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Effect of copper, zinc, and manganese supplementation and source on reproduction, mineral status, and performance in grazing beef cattle over a two-year period^1,2,3

J. K. Ahola^*, D. S. Baker^*, P. D. Burns^*, R. G. Mortimer, R. M. Enns^*, J. C. Whittier^*, T. W. Geary and T. E. Engle^*,4

^* Department of Animal Sciences and and Department of Clinical Sciences, Colorado State University, Fort Collins 80523 and and ARS, USDA, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301

	Abstract

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Crossbred, multiparous beef cows (n = 178 in Year 1; n = 148 in Year 2) were used to evaluate the effects of Cu, Zn, and Mn supplementation and source on reproduction, mineral status, and performance in grazing cattle in eastern Colorado over a 2-yr period. Cows were stratified by expected calving date, age, BW, BCS, and liver mineral status and assigned to the following treatments: 1) control (no supplemental Cu, Zn, or Mn); 2) organic (ORG; 50% organic and 50% inorganic Cu, Zn, and Mn); and 3) inorganic (ING; 100% inorganic CuSO₄, ZnSO₄, and MnSO₄). Free-choice mineral feeders were used to provide current NRC-recommended concentrations of Cu, Zn, and Mn from 82 d (Year 1) and 81 d (Year 2) before the average calving date of the herd through 110 d (Year 1) and 135 d (Year 2) after calving. At the end of Year 1, supplemented cows had greater liver Cu (P < 0.01), Zn (P < 0.05), and Mn (P < 0.01) concentrations compared with controls, whereas liver Cu concentration was greater (P < 0.01) in ORG vs. ING cows. At the end of Year 2, supplemented cows had greater (P < 0.01) liver Cu concentrations relative to controls, whereas control cows had greater (P < 0.02) liver Mn concentration than did supplemented cows. In Year 1, pregnancy rate to AI in control cows did not differ (P = 0.47) from supplemented cows, but there was a trend (P < 0.08) for pregnancy rate to be higher for ORG than ING cows. In Year 2, supplemented cows had a higher (P < 0.02) pregnancy rate to AI than controls. In both years, when cows were inseminated after an observed estrus, supplemented cows had a higher (P < 0.04) pregnancy rate than did controls. Also, for both years, overall 60-d pregnancy rate tended (P = 0.10) to be higher for supplemented cows than for controls. In Year 1, kilograms of calf weaned per cow exposed was greater (P < 0.02) in controls than in supplemented cows, and kilograms of calf weaned per cow exposed was greater (P < 0.01) in ING than ORG treatments. However, in Year 2, kilograms of calf weaned per cow exposed was greater (P < 0.02) in controls than in supplemented cows, and tended (P = 0.09) to be greater in ORG than ING treatments. Results indicate that supplementation and source of trace minerals affected mineral status and kilograms of calf weaned per cow exposed in grazing beef cows. Supplementation also improved pregnancy rate to AI compared with cows not supplemented with Cu, Zn, or Mn for more than 1 yr. Furthermore, mineral source may influence pregnancy rate to AI.

Key Words: Beef Cattle • Performance • Reproduction • Trace Minerals

	Introduction

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Responses in reproduction and performance to Cu, Zn, and Mn supplementation in ruminants have been variable (Underwood and Suttle, 1999). Olson et al. (1999) reported no difference in reproductive performance in cows supplemented with organic vs. inorganic forms of trace minerals. However, supplemented cows had lower pregnancy rates compared with controls, most likely because supplementation levels were two times NRC (1996) recommendations. In contrast, Stanton et al. (2000) reported that cows receiving organic trace minerals exhibited higher pregnancy rates to AI than those receiving inorganic trace minerals. Improved reproductive performance has been reported in dairy cows receiving organic mineral supplements (Manspeaker et al., 1987), which was attributed to improved repair of damaged uterine tissue following calving.

The increase in performance associated with organic trace minerals may be due to increased bioavailability that has been reported in several species. Du et al. (1996) and Kegley and Spears (1994) suggested that organic Cu sources are equally, if not more, bioavailable than CuSO₄ in rats and growing calves, respectively. Conversely, Cao et al. (2000) compared eight organic Zn products in chicks and lambs and found that only one was more bioavailable than ZnSO₄.

There has been tremendous variability in the concentration and source of trace mineral supplementation used in different studies, making interpretation of data and ability to reach a concise conclusion challenging. We hypothesized that not supplementing trace minerals would be detrimental to cow reproductive performance and that source of trace mineral supplementation may have different effects on reproductive performance in cows and on calf performance. The objective of this study was to determine the effect of the supplementation and source of Cu, Zn, and Mn at levels recommended by NRC (1996) on reproductive performance, mineral status, and performance in grazing beef cattle.

	Materials and Methods

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Experimental Design
Before starting this study, all animal use, handling, and sampling techniques described herein were approved by the Colorado State University Animal Care and Use Committee.

Twelve days before the start of this experiment (January 17, 2001), 178 crossbred (Red Angus-based), multiparous beef cows at the Eastern Colorado Research Center (Akron, CO) were stratified based on age, expected calving date, BW, BCS, and liver mineral status (Table 1) and randomly assigned to one of nine replicates. The replicates were then assigned to one of three treatments (n = 19 to 20 cows per replicate), resulting in three replicates per treatment in each year. Treatments were as follows: 1) control (no supplemental Cu, Zn, or Mn; n = 59); 2) organic (ORG; 50% organic and 50% inorganic Cu, Zn, and Mn; n = 60); and 3) inorganic (ING; 100% inorganic Cu, Zn, and Mn; n = 59).

Mineral treatments were provided at a single location in each pasture in free-choice mineral feeders beginning 82 (d –82; Year 1) and 81 d (d –81; Year 2) before the average expected calving date (d 0) of the cowherd. Mineral treatments remained available for ad libitum consumption until 110 (d +110; Year 1) and 135 d (d +135; Year 2) after the average calving date of the cowherd. Mineral treatments were also made available exclusively to the calves via creep feeders in each pasture when calves within the pasture averaged 90 (Year 1) and 99 d (Year 2) of age. All calves received the same respective mineral treatment as their dam. In Year 1, on d +110 after the average calving date, treatments were discontinued and all cows within a treatment were combined and given access to the control (basal) mineral supplement, which did not contain any supplemental Cu, Zn, or Mn (Table 2) for a period of 160 d. Calves continued to have access to their respective mineral treatments until weaning at an age of 185 and 164 d in Years 1 and 2, respectively. Beginning 81 d before the average expected calving date of the cowherd in Year 2, cows were sorted into the same respective treatment groups as in Year 1, assigned to new replicates, and maintained on treatments until d +135 of Year 2. At the beginning of Year 2, there were fewer cows in the experiment (n = 148) than at the beginning of Year 1 (n = 178) because cows that were not pregnant after the final pregnancy rate was determined in Year 1 were removed from the experiment.

Replicates were rotated among pastures approximately every 28 d in order to minimize pasture effects. In conjunction with pasture rotations, mineral weigh-backs were performed at each rotation in order to calculate mineral disappearance. Replicates were pooled but remained within treatments for approximately 30 d around the time of calving and 60 d during the time of breeding to allow for more intensive management of the cattle. Mineral treatments continued to be available at these times, although mineral disappearance within replicates could not be monitored.

Mineral Status.
Mineral status of the cows was measured using two methods. The first method involved the collection of a liver biopsy sample from every cow before the start of the experiment and then from a subgroup of animals (five per replicate) at the end of the supplementation period in Year 1 and at the beginning and end of the supplementation period in Year 2. This was accomplished by using the true-cut technique described by Pearson and Craig (1980), as modified by Engle and Spears (2000b). Following collection, samples were immediately rinsed with 0.01 M PBS, placed in an acid-washed polypropylene tube, capped, and placed on ice for 5 h before storing at –20°C. Liver samples were analyzed for trace mineral concentrations as described by Engle et al. (1997).

The second method used to determine mineral status involved the collection of whole blood via jugular venipuncture in heparinized, trace-mineral-free Vacutainer tubes (Becton Dickinson Co., Franklin Lakes, NJ) at the beginning, midpoint, and end of mineral supplementation from a subgroup of animals (five per replicate), the same animals from whom liver biopsies were obtained. Once collected, samples were placed on ice for 5 h before being centrifuged at 2,000 x g for 15 min. Plasma was then transferred to acid-washed storage vials and stored at –20°C.

Plasma Cu and Zn concentrations were measured after the samples were thawed at room temperature for 3 to 4 h. For Cu analysis, 1 mL of a 10% (wt/vol) trichloroacetic acid solution was added to 1 mL of plasma or standard and then vortexed vigorously for 20 s. To aid in precipitation, the sample was placed in a –20°C freezer for 30 min, and then centrifuged at 1,200 x g for 10 min at room temperature. The supernatant was removed and diluted in deionized water to concentrations that fit within a linear range of a standard curve generated by linear regression of known concentrations. Plasma Cu concentrations were read at 324.7 nm using a flame atomic absorption spectrophotometer (model 1275, Varian, Walnut Creek, CA). Plasma Zn concentrations were analyzed in a similar manner as described above. Five hundred microliters of plasma was diluted to a 1:5 ratio (plasma:deionized water). This dilution was then read at 213.9 nm using the same atomic absorption spectrophotometer previously mentioned. Only samples from Year 1 were analyzed for Cu and Zn concentrations.

Cow Performance.
To determine the effects of trace mineral supplementation and source on reproductive performance, every cow was inseminated once following a modified Select-Synch (Geary et al., 2000) estrus synchronization protocol. Beginning on d +55 (Year 1) and d +69 (Year 2) after the average calving date of the cowherd, all cows were injected with 100 µg of GnRH (Fertagyl, Intervet, Millsboro, DE; i.m.), and then injected with 25 mg of PGF₂ 7 d later (Lutalyse, Pharmacia & Upjohn, Kalamazoo, MI; i.m.). Calves were removed for 72 h from all cows at the time of PGF₂ administration in an effort to increase estrous response. For management reasons, all cows were pooled and treatments were withheld during the 72-h synchronized AI breeding period. Cattle were observed for signs of behavioral estrus from 12 h before PGF₂ injection through 72 h after PGF₂ injection. Detection of estrus was accomplished by visual observation for a minimum of 1 h at dawn, noon, and dusk. Animals observed in behavioral estrus were inseminated approximately 12 h after first detected in estrus. At 72 h after PGF₂, all females that had not been observed in estrus were given a second injection of GnRH (100 µg; i.m.) and mass inseminated. Calves were returned to their dams following AI, and all cattle were sorted back into treatments and returned to appropriate pastures after mass insemination.

To minimize variation in breeding measurements, an effort was made to decrease confounding variables. Two AI technicians performed the inseminations, equally represented within each replicate, and one technician thawed semen. Semen from one purebred Charolais sire that had been acquired from a single collection was used to avoid a sire effect. To allow for the accurate differentiation between pregnancy to AI vs. pregnancy to natural service, cows were not exposed to bulls until 14 d after mass insemination. Six Red Angus-based composite bulls that had previously passed a breeding soundness evaluation were exposed to the cows for 46 d (two bulls per treatment).

In Year 2 only, immediately before the initiation of the synchronization protocol, two blood samples were collected via jugular venipuncture at 10-d intervals and progesterone concentrations were determined to identify cows that had cycled since parturition. Cows were identified as cycling if at least one blood sample had a serum progesterone concentration greater than 1.0 ng/mL, indicating the presence of a functional corpus luteum. Serum was harvested and stored at –20°C until samples were evaluated in duplicate for progesterone concentration by solid-phase RIA (Coat-a-Count kit; Diagnostic Products Corp., Los Angeles, CA), as described by Bellows et al. (1991). The intra- and interassay CV for the two assays were 5.9 and 2.2%, respectively. The sensitivity of the assay was 0.04 ng/mL of serum.

To determine pregnancy rate to AI, cattle were examined via rectal ultrasonography (Aloka 500V equipped with 5.0-MHz linear array transducer, Corometrics Medical Systems, Wallingford, CT) by a state-licensed veterinarian 40 d after mass mating. Cows with fetuses that were approximately 40 d old were classified as pregnant to AI, whereas all other cattle were classified as not pregnant to AI. Final pregnancy rates following the 60-d breeding season were determined via palpation per rectum with the aid of ultrasonography approximately 40 d after bull removal.

Cow performance was also monitored by collection of BW and BCS data at approximately 56-d intervals throughout the duration of the supplementation period in each year. The BCS measurements collected were on a scale of 1 to 9 (1 = emaciated, 9 = obese; Richards et al., 1986) and were assigned by the same technician throughout the study.

Calf Performance.
Calf performance was measured in both years based on kilograms of calf weaned per cow exposed. Weaning weights were collected on each calf before shipment to the feedlot. At the end of Years 1 and 2, calves averaged 185 and 164 d of age, respectively.

Statistical Analysis
Cow performance, mineral status, and calf performance data (including BCS; BW; liver Cu, Zn, and Mn concentrations; plasma Cu and Zn concentrations; and weaning weight) were assessed using a restricted maximum likelihood-based, mixed-effects model, repeat-measures analysis (PROC MIXED; SAS Inst., Inc., Cary, NC). Initial cow performance and mineral status models contained fixed effects of treatment, time, and treatment x time interaction. Initial calf performance models included fixed effects of treatment, year, age of dam, age of calf, sex of calf, and all relevant two- and three-way interactions. A spatial power covariance structure was used in the analysis and the containment approximation was used to calculate denominator degrees of freedom. Pasture was used as the experimental unit. Reproductive response data (including estrous cyclicity, estrous response, pregnancy to a synchronized AI, and final pregnancy throughout the 60-d breeding season) were analyzed using logistic regression (PROC GENMOD of SAS). Initial models for reproductive response contained fixed effects of treatment, postpartum interval, BCS, BW, year, and AI technician, in addition to all relevant two- and three-way interactions. When an interaction was not significant, it was removed from the model. If the interaction of year x treatment was not significant, data were pooled across years; otherwise, data were reported for each year separately. Differences among means were determined using single-df contrasts; comparisons made were as follows: 1) control vs. supplemented and 2) ORG vs. ING.

	Results and Discussion

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Mineral Consumption
Average mineral disappearance for the cows during Year 1 was 0.12 ± 0.06, 0.11 ± 0.07, and 0.12 ± 0.07 kg•cow^–1•d^–1 for the ORG, ING, and control treatments, respectively. Average mineral disappearance during Year 2 was 0.13 ± 0.06, 0.12 ± 0.06, and 0.13 ± 0.06 kg•cow^–1•d^–1 for the ORG, ING, and control treatments, respectively. In both years, the amount of daily mineral disappearance indicated that the ORG and ING supplemented cattle consumed enough Cu, Zn, and Mn to meet their NRC-recommended requirements of 10 mg of Cu/kg of DM, 30 mg of Zn/kg of DM, and 40 mg of Mn/kg of DM (NRC, 1996; estimated DMI = 10.9 kg). Because forage basal trace minerals were at low concentrations relative to NRC (1996) recommendations, no effects on performance were attributed to excess supplementation of Cu, Zn, or Mn.

Liver Mineral Status
Liver mineral status for cows in this experiment was affected (P < 0.05) by trace mineral supplementation and source (Table 3). A year x treatment interaction was detected (P = 0.03) for liver Cu concentration. At the end of both years, liver Cu concentration was greater (P < 0.01) in supplemented vs. control cows; however, liver Cu concentration in control cows decreased in Year 2. In addition, at the end of Year 1, ORG cows had greater (P < 0.01) liver Cu concentration than did ING cows, whereas at the end of Year 2, liver Cu concentration did not differ (P > 0.38) between ORG and ING cows. Liver Cu results are consistent with those reported by other researchers (Du et al., 1996; Olson et al., 1999), wherein liver Cu concentrations increased with mineral supplementation when compared with controls. Although an increase in liver Cu concentration is often observed in conjunction with trace mineral supplementation, and in this case with source in Year 1, it is not clear as to the positive or negative effects of liver Cu retention on reproduction and/or calf performance.

There was no year x treatment interaction (P = 0.11) for liver Zn concentration. Liver Zn concentration was greater (P < 0.05) in supplemented than in control cows at the end of Year 1; however, liver Zn concentration did not differ (P > 0.56) between supplemented and control cows at the end of Year 2. Liver Zn concentration did not differ (P > 0.86) between ORG and ING treatments at the end of Year 1, but ING cows tended (P < 0.07) to have greater liver Zn concentrations than ORG cows at the end of Year 2.

Plasma Mineral Status
Plasma trace mineral concentrations were impacted by trace mineral supplementation, but not by source. Plasma Cu concentrations at the end of the supplementation period in Year 1 tended (P = 0.08) to be greater in supplemented vs. control cows, and plasma Zn concentrations were greater (P < 0.01) in the supplemented cows vs. controls (data not shown).

Cow Performance
Mean BW and BCS did not differ among treatments throughout the 2-yr experiment (data not shown). However, there was a main effect of time (P < 0.05). Both BW and BCS declined following calving but returned to precalving levels by mid-summer. Similarly, previous experiments have reported that neither trace mineral supplementation nor source affected cow BW or BCS (Olson et al., 1999; Stanton et al., 2000; Muehlenbein et al., 2001).

Reproductive performance data throughout the experiment are reported in Table 4. There were no differences between control and supplemented cows, or between ORG and ING supplemented cows for the rate of estrous cyclicity at the start of the breeding season (Year 2 only) or percentage of cows exhibiting behavioral estrus in response to the synchronization protocol in either year. A year x treatment interaction (P < 0.02) for pregnancy rate to a synchronized AI was present, and therefore data were reported individually for each year. In Year 1, pregnancy rate to AI did not differ between control and supplemented cows; however, ORG-supplemented cows tended (P < 0.08) to have a higher pregnancy rate to AI than did ING-supplemented cows. Unlike Year 1, control cows had a lower (P < 0.02) pregnancy rate to AI than did supplemented cows in Year 2. Additionally, ORG- and ING-supplemented cows did not differ in pregnancy rate to AI in Year 2. To further evaluate the pregnancy response to AI data, time of AI was classified as being administered either 12 h after an observed estrus (EAI) or via mass insemination at 72 h after PGF₂ injection. Within the EAI group, supplemented cows had a higher (P < 0.04) pregnancy rate to AI than did control cows, and ORG-supplemented cows tended (P = 0.13) to have a higher pregnancy rate compared with ING-supplemented cows. Conversely, no differences were observed within the mass-insemination group; however, a much smaller number of cows were mass inseminated due to the high estrous response (approximately 80%) that occurred in both years.

Pregnancy rates collected at the end of the 60-d breeding season included pregnancies to both the synchronized AI and natural service. Supplemented cows tended (P = 0.10) to have a higher pregnancy rate compared with control cows; however, there was no difference between ORG- and ING-supplemented cows. To our knowledge, this is the first study to report that trace mineral supplementation may affect season-long reproductive performance in beef cattle. Trace mineral supplementation affected pregnancy rates to AI (as reported above) and possibly also natural service pregnancy rates during the 46-d period of bull exposure. Earlier studies have reported no differences at the end of a 60-d breeding season due to trace mineral supplementation (Arthington et al., 1995; Muehlenbein et al., 2001). However, Olson et al. (1999) reported a depressed 60-d pregnancy rate in cows that received supplemental Cu, Zn, and Mn in organic or inorganic forms at twice the NRC (1996) recommended levels compared with controls. The beneficial reproductive performance observed in the current study, when supplementation was provided at NRC (1996) recommended levels for 2 yr, suggests that trace minerals impact reproductive performance positively when supplementation follows NRC recommendations.

Calf Performance
From d +90 until weaning in Year 1, mineral disappearance from calf creep feeders was 0.010 ± 0.003, 0.014 ± 0.004, and 0.011 ± 0.004 kg•calf^–1•d^–1 for ORG, ING, and control treatments, respectively. Conversely, from d +99 until weaning in Year 2, mineral disappearance from creep feeders of calves was 0.048 ± 0.012, 0.042 ± 0.12, and 0.061 ± 0.018 kg•calf^–1•d^–1 for ORG, ING, and control treatments, respectively. In Year 2, mineral disappearance from creep feeders was greater (P < 0.01) than in Year 1. It is not clear why calves consumed more supplement in the second year of this experiment.

There was a year x treatment interaction (P < 0.05) for calf performance, as measured by kilograms of calf weaned per cow exposed, and therefore data were reported separately for each year (Table 5). In both years, trace mineral supplementation and source affected kilograms of calf weaned per cow exposed. In Year 1, there were more (P < 0.02) kilograms of calf weaned per cow exposed in the control treatment than in supplemented cows, and more (P < 0.01) kilograms of calf weaned per cow exposed in the ING treatment than in ORG. Year 2 results for the effect of supplementation were similar to Year 1 because controls had more (P < 0.02) kilograms of calf weaned per cow exposed vs. supplemented; however, unlike Year 1, the ORG treatment tended (P = 0.09) to have more kilograms of calf weaned per cow exposed than ING.

The effect of Cu on postweaning feedlot calf performance in the growing and finishing phases has been evaluated in several experiments. However, much like preweaning performance, postweaning performance has been shown to be positively affected, negatively affected, and also not affected by trace mineral supplementation. It has been reported that Cu supplementation does not impact the performance of growing or finishing steers (Engle and Spears, 2000a, 2001). However other experiments have shown both positive and negative effects of Cu on feedlot performance during the finishing phase. Copper has been shown to decrease feed intake, feed efficiency, and ADG when supplemented at 20 or 40 mg of Cu/kg of DM (Engle and Spears, 2000b). Conversely, Cu supplementation at 10 or 40 mg of Cu/kg of DM improved ADG and daily feed intake (Engle et al., 2000).

It seems that Cu, Zn, and Mn supplementation and source affect calf performance; however, based on the highly variable results of other studies, it is not known why this might be occurring. Further research into the effect of trace mineral supplementation and source on calf performance is needed.

	Implications

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Both supplementation (at concentrations recommended by the NRC) and source of copper, zinc, and manganese affected the concentration of these minerals in the liver and plasma of multiparous beef cows, although the effect of these differing body mineral concentrations on reproduction or calf performance is unclear. After 1 yr without supplemental copper, zinc, and manganese, pregnancy rate to a synchronized artificial insemination may be decreased, and season-long reproductive performance also may be affected. Additionally, trace mineral source may affect pregnancy rate to a synchronized artificial insemination, particularly if the insemination was administered based on an observed estrus. Finally, under certain conditions, kilograms of calf weaned per cow exposed may be affected by both trace mineral supplementation and source, although the effects are not well understood and deserve further attention in future research.

	Footnotes

 
¹ Use of trade names in this publication does not imply endorsement by Colorado State Univ. or criticism of similar products not mentioned.

² Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA, Montana Agric. Exp. Stn., or the authors, and does not imply its approval to the exclusion of other products that may also be suitable.

³ This research was supported in part by grants from the Colorado State Univ. Agric. Exp. Stn. and Alltech, Inc. (Nicholasville, KY). Appreciation is also extended to Alltech, Inc., for donation of the Bioplex trace minerals.

⁴ Correspondence: 208A Animal Sciences Bldg. (phone: 970-491-3597; fax: 970-491-5326; e-mail: engle{at}lamar.colostate.edu).

Received for publication November 5, 2003. Accepted for publication April 14, 2004.

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