breeding. This type of heifer development evolved because producers wished to achieve maximum pregnancy rates in the first breeding season, and dormant forages do not contain the nutrients necessary to support this level of performance. Drylot systems may attain maximum pregnancy rates, but not necessarily optimum performance in terms of profit or sustainability. The energy demands for this system are high because of the requirement for fuel to harvest the feed and then deliver it to the cattle. Cereal grains are often used as a major source of energy in the diet, detracting from the sustainability of this system due to growing demand for cereal grains for human food and ethanol production.
Recent research efforts in Nebraska are focused on developing a systems approach and mentality towards heifer development, where profit and sustainability are the indicators of success. Heifers have been developed to lighter than previously recommended target weights with no adverse affects on reproduction or production. A system for developing heifers on range while producing acceptable pregnancy rates, during a short breeding season has not yet been demonstrated. Ovum fertilization rates in heifers are routinely near or above 90% in controlled research; however, pregnancy rates to d 18 are 30-40% lower. The logical target for improving pregnancy rates in heifers is to improve embryo survivability and increase the percentage of pregnant heifers that remain pregnant.
Overcoming embryo loss in heifers with targeted, short-duration energy supplementation should achieve our goal of demonstrating acceptable pregnancy rates in heifers developed on range.
In order to realize progress toward improving sustainability and reducing risk for ranchers developing heifers, it is essential that a low-cost energy source be available for supplementation. Dried distillers grains (DDG) are a by-product of ethanol production that provide approximately 120% the energy value of corn for cattle on forage diets. Therefore, DDG are a logical and sustainable option for short-term supplementation of heifers developed on range.
Replacement heifer development is critical for the future reproductive success and profitability of the cowherd. Heifer development should be achieved at low cost in order to manage financial risk and prevent over-investment in a non-productive female. Previous studies (Patterson et al., 1992) indicated that puberty occurs at a genetically predetermined size, and only when heifers reach their target weight can high pregnancy rates be obtained. Recommended guidelines generally have been 60 to 65% of mature WEIGHT in beef heifers (Patterson et al., 1992). This development program requires substantial resources because an accelerated rate of gain is needed to reach the target weight. Development programs that allow heifers to conceive early as yearlings at the lowest cost possible are needed.
Funston and Deutscher (2004) proved that heifers developed to only 53% of mature weight could achieve similar pregnancy rates compared to heifers developed to 58% of mature weight. Further research indicates that heifers developed to 50% of mature weight attained similar pregnancy rates as those developed to 55% of mature weight, but the lighter heifers were allowed a 33% longer breeding season (NE Beef Report, 2005). These data were generated using drylot heifer development regimes, rather than developing heifers on range. Demonstration of a system for developing heifers on native range with targeted supplementation to achieve high pregnancy rates is needed to motivate producers to develop heifers in a more cost-effective way.
Embryonic mortality refers to losses that take place from fertilization until the period of differentiation, at approximately 42 days of gestation (Committee on Reproductive Nomenclature, 1972). Embryo loss is the major cause of reproductive failure in cattle and represents significant economic loss to the industry (Dunne et al., 2000). Numerous studies report a fertilization rate of 80-90% in beef heifers (Henricks et al., 1971; Diskin and Sreenan, 1980; and Roche et al., 1981). However, these studies also report that at 42 days following insemination, embryo survival rate was only 60% of initial. Furthermore, Roche et al (1981) demonstrated that embryo loss occurs primarily in the first 16 days following breeding. Dunne et al (2000) found that embryo survival rates decreased to day 14 with no further reduction. These studies clearly demonstrate that embryo loss occurs primarily in the first 14-16 days following insemination.
Research in suckled beef cows, which may represent a similar nutritional demand as growth in heifers, indicates a positive influence of concentrate supplementation three weeks prior to and three weeks following breeding (Khireddine et al., 1998). In this study, all cows were restricted to 70% of maintenance requirement from calving until three weeks prior to breeding. Subsequently, cows were either maintained on the restricted diet or placed on a supplemented diet. On day 21, the supplemented group had a greater pregnancy rate (100% vs. 20%) compared to the nonsupplemented group. This indicates a positive effect of supplemental nutrition on embryo survival.
Supplementation may be more beneficial in heifers that are nutritionally challenged or on a low plane of nutrition (Funston, 2004). Supplementation with a high-energy feedstuff containing a modest level of fat, such as DDG, may be beneficial. Ciccoli et al. (2005) found heifers that were supplemented in a drylot with corn and soybean meal or corn and distiller’s grain reached puberty at a younger age than control counterparts grazing pasture and supplemented only with soybean meal. However, data demonstrating the effects of short-term supplementation with distillers' dried gram (DDG) as a component of a low-cost heifer development system is needed.
The primary expected short-term outcomes of this research are to demonstrate the feasibility and sustainability of a low-cost heifer development strategy to beef producers in the north central region. This short-term outcome includes not only demonstrating that heifers can be developed successfully on range, but also that targeted energy supplementation from one week before breeding until 18 after should improve pregnancy rates. For ranchers, this should result in more heifers pregnant in a shorter time, with reduced cost compared to feedlot heifer development.
Our projected intermediate outcomes include ranchers adopting this strategy for developing heifers on range, reduced heifer development costs, diminishing use of fossil fuels and cereal grains, and lower capital investment in heifer development enterprises. Additionally, we expect to increase awareness of embryonic loss in beef heifers and educate producers about strategies to control early pregnancy loss in developing heifers.
Long term, these results may effect how beef females are managed prior to the breeding season. The periods immediately before and after breeding are novel in a heifer’s development and represents new nutritional demands. Implementation of a system that promotes a targeted, cost effective supplementation program could improve pregnancy rate and reduce economic risk for livestock producers. This short-term supplementation system could be incorporated on ranches with widely varying forage resources.
Cooperators
Research
Four experiments have been conducted to evaluate the effect of grazing crop aftermath or standing winter forage on heifer development and reproduction. Furthermore, these studies evaluated the effect of short-duration supplementation around breeding on AI conception and pregnancy rates. Two experiments over two years were conducted at the University of Nebraska’s Gudmundsen Sandhills Laboratory (Experiments 1 and 2). This is a working-ranch style research institution in the Nebraska Sandhills. Data collected at Gudmundsen is directly applicable by ranchers in the Sandhills region. A third study was conducted at the University of Nebraska Agricultural Research Division in eastern Nebraska (Experiment 3). Producers in the eastern half of Nebraska can apply this research. Finally, a study was conducted at the Rex Ranch located near Lakeside, Nebraska (Experiment 4). The Rex Ranch, Joy and Star units, is a privately held ranch. The Rex Ranch has adopted a heifer development system created by researchers at the University of Nebraska.
Experiment 1
Weaned heifer calves (n = 96) were blocked by initial weight (489 ± 7 lb) and assigned randomly to graze either corn stalks (Stalk) or dormant native Sandhills range (Range) from November through March. The heifers were weaned at an average age of 209 days and began treatment at an average age of 240 days. A daily supplement was offered (1 lb/hd). Both the Stalk and Range groups grazed for a period of 138 days. Subsequently, all heifers were recombined and grazed a common pasture for 48 days with a supplement (1 lb/hd). After the 48 day grazing period, heifers were reassigned to breeding treatments within winter development treatment randomly by weight. The breeding treatments included offering heifers a supplement (Supp; 3 lb/hd) for 7 days prior to and for 14 days following prostaglandin injection or not offering a supplement (NoSupp). Body weight was measured after 138, 160, and 170 days on trial for both the Stalk and Range groups. Blood samples were collected 26 and 14 days prior to prostaglandin injection. Progesterone concentrations were quantified using radioimmunoassay and concentrations > 1 ng/ml were interpreted to indicate luteal activity and hence, attainment of puberty.
Estrus was synchronized using a single injection of prostaglandin (day 0). A progestin was not utilized to avoid confounding the effect of winter development on age at puberty. Five days prior to prostaglandin, fertile bulls were turned in with both groups of heifers for a period of 45 days. The heifers were recombined after the supplementation period. Pregnancy rate was determined via transrectal ultrasonography 40 days after bull removal. At approximately 23 months of age, heifers were weighed and BCS was assessed. At parturition, calf birth date, birth weight, calving ease score, and sex were recorded.
The heifer weight, average daily gain (ADG), calf birth date, birth weight, and calving ease score data were analyzed using Proc Mixed of SAS. In addition, the percentage of heifers pubertal, pregnancy rate and calf sex were analyzed using Proc Genmod of SAS. The interactions between winter development and short-term supplementation were also tested for each variable using the respective procedure (P ≤ 0.05).
Experiment 2
Weaned heifer calves (n = 83) were blocked by initial weight (464 ± 11 lb) and assigned randomly to graze either corn stalks (Stalk) or dormant native Sandhills range (Range) from November through February. The heifers began treatment at an average age of 280 days. A daily supplement was offered (1 lb/hd) while grazing. Both the Stalk and Range groups grazed for a period of 83 days. Subsequently, all heifers were recombined and grazed a common pasture for 98 days with a daily supplement (1 lb/hd). After the 98 days grazing period, heifers were reassigned to breeding treatments within winter development treatment randomly by weight. The breeding treatments included offering heifers a daily supplement (Supp; 3 lb/hd) that provided 100 g/day of Ca propionate, a gluconeogenic precursor, for 7 days prior to and for 14 days following prostaglandin injection or not offering a supplement (NoSupp). Body weight was measured after 148, 175, and 185 days on trial for both the Stalk and Range groups. Blood samples were collected 21 and 11 days prior to prostaglandin injection. Progesterone concentrations were quantified using radioimmunoassay and concentrations > 1 ng/ml were interpreted to indicate luteal activity and hence, attainment of puberty.
Estrus was synchronized using a single injection of prostaglandin (day 0). A progestin was not utilized to avoid confounding the effect of winter development on age at puberty. Five days prior to prostaglandin, fertile bulls were turned in with both groups of heifers for a period of 45 days. Pregnancy rate was determined via transrectal ultrasonography 40 days after bull removal.
The heifer weight and ADG data were analyzed using Proc Mixed of SAS. In addition, the percentage of heifers pubertal and pregnancy rate were analyzed using Proc Genmod of SAS. The interactions between winter development and short-term supplementation were also tested for each variable using the respective procedure (P ≤ 0.05).
Experiment 3
Weaned heifer calves (n = 89) were blocked by initial weight (464 ± 11 lb) and assigned randomly to graze either corn stalks (Stalk) or dormant stockpiled brome grass pasture (Pasture) from November through February. The heifers began treatment at an average age of 226 d. A daily supplement was offered (1 - 2 lb/hd). Both the Stalk and Pasture groups grazed for a period of 103 days. Subsequently, all heifers were recombined and grazed a common pasture for 91 days with a daily supplement (2 lb/hd). After the 91 day grazing period, heifers were reassigned to breeding treatments within winter development treatment randomly by weight. The breeding treatments included offering heifers a daily supplement (Supp; 3 lb/hd) that provided 100 g/day of Ca propionate, a gluconeogenic precursor, for 7 days prior to and for 14 days following the beginning of artificial insemination (AI) or not offering a supplement (NoSupp). The heifers were recombined after the supplementation period. Body weight was measured monthly while on trial for both the Stalk and Pasture groups. Blood samples were collected every two weeks, beginning 90 days prior to AI to determine luteal activity. Progesterone concentrations were quantified using radioimmunoassay and concentrations > 1 ng/ml were interpreted to indicate luteal activity and hence, attainment of puberty.
Estrus was synchronized using two injections of prostaglandin 13 days and 2 days prior to the beginning of AI. A progestin was not utilized to avoid confounding the effect of winter development on age at puberty. Artificial insemination was performed 12 hours after the first detection of estrus and estrus detection was continued for six days. Due to a poor response to synchronization, the heifers not detected in estrus were reinjected with prostaglandin 10 days after the previous injection. Estrus detection and AI were continued for an additional 5 days after injection. One day after the final insemination, fertile bulls were turned in with both groups of heifers for a period of 45 days. Transrectal ultrasonography was performed 43 days after the final insemination to determine AI conception rate and pregnancy rate was determined via ultrasonography 40 days after bull removal.
The heifer weight and ADG data were analyzed using Proc Mixed of SAS. In addition, the percentage of heifers pubertal and pregnancy rate were analyzed using Proc Genmod of SAS. The interactions between winter development and short-term supplementation were also tested for each variable using the respective procedure (P ≤ 0.05).
Experiment 4
Weaned heifer calves (n = 1230) grazed native Sandhills range at two locations from November through June with a supplement. Seven days prior to prostaglandin injection, the treatment group (Supp vs. NoSupp) received a daily supplement (3 lb/hd) and continued to receive the supplement for 14 days after the prostaglandin injection. In addition to supplement, half of the heifers in each treatment at each location were injected with prostaglandin (PGF vs. NoPGF).
On the day of prostaglandin injection, 3 esophageally fistulated cows were allowed to graze with each treatment group at each location. The esophageal samples were analyzed to determine the NDF, ADF, CP, and ash contents of the range grasses. Five days prior to prostaglandin injection, 11 fertile bulls were turned in with each group of approximately 308 heifers. The bulls remained with the heifers for 20 days following prostaglandin injection. Pregnancy rate was determined via transrectal ultrasound 47 days following bull removal. The heifer weight data were analyzed using Proc Mixed of SAS and the pregnancy rate data were analyzed using Proc Genmod of SAS.
Experiment 1
The Stalk heifers were lighter after grazing (day 203; P < 0.001), at prostaglandin injection (day 236; P < 0.001), and at pregnancy diagnosis (day 330; P = 0.004) when compared to the Range heifers. The BCS at pregnancy diagnosis was not different (P > 0. 10). At breeding the Stalk heifers were approximately 50% of mature weight and the Range heifers were approximately 55% of mature weight, assuming a mature cow weight of 1200 lb. The Stalk heifers (0.44 ± 0.01 lb/day) had a lower (P < 0.001) ADG prior to breeding compared to Range heifers (0.71 ± 0.01 lb/day). However, Stalk heifers had a greater (P < 0.001) ADG after breeding, which indicates compensatory gain. Supplemental nutrition did not affect (P > 0. 10) weight nor BCS at pregnancy diagnosis or ADG after breeding. There were a greater (P < 0.001) percentage of Range heifers pubertal prior to breeding than Stalk heifers (73 vs. 33%). However, neither winter development nor supplemental nutrition affected (P > 0. 10) overall pregnancy rate (86%).
Neither winter development nor supplemental nutrition affected (P > 0. 10) BW prior to parturition. However, Stalk heifers tended to have a greater (P = 0.06) BCS than Range heifers. Winter development and supplement treatments did not affect (P > 0. 10) calf birth weight or calving ease score. Heifers that were not supplemented around breeding gave birth to twice as many bulls as heifers (P < 0.001) in comparison to supplemented heifers that gave birth to an equal number of sexes. There was not (P > 0. 10) a greater percentage of Range heifers that calved in the first 21 days of the season. Finally, second calf pregnancy rates were not affected (P > 0. 10) by previous winter treatments.
Experiment 2
The Stalk heifers were lighter after grazing (day 153; P = 0.03), at breeding (day 190; P = 0.01), and tended to be lighter at pregnancy diagnosis (day 281; P = 0.09) when compared to the Range heifers, although BCS at pregnancy diagnosis was not different (P > 0. 10). At breeding the Stalk heifers were approximately 54% of mature weight and the Range heifers were approximately 57% of mature weight, assuming a mature cow weight of 1200 lb. The Stalk heifers (0.57 ± 0.03 lb/day) had a lower (P < 0.001) ADG prior to breeding compared to Range heifers (0.78 ± 0.03 lb/day). However, the Stalk heifers compensated after breeding and tended to have a greater ADG than the Range heifers (P = 0.10). Supplemental nutrition did not affect (P > 0. 10) weight or BCS at pregnancy diagnosis or ADG after breeding. The percentage of heifers pubertal prior to breeding was not significantly effected by Stalk or Range development (61 vs. 76 %; P = 0.13; respectively). The difference in pubertal development between experiments 1 and 2 may be explained by the differences in percentage of mature weight at breeding. The heifers in experiment 2 had weights 3 to 4 percent greater than the weights in experiment 1. Finally, winter development did not affect (P > 0. 10) overall pregnancy rate (86%) nor did supplemental nutrition around breeding.
Experiment 3
Stalk development did not significantly affect (P > 0. 10) weight after winter grazing (day 103), at breeding (day 187) or at pregnancy diagnosis (day 251). The Stalk heifers (0.48 ± 0.04 lb/day) had a lower (P < 0.001) ADG on corn stalks compared to Pasture heifers (0.62 ± 0.04 lb/day) on winter pasture. However, the Stalk heifers compensated (P = 0.06) following removal from corn stalks and gained more weight (1.88 vs. 1.22 ± 0.06 lb/day) than the Pasture heifers prior to breeding. The Stalk heifers achieved 60% of mature weight by breeding while Pasture heifers reached 61%. The percentage of heifers that were pubertal (53%) by breeding was similar between development systems. Body condition score of the Stalk heifers were similar (P > 0.10) to Pasture heifers at breeding.
The ADG from breeding until the initial pregnancy diagnosis was similar (P > 0.10) between winter development groups as was weight at pregnancy diagnosis. The Supp heifers were heavier (P = 0.04) than the NoSupp heifers at pregnancy diagnosis. The Supp heifers also tended to have greater BCS (P = 0.06) compared to the NoSupp heifers at pregnancy diagnosis. Neither AI conception rate nor AI pregnancy rate were different (P > 0.10) between winter development group or breeding supplement group. Final pregnancy rate was similar (P > 0.10) between winter development groups and breeding supplement groups.
Experiment 4
The Supp heifers were heavier (P ≤ 0.05) at pregnancy diagnosis and had greater BCS compared to the NoSupp heifers. In addition, the NoPGF heifers had a greater BCS (P = 0.02) compared to the PGF heifers. Furthermore, the Supp heifers that did not receive prostaglandin tended to have greater BCS (P = 0.08) compared to Supp heifers that did receive prostaglandin and both the NoSupp/PGF and NoSupp/NoPGF. The supplementation scheme did not effect (P > 0.10) pregnancy rate, but pregnancy rate was greater (P = 0.03) for NoPGF heifers compared with PGF heifers.
Discussion/Milestones
Post-weaning ADG dictates what percentage of mature weight a heifer will attain prior to breeding. Guidelines suggest that heifers should reach 60-65% of mature weight prior to breeding. Patterson et al (1992) concluded that maximal pregnancy rate can only be achieved if heifers are developed to this end-point. These data challenge that hypothesis. The heifers in Exp.1 and 2 were developed to as light as 56% of mature body weight at breeding. Thus, a paradigm shift may need to occur, as numerous studies including this one demonstrate no negative effects of light development. Numerous studies have related low post-weaning ADG to an increased age at puberty (Short and Bellows, 1971, Wiltbank et al. 1985). Furthermore, the number of estrous cycles a heifer experience prior to breeding is positive related to pregnancy rate, as reported by Byerley et al. (1987). Thus, it seems intuitive that reducing the percentage of heifers that are pubertal prior to breeding will result in lower pregnancy rates. However, that is not the case in these experiments.
Funston and Deutscher (2004) developed heifers to 53 and 58% of mature weight and reduced the percentage of heifers cyclic before breeding. However, pregnancy rate was unchanged. Martin et al. (2007a), using heifers from the same herd as Funston and Deutscher (2004) reported that heifers reaching 51 % of mature weight before breeding did not have reduced pregnancy compared with those reaching 57%, even though puberty was delayed in those heifers. Heifers developed over the winter on pasture fed 1.5 lb/day of a 42% CP supplement were compared to heifers offered a high or low starch in a drylot for 30 or 60 days prior to breeding (Ciccoli et al., 2005). Developing heifers solely on pasture delayed puberty. However, pregnancy rates were unaffected unless heifers gained less than 1.10 lb/day over the treatment period. In this case, heifers developed on pasture gained 0.50 lb/day until breeding and pregnancy rate was reduced compared to heifers that received supplemental energy for 60 days before breeding. Similarly, in Experiment 3, gain was partially delayed until the last 60 days prior to breeding.
In Experiment 3, heifers were allowed to gain 0.48 lb/day grazing corn stalks compared to 0.62 lb/day for those heifers grazing winter pasture. Subsequent ADG during spring regrowth was substantially greater for Stalk heifers than Pasture heifers. Compensatory gain did not reduce pregnancy rates, which is similar to previous research. Earlier data suggest that delaying gain and subsequent puberty does not reduce pregnancy rate, provided the heifers become pubertal prior to the breeding season (Freetly et al., 1997 and Lynch et al., 1997). While the heifers compensated and age at puberty was not different, only 50 and 55% of the heifers in Experiment 3 were pubertal prior to AI. However, 70% of the heifers (unpresented data) were detected in estrus during the 10-day AI period. Therefore, approximately 20% of the heifers may have been bred on their pubertal estrus. Byerly et al. (1987) suggest that the initial estrus is less fertile than the second estrus. However, recent data challenge that hypothesis and indicate that the first post-partum estrous cycle is as fertile as subsequent cycles (Cushman et al., 2007).
As Stalk development reduced the percentage of heifers cyclic prior to breeding in Experiments 1 and 2, increased supplementation around breeding may improve pregnancy rate. Range grasses are typically deficient in the ruminally undegradable fraction of crude protein (UIP), which can result in a metabolizable protein (MP) deficiency. While ruminally degradable protein (DIP) may be a limiting nutrient in dormant forage-based systems as well, supplementation of UIP to nutritionally restricted animals improves performance (reviewed by Patterson, 2001). Martin et al. (2007b) supplemented pre-pubertal heifers consuming grass hay ad-libitum with approximately 4 lb/day of DDG, which contains approximately 30% CP, of which about 50% is UIP. The DDG supplementation was began shortly after weaning and continued through the AI period. While age and weight at puberty were unaffected by supplementation, AI pregnancy rate was 17% greater for supplemented heifers. Dried distillers grains also have a energy value approximately 120% that of corn. Therefore, providing DDG around breeding will increase both energy and MP. However, heifers supplemented with 3 lb/day of a high-protein range cube around breeding did not have greater pregnancy rates than those that did not receive supplement. Potentially, this was due to the low level supplementation during the winter and the short duration of increased supplementation around breeding.
While heifers may receive adequate MP from grazing spring forage, the type of ruminal fermentation promoted by forage consumption results in an acetate:propionate ratio that is unfavorable for high glucose production. Adequate glucose concentration is potentially important to reproductive function, mitigated through the insulin-like growth factor-1 (IGF-1) system. IGF-1 is related to the steroidogenic potential of the ovarian follicle and in vitro oocyte maturation in cattle (Gong et al., 1993; Spicer et al., 1993). Therefore, a supplement designed to increase the acetate:propionate ratio may improve steroidogenic capacity and/or oocyte quality. Khirenne et al. (1998) supplemented severely nutritionally restricted suckled beef cows for 11 days prior to and for 21 days following AI with approximately 4.5 lb/day of triticale, which is a high-energy supplement. Twenty-one day pregnancy rates were 63% greater for supplemented cows. While the current study evaluated heifers, a growing heifer experiences nutritional stress similar to post-partum cows. Two-year old post partum cows consuming low CP range were supplemented with a range cube containing a Ca salt of propionate (NutroCal; 100 g/day) and providing excess UIP (Waterman et al., 2006). Supplementation began ten days postpartum and continuing through approximately day 35 of the breeding season. Supplementation with NutroCal and excess UIP increased peak insulin time and glucose disappearance, indicating an improvement in insulin sensitivity. Cattle grazing forage may be partially insulin insensitive and therefore, have impaired glucose uptake. Thus, providing a glucogenic feed source might improve reproductive function, especially in heifers grazing low-quality forage. However, the lack of a substantial response to supplemental energy in the current experiments indicates that these heifers were not sufficiently restricted prior to breeding. Ciccoli et al., (2005) found that pregnancy rates were unaffected unless heifers gained less than 1.10 lb/day prior to breeding. This occurred in heifers developed on pasture that gained 0.50 lb/day until breeding and pregnancy rate was reduced compared to heifers that received supplemental energy for 60 days before breeding. In our experiments, the heifers in both experiments at least partially compensated prior to breeding and were not sufficiently restricted at breeding.
Although these winter treatments delay puberty, they do not reduce pregnancy rate. However, a secondary concern is delaying age at puberty may reduce the percentage of heifers that calve early in the season. Potentially, heifers that calves later will wean lighter calves. That is also not the case in Experiment 1, where Stalk developed delayed age at puberty. However, there were no difference in the percentage of heifers calving in the first 21 days nor was there a difference in calf weaning weight. In addition, if these heifers are nutritionally restricted, they may have delayed second calf rebreeding, provided they rebreed. However, second calf pregnancy rates were also similar between these groups. Cushman et al. (2007) determined that primiparous cows with one estrous cycle prior to breeding had comparable pregnancy rates to those with multiple cycles. Therefore, if BCS is adequate, reproductive success may not be limited by the timing of resumption of estrous. In these studies, heifers had a BCS greater than 5, indicating an adequate energy balance. Thus, one would not expect to change the timing of parturition, as was the case.
These studies demonstrate, a post-weaning heifer development system that utilizes crop-residue and standing forages is not only sustainable, but also viable. While this system reduces ADG and delays age at puberty, it does not reduce pregnancy rate. Furthermore, calf production and rebreeding ability are unaffected by winter development and the supplementation strategy around breeding. These development systems minimize the use of harvested forages in lieu of grazing. This method will help maximize profitability of beef cattle operations by reducing the cost associated with hay production. In addition, this system promotes the utilization of co-products from other industries, adding to the sustainability of this system. Therefore, this style of heifer development is both novel and applicable for many beef producers.
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Educational & Outreach Activities
Participation Summary:
Larson D., R. Richardson, and R. Funston. 2008. Effect of wintering system, nutrition around breeding, and prostaglandin on gain and reproduction in beef heifers. J. Anim. Sci. (Abstr. No.)
Larson D., R. Richardson, and R. Funston. 2008. Effect of wintering system, nutrition around breeding and prostaglandin on gain, reproduction, and calf production in beef heifers. J. Anim. Sci. (Abstr. No.)
Larson D. M, J. L. Martin, and R. N. Funston. 2007. Effect of winter system and nutrition around breeding on gain and reproduction in heifers. In press. Nebraska Beef Report.
In addition to these reports, these data have been presented by Dr. Rick Funston and others at numerous producer meetings. In addition, the data were recently presented at the Range Beef Cow Symposium, which is a livestock producer oriented gathering.
Project Outcomes
The major implication of this research is a system that develops heifers to less than 65% of mature weight prior to breeding is not detrimental. We demonstrate that reducing ADG prior to breeding and increasing age at puberty does not reduce pregnancy rate or delay calving. By increasing age at puberty, one would expect that timing of parturition would be shifted later in the calving season. This hypothesis resulted from the theory that the first estrus cycle is infertile. These data challenge that hypothesis as well. Combined with previous research, we demonstrate low input preweaning development does not influence reproductive efficiency.
This type of development system also represents a significant cost-savings over traditional development in a feedlot. Simply reducing the percentage of mature weight a heifer reaches prior to breeding will result in savings. Funston and Deutscher (2004) report that the feed cost of developing heifers to 53% of mature weight prior to breeding was $85/heifer while developing heifers to 58% of mature weight cost $107/heifer. The 5% reduction in weight resulted in a $22 feed cost saving per heifer. Alternatively, Martin et al. (2007) calculated the net total cost of a pregnant second-calf cow that was developed to achieve 51 or 57% of mature weight prior to the first breeding season. This cost accounted for not only feed cost and the value of an open heifer, but also for the cost associated with nonpregnant cows. Here the two-year-old cost of developing a heifer to 51% of mature weight was $577/heifer and the cost of developing to 57% of mature weight was $594/heifer. This resulted in a significant (P < 0.001) cost savings of $17/heifer. These experiments cannot be directly compared to the current study because they took place in a drylot setting using harvested forages. However, they demonstrate the economic benefit of reduced weight at breeding.
Economic Analysis
To conduct the economic analysis of this research, we have made the following assumptions:
We assume that the cost for corn stalks is approximately $0.13 per heifer per day and winter pasture cost is approximately $0.25 per heifer per day. The cost of the supplement fed during grazing is $0.08 per heifer per day. The overall costs of winter development in Experiment 1 and 2 were $55.83 for Stalk development and $106.89 for Range development. In Experiment 3, Stalk development cost $63.05 per heifer and Pasture development $114.11 per heifer. We do not have a winter development cost analysis for Experiment 4. However, it would be very similar to the Range development costs for Experiments 1 and 2. The cost of the breeding supplement in Experiment 1 was three times the cost of winter supplement per day. The cost of the breeding supplements in Experiments 2 and 4 were $0.48 per heifer per day. The cost of the breeding supplement for experiment 3 was $0.47 per heifer per day.
Farmer Adoption
The Rex Ranch (Ashby, NE) where Experiment 4 was conducted has adopted the winter range development system which was developed from this research. In addition, the breeding supplementation scheme has been used at this location for two years. It appears the breeding system will be useful when heifers have less than an adequate body condition score (BCS) prior to breeding.
Areas needing additional study
Clearly, winter development utilizing dormant standing forage or crop residues reduces ADG post-weaning. However, it is also evident that reducing a heifers percentage of mature weight at breeding to as low as 53% does not reduce pregnancy rate. More research is needed to determine how light heifer can be developed prior to the breeding season. In these experiments, supplementation around breeding did not have a positive effect on pregnancy rate. It seems intuitive that these heifers were still on too high a plane of nutrition for short duration supplementation of energy or protein to have an effect. In the future, further reducing weight at breeding and plane of nutrition might reveal the benefits of additional energy or protein around breeding. As this system gains more industry acceptance, the needs of producers will necessitate further research in these areas.
Furthermore, research is needed to determine the changes caused by this system at the hormonal level. By understanding the biological changes that are occurring, we could better tailor a supplementation scheme. A well-controlled grazing simulation using individual feeding with hormonal analysis would provide the necessary evidence. Using these data, we could further characterize the composition of the winter forage and identify the specific limiting nutrients. This would facilitate designing a feeding program that is more specific and would be most economically efficient.