J. Anim Sci. 2007. 85:2304-2313. doi:10.2527/jas.2007-0167
© 2007 American Society of Animal Science
Effects of source of supplemental zinc on performance and humoral immunity in beef heifers1
G. A. Nunnery*,2,
J. T. Vasconcelos*,3,
C. H. Parsons*,2,
G. B. Salyer*,2,
P. J. Defoor*,2,
F. R. Valdez
and
M. L. Galyean*
* Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409; and
Kemin Industries, Des Moines, IA 50306
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Abstract
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Two experiments were conducted to evaluate receiving-period performance, morbidity, and humoral immune response, as well as finishing performance and carcass characteristics of heifers fed different sources of supplemental Zn. In Exp. 1, 97 crossbred beef heifers (initial BW = 223.4 kg) were fed a 65% concentrate diet with no supplemental Zn (control) or 75 mg of supplemental Zn/kg of DM from Zn sulfate, Zn methionine, or Zn propionate. During a 35-d receiving period, heifers were monitored daily for signs of bovine respiratory disease. Serum samples were collected for Zn analysis on d 0, 14, and 28. After the receiving period, heifers were adapted to and fed a high-concentrate diet with no supplemental Zn for 42 d. Heifers were then assigned to finishing diet treatments, with the same concentrations and sources of supplemental Zn as during the receiving period and fed for an average of 168 d. Serum samples also were obtained on d 0 and 56 of the finishing period and at the end of the study. During the receiving period, control heifers had a greater (P 
0.05) BW and G:F on d 35 than heifers in the other treatments, but no differences were observed among treatments for morbidity or serum Zn concentrations (P 
0.50). For the finishing period, DMI and ADG did not differ among treatments; however, overall G:F tended (P = 0.06) to be less for control heifers than for heifers in the 3 supplemental Zn treatments. On d 56 of the finishing period, control heifers tended (P = 0.06) to have a lower serum Zn concentration than heifers in the 3 supplemental Zn treatments. In Exp. 2, 24 crossbred beef heifers (initial BW = 291.1 kg) were fed the same 4 treatments as in Exp. 1 for a 21-d period. The humoral immune response to treatments was determined by measuring specific antibody titers after s.c. injection of ovalbumin on d 0 and 14. Body weights and blood samples for serum Zn concentration and ovalbumin IgG titers were collected on d 0, 7, 14, and 21. Serum Zn concentration and specific ovalbumin IgG titers did not differ (P > 0.10) among the 4 treatments on any sampling day. Results from these 2 studies showed no major differences among the sources of supplemental Zn for receiving period morbidity, ADG, DMI, and humoral immune response of beef heifers; however, a lack of supplemental Zn during an extended finishing period tended to negatively affect G:F.
Key Words: beef cattle • humoral immune response • morbidity • performance • zinc
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INTRODUCTION
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Stresses associated with weaning and transportation negatively affect the immune system of beef cattle, resulting in an increased incidence of infectious diseases (Spears et al., 1991
). In addition, DMI by newly received cattle is typically low for the first 2 wk after arrival (Galyean et al., 1999), which can exacerbate nutrient deficiencies during periods of stress. Zinc has a major role in immune responsiveness (Keen and Gershwin, 1990), and a nutritional deficiency of Zn has been associated with increased morbidity and mortality (Kincaid et al., 1997). During times of stress and suboptimal feed intake, supplementation with an organic source of Zn might be beneficial. Nonetheless, effects of organic sources of Zn on cattle health have been equivocal (Duff and Galyean, 2007). For example, supplementation with an organic Zn source was beneficial to newly received (Johnson et al., 1988) and morbid (Chirase et al., 1991; Spears et al., 1991) cattle, whereas Galyean et al. (1995), Gunter et al. (2001), and Salyer et al. (2004) reported no effect of Zn source on cattle morbidity.
Use of organic sources of Zn in cattle finishing diets as a means of increasing feedlot performance and carcass quality also has been studied. Spears (1989) reported that heifers fed Zn methionine had greater ADG and improved feed efficiency than heifers receiving no supplemental Zn. Rust (1985) and Greene et al. (1988) found that supplementation with Zn methionine increased the percentage of cattle grading USDA Choice compared with controls receiving no supplemental Zn. In contrast, however, numerous researchers have not observed benefits in performance and carcass characteristics as a result of source of supplemental Zn (Martin et al., 1987; Galyean et al., 1995; Nunnery, 1998).
Given the conflicting data regarding effects of Zn supplementation and Zn source on health and performance of cattle, the current study evaluated receiving-period performance and morbidity, humoral immune response, finishing performance, and carcass characteristics of beef heifers in response to different sources of Zn.
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MATERIALS AND METHODS
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All procedures involving live animals were conducted within the guidelines of and approved by the Texas Tech University Animal Care and Use Committee.
Experiment 1
Receiving Period.
Ninety-seven beef heifers (British x Continental; average initial BW = 223.4 ± 2.59 kg) were purchased at livestock auctions by an order buyer and delivered to the Texas Tech University Burnett Center. On arrival (d 0), heifers were weighed (squeeze chute mounted on 4 load cells; Rice Lakes Weighing Systems, Rice Lake, WI), given a uniquely numbered ear tag, vaccinated with a Fortress 7 and Bovishield 4 + Lepto vaccines (Pfizer Animal Health, New York, NY), treated for internal and external parasites with Dectomax (Pfizer Animal Health), injected with tilmicosin phosphate (Micotil, Elanco Animal Health, Indianapolis, IN), and assigned randomly to 1 of 12 soil-surfaced-floor pens (3 pens/treatment with 8 or 9 heifers/pen). Pens were approximately 5.5 x 30.5 m, with 4.57 m of linear bunk space. Once in the pens, heifers were allowed ad libitum access to a 65% concentrate diet with no supplemental Zn (Con) or a formulated supplementation of 75 mg of Zn/kg from Zn sulfate (Sul), Zn methionine (Met), or Zn propionate (Prop; Table 1). In addition to the 65% concentrate diet, heifers were allowed ad libitum access to alfalfa hay for the first 5 d after arrival. On d 14 of the study, all heifers were given a booster vaccination with Bovishield-4 + Lepto and implanted with Ralgro (Schering-Plough Animal Health, Union, NJ).
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Table 1. Composition and analyzed nutrient content (DM basis) of diets containing different sources of supplemental Zn1
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Heifers were monitored daily at 0730 for signs of bovine respiratory disease (BRD). Visual signs of BRD included nasal and ocular discharge, depression, and anorexia. Suspect heifers were removed from their pens for evaluation of rectal temperature, and those with rectal temperatures 39.7°C received an i.m. injection of Nuflor (6 mL/45.4 kg; Schering-Plough Animal Health) as the first medical treatment. After treatment, heifers were returned to their pen and removed 48 h later for reevaluation of their health status. If after reevaluation, the rectal temperature of treated heifers had not fallen below 39.7°C, the second medical treatment consisted of Micotil 300 (s.c., 1.5 mL/45.4 kg; Elanco Animal Health) plus penicillin G (s.c., 2 mL/45.4 kg; Aspen Veterinary Resources, Kansas City, MO). Heifers that required a third medical treatment received Liquamycin LA-200 (s.c., 4.5 mL/45.4 kg; Pfizer Animal Health) and Albon SR boluses (1 oral bolus/90.7 kg; Pfizer Animal Health). In addition to the antibiotic therapy, when heifers had a rectal temperature 40.3°C, Banamine (i.v., 1.1 mg/kg; Schering-Plough Animal Health) was administered.
Feed bunks were visually evaluated between 0730 and 0800 every day for estimation of the daily feed allotment. The quantity of unconsumed feed remaining in each bunk was estimated, and the quantity of feed delivered was adjusted to ensure ad libitum feed intake. Feed samples of each treatment were obtained weekly for DM determination, dried in a forced-air oven (100°C) overnight, and stored for chemical analyses. After completion of the 35-d receiving study, feed samples from each week were composited by treatment, ground to pass a 2-mm screen in a Wiley mill, and analyzed for DM, CP, ash, Ca, P, and Zn (AOAC, 1990). When cattle were weighed, feed bunks were swept, and orts were removed from the bunk, weighed, and sampled for DM content. The DM content of the orts was determined in the same manner as for the weekly feed samples, and the quantity of dry orts was subtracted from the total dry feed delivered to obtain a measurement of DMI. All heifers were weighed individually on d 14, 28, and 35 of the receiving period.
Nine animals per treatment (3/pen) were selected randomly at the time of arrival processing to measure serum Zn concentration. On d 0, 14, and 28, a 10-mL blood sample was obtained from the selected heifers via jugular venipuncture using evacuated serum-separator tubes (product No. 02-657-13, Fisher Scientific, Houston, TX). The tubes were centrifuged at 1,000 x g, and the serum was decanted into storage vials at –4°C until Zn analysis was conducted using atomic absorption spectroscopy (model 2380, Perkin-Elmer, Norwalk, CT). Serum samples were diluted 1:3 (serum:deionized water, vol/vol) and analyzed at a wavelength of 213.9 nm with a 0.7-nm slit width. Standards were prepared in a 10% (vol/vol) glycerin solution.
Finishing Period.
At the end of the 35-d receiving period, all heifers were housed in a large pen and fed the Con diet for 42 d. This period was used to gradually increase the concentrate level of the diet to 88%. After this period, heifers were implanted with Revalor H (140 mg of trenbolone acetate and 14 mg of estradiol; Intervet, Millsboro, DE), blocked by BW within original treatment, and sorted to concrete, partially slotted-floor pens (4 heifers/pen with 6 pens/treatment). Pens were 2.9 m wide x 5.6 m deep with 2.4 m of linear bunk space. On d 0 of the finishing trial, the heifers were weighed individually and received the same treatments they had been assigned during the receiving period. The ingredient and chemical composition data for the diets fed during the finishing trial are shown in Table 1
. Heifers were weighed every 28 d thereafter for the remainder of the feeding period. On d 0 and 56 of the finishing period and at the end of the study when the heifers were shipped to slaughter, serum samples were obtained from the same heifers that had been sampled during the receiving period. All feeding and weighing procedures and serum sample collection and analysis were performed as described for the receiving period.
When approximately 60% of the heifers in each weight block were deemed by visual evaluation to have sufficient finish to grade USDA Choice, they were shipped to a commercial slaughter facility. Heifers in blocks 4, 5, and 6 were on feed for 154 d before shipment, those in block 3 were shipped after 168 d on feed, and those in blocks 1 and 2 were shipped after 182 d on feed. Carcass measurements were obtained 48 h after slaughter by the West Texas A&M University Cattlemen’s Carcass Data Service and included HCW, dressing percent, external fat thickness at the 12th rib, marbling score, LM area, KPH, calculated yield grade, and USDA quality grade.
Experiment 2
Twenty-four beef heifers (British x Continental; average initial BW = 291.1 ± 9.5 kg) that had been previously received (as described for Exp. 1) at the Texas Tech University Burnett Center were assigned to 4 dietary treatments in a completely random design. Treatments included a 70% concentrate diet with no supplemental Zn or with the same 3 supplemental Zn treatments used in Exp. 1 (Table 1). Before initiation of the study, all heifers were fed the Con diet for 30 d. Five days before beginning the trial, heifers were sorted into treatment groups and placed in the concrete, partially slotted-floor pens described for Exp. 1 (3 heifers/pen, with 2 pens/treatment). Beginning on d 0 and for the remainder of the 21-d trial, each pen of heifers was fed their assigned treatment diet. Overall cattle management, bunk and feeding management, and weighing procedures were as described for Exp. 1. Weekly samples of diets were composited and analyzed for DM, CP, ash, Ca, P, and Zn (AOAC, 1990). All heifers were weighed individually on d 0, 7, 14, and 21 of the study.
To evaluate the effects of the supplemental Zn sources on the humoral immune response, heifers were initially injected with 4 mL of a solution containing 4 mg of ovalbumin (chicken egg albumin, grade V, product No. A-5503, Sigma Chemical, St. Louis, MO) on d 0, followed by a 4-mL booster injection on d 14. The suspension and injection of the ovalbumin and the ELISA procedure used to measure specific IgG titers were adapted from Burton et al. (1989) and Henning (1992), respectively. Each ovalbumin injection was administered s.c. in the area of the point of the shoulder. The ovalbumin solution was prepared from crystallized ovalbumin suspended in nonsterile 0.01 M PBS at a concentration of 2 mg of crystallized ovalbumin/mL of PBS. The ovalbumin-PBS solution was then further diluted 1:1 with Freund’s incomplete adjuvant (product No. F-5506, Sigma Chemical), resulting in the final ovalbumin suspension used for injection. The ovalbumin solution was then emulsified and stored at –4°C until injection.
On d 0, 7, 14, and 21, two blood samples were obtained from each heifer. Approximately 10 mL of blood was collected via jugular venipuncture into evacuated serum-separator tubes (Fisher Scientific) for determination of the serum Zn concentration. An approximately 10-mL sample of blood was collected into heparinized, evacuated tubes (Fisher Scientific) for determination of the ovalbumin-specific IgG titer response. Blood tubes were centrifuged at 1,000 x g for 20 min for separation of the serum and plasma. The serum and plasma for each evaluation were decanted into storage vials, labeled, and stored at –4°C. Serum Zn analyses were conducted following the same procedures described for Exp. 1. Plasma samples obtained on each of the sampling days were analyzed for specific IgG titers to the ovalbumin antigen using an ELISA procedure, as described by Rivera et al. (2002) and Salyer et al. (2004).
Statistical Analyses
Performance and serum Zn data from Exp. 1 (receiving and finishing) and carcass data (except USDA quality grade) were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC). The MIXED procedure also was used for the analyses of performance data in Exp. 2. Serum Zn concentrations and specific ovalbumin titer values were analyzed with repeated measures using the MIXED procedure of SAS, with the subject defined as heifer nested within pen. For the receiving period of Exp. 1, data were analyzed as completely randomized designs, with pen as the experimental unit. The model statement included the fixed effect of treatment. For the finishing period of Exp. 1, pen-based performance and carcass data were analyzed as a randomized complete block design. The model statement included the fixed effect of treatment, and the random effect was block. Morbidity data, as well as carcass quality grade data from Exp. 1, were analyzed using the GLIMMIX procedure of SAS using a completely randomized design for the morbidity data and a randomized block design for the quality grade data. In Exp. 2, specific ovalbumin titers values were converted to the log2 to normalize the data before analysis. In both experiments, the following specific orthogonal contrasts were used to test treatment responses: 1) Con vs. the average of the 3 Zn sources; 2) Sul vs. the average of the Met and Prop; and 3) Met vs. Prop.
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RESULTS AND DISCUSSION
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Chemical composition of the experimental diets is presented in Table 1. Zinc concentrations were lower than formulated values and more variable than expected in all diets in which Zn was supplemented. The intermediate premixes (Table 1) that supplied the Zn sources were prepared by weighing the premix to the nearest gram, and the premixes were subsequently delivered accurately to the mixer by the feed mill batching system. Weekly samples were composited by mixing grab samples from each week and obtaining a grab sample of the mixture. Because the premixes were in a relatively dense, loose-meal form, we believe that the variable and lower than expected Zn concentrations were most likely a result of difficulty in obtaining representative samples of the finished feed because of settling of dense mineral components of the premix when weekly samples were composited for analysis. We have subsequently modified our compositing procedures to eliminate the use of grab samples (e.g., weekly samples are mixed, and half the complete mixture rather than a grab sample is used for compositing) and have found this method to decrease variability in nutrient concentrations.
Experiment 1
Initial BW, final BW, and heifer performance for the 35-d receiving period are shown in Table 2. Heifers fed the Con treatment had a greater (P = 0.05) BW on d 35 than heifers in the other 3 treatment groups. This increased final BW was reflected in a tendency (P = 0.11) for Con heifers to have an increased ADG for the 35-d receiving period compared with heifers in the other 3 treatments; however, ADG by heifers in all treatments did not differ from d 0 through 14 (P = 0.42) or from d 0 through 28 (P = 0.48). Average daily DMI did not differ among treatments (P 0.20). The tendency for Con heifers to gain faster during the overall receiving period, coupled with the similar DMI among treatments, resulted in an increased (P = 0.04) G:F for Con heifers compared with those in the other 3 treatments for d 0 to 35. The reason for the increased BW and G:F of heifers fed the Con diets during the growing period is not clear. As with ADG, treatments did not affect G:F for d 0 through 14 (P = 0.22) or for d 0 through 28 (P = 0.38) of the receiving period. Galyean et al. (1995) evaluated the effects of dietary Zn source (sulfate or methionine) and concentration (35 or 70 mg of supplemental Zn/kg) and supplemental Cu lysine on performance and health during receiving, growing, and finishing periods. Their results indicated that performance during the 28-d receiving period was not affected by Zn source or concentration. Similarly, Salyer et al. (2004) examined the effects of source of Cu and Zn (sulfate or polysaccharide complexes) on the performance and health of newly received heifers and reported that the source of supplemental Zn (75 mg/kg) had no effect on ADG, DMI, or G:F. Neither of these previous studies included an unsupplemented control treatment as did the current study. Ignoring the unsupplemented control treatment, results of the current study agree with those of both Galyean et al. (1995) and Salyer et al. (2004).
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Table 2. Effects of source of supplemental Zn on performance of newly received beef heifers during a 35-d receiving period (Exp. 1)
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Morbidity data from d 0 through 35 are presented in Table 3. Percent morbidity did not differ (P 0.50) among the 4 treatments, averaging 20.8, 26.2, 16.7, and 20.8% for Con, Sul, Met, and Prop treatments, respectively. Galyean et al. (1995) and Salyer et al. (2004) also reported no differences in morbidity of newly received beef cattle as affected by supplemental Zn source. Johnson et al. (1988) reported that supplementing steer and bull calves with 360 mg of Zn/d from Zn methionine decreased (P < 0.03) the number of medical treatments per steer from 4.94 to 4.45 when all cattle were included in the analyses, but when cattle that were detected as sick during the first 2 d of the study were eliminated from the analysis, overall morbidity did not differ between the control (46%) and Zn methionine-supplemented (51%) groups (Johnson et al., 1988).
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Table 3. Effects of source of supplemental Zn on morbidity and serum Zn concentrations of heifers during receiving and finishing periods (Exp. 1)1
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Mean serum Zn concentrations of heifers measured on d 0, 14, and 28 during the receiving period of the current study are shown in Table 3. No day x treatment interactions (P = 0.60) were observed, and overall serum Zn concentration did not differ (P 0.87) among treatments. Similarly, Salyer et al. (2004) also reported no differences among Zn sources in serum Zn concentration of newly received heifers supplemented with 75 mg of Zn/kg of dietary DM.
Heifer performance data for the finishing period is shown in Table 4. The Con heifers had a greater (P = 0.03) initial BW than heifers in the other treatments, reflecting their greater BW at the end of the receiving period. At the end of the finishing trial (average of 168 d on feed), there were no differences (P 0.24) among treatments for final BW; however, Con heifers averaged less final BW than heifers in the other treatment groups. Daily DMI did not differ among treatments for the overall finishing period, but ADG tended (P = 0.11) to be least for Con heifers. Gain efficiency did not differ (P > 0.88) among treatments for the d 0 through 56 or for the d 0 through 112 periods; however, for the entire trial, Con heifers tended to have lower (P = 0.06) G:F than heifers in the other 3 treatment groups. The finishing period G:F data do not agree with results from the receiving period, in which Con heifers had increased G:F compared with the other 3 treatments. The reason for the contrasting results between these 2 periods is not clear. Because heifers in all treatment groups were fed a diet with no supplemental Zn for 42 d between the receiving and finishing periods, it was not possible to determine exactly when differences in G:F noted in the receiving period began to diminish. Nonetheless, Con heifers no longer had any advantage in G:F for the first 56 d of the finishing period (Table 4), suggesting that effects on G:F noted in the receiving period had already diminished by early in the finishing period. The days on feed in other trials that have included a no supplemental Zn treatment have typically ranged from 112 to 128 d; thus, the lower G:F for heifers fed the Con diet in the current study might reflect a greater length of the feeding period. Spears (1989) found that heifers supplemented with Zn methionine (addition of 25 mg of Zn/kg to a corn silage-based diet containing 24 mg of Zn/kg) gained faster and more efficiently than control heifers fed no supplemental Zn for the first 56 d of a 126-d study. Average daily gain and G:F of heifers supplemented with ZnO did not differ from control- or Zn methionine-supplemented heifers during the first 56 d of the study, but similar to our results, over the entire study, Zn supplementation, regardless of source, tended to increase ADG and increase G:F compared with the control treatment. Greene et al. (1988), however, reported no differences in ADG, DMI, or G:F in cattle receiving no supplemental Zn, Zn methionine, or ZnO. Similarly, Galyean et al. (1995) observed no differences for ADG or feed efficiency among steers fed low Zn methionine (35 mg of supplemental Zn/kg), high Zn sulfate (70 mg/kg), high Zn methionine (70 mg/kg) diets, and a control diet (supplemented with 30 mg/kg of Zn from ZnO). Dry matter intake was less by steers fed the control diet than by steers fed the other dietary treatments (Galyean et al., 1995). Nunnery (1998) reported that ADG by steers supplemented with Zn sulfate was 7.4% greater than those supplemented with Zn methionine. Likewise, Rust and Schlegel (1993) observed that steers supplemented with Zn tended to have greater ADG than steers not receiving supplemental Zn, but G:F and DMI did not differ among treatments. Gunter et al. (2001) supplemented grazing steers with 103 mg/d of Zn from Zn sulfate, Zn-amino acid complex, or Zn-polysaccharide during a 116-d period, after which the steers were shipped to a research feedlot where they were assigned to the same treatments they received during grazing. No differences were observed for grazing and feedlot performance of steers fed the different Zn sources. Spears and Kegley (2002) fed growing (silage base) and finishing (corn base) diets with no added Zn (Zn content of the control diet for the growing phase was 33 mg/kg, and the control finishing diet contained 26 mg Zn/kg) or 25 mg of supplemental Zn/kg from ZnO or 2 different types of Zn proteinate to Angus and Angus x Hereford steers. Supplemental Zn provided by the different sources increased ADG during the growing period. During the finishing period, ADG and G:F tended to be greater by steers fed Zn proteinate than by those fed ZnO, but ADG and G:F did not differ between the control and ZnO treatments during the finishing phase.
Serum Zn concentrations of heifers during the finishing period (d 0, 56, and end) are shown in Table 3. A treatment x day interaction was observed (P = 0.04). On d 56, Con heifers tended (P = 0.06) to have a lower serum Zn concentration than heifers in the other 3 treatment groups. Greene et al. (1988) measured serum Zn concentration of steers fed no supplemental Zn, Zn methionine, or ZnO, with samples obtained on d 1, 28, 56, 84, and 112 of the feeding period; serum Zn concentration did not differ among treatments on any of the sampling days. Spears (1989) measured serum Zn of heifers fed no supplemental Zn or supplemental Zn from ZnO or Zn methionine on d 42 and 126 of a 126-d study, and also reported no difference among treatments on either sampling date. Similarly, Malcolm-Callis et al. (2000) reported no differences in serum Zn concentrations measured on d 28 or 112 among steers supplemented with Zn sulfate, Zn polysaccharide, or a Zn amino acid complex, and Spears and Kegley (2002) reported no difference in serum Zn concentrations between steers supplemented with Zn proteinate or ZnO.
The effect of Zn source on carcass characteristics is shown in Table 5. With the exception of KPH, carcass characteristics were not affected by source of Zn. Heifers supplemented with Zn sulfate had a lower (P = 0.03) percentage of KPH than heifers supplemented with Zn methionine and Zn propionate, but reasons for this effect on KPH are not readily evident. The effects of Zn source on carcass characteristics have not been consistent. Several researchers have observed no effects on carcass traits as a result of source of supplemental Zn (Martin et al., 1987; Galyean et al., 1995; Nunnery, 1998). Spears and Kegley (2002) noted that steers fed Zn proteinate had heavier HCW and slightly greater dressing percents than those in control or ZnO-supplemented treatments, although other carcass traits were not affected by source of Zn in their study. Rust and Schlegel (1993) reported that carcass characteristics did not differ among Zn sources, but that supplemental Zn increased subcutaneous fat thickness and calculated yield grade and decreased LM area compared with steers not receiving supplemental Zn. In contrast to our results, Malcolm-Callis et al. (2000) reported that steers supplemented with Zn sulfate had greater KPH and less external fat than steers receiving Zn polysaccharide or a Zn amino acid complex, but no other carcass measurements collected in their study differed as a result of source of supplemental Zn. Greene et al. (1988) observed that steers fed Zn methionine had more subcutaneous fat than control steers receiving no supplemental Zn, with steers supplemented with ZnO being intermediate to the other 2 treatments. Furthermore, Zn methionine supplementation increased KPH compared with the ZnO and control treatments. Greene et al. (1988) also reported that steers supplemented with Zn methionine had more marbling than control or ZnO-supplemented steers, resulting in a greater percentage of steers supplemented with Zn methionine grading Choice compared with the other 2 treatments. Similarly, Rust (1985) reported a greater percentage of steers grading USDA Choice when supplemented with Zn methionine compared with no supplemental Zn. Why source or concentration of supplemental Zn might occasionally affect carcass fatness or marbling score remains unclear, but such effects do not seem to be repeatable.
Experiment 2
Source of Zn did not affect performance of the heifers injected with ovalbumin to assess humoral immune function. No differences (P > 0.10) were observed among treatments for initial (291 ± 13 kg) and final BW (314 ± 13 kg), ADG (1.13 ± 0.17 kg), DMI (6.86 ± 0.29 kg), or G:F (0.164 ± 0.6 kg) during the 21-d trial. These heifers were intentionally not subjected to any additional stressors besides the ovalbumin injection. Hence, only the humoral immune response should have been affected, which would seem unlikely to markedly affect performance variables. Spears et al. (1991) conducted a study in which steers were offered either no supplemental Zn or 25 mg of supplemental Zn/kg from ZnO or Zn methionine and stressed by vaccination for bovine herpes virus-1 (BHV-1 or IBRV) and parainfluenza-3 (PI3). Similar to our results, ADG by steers for the 28-d study did not differ among treatments; however, DMI tended to be less by steers in the control treatment than for those in the other 2 treatments during the 28-d trial. Feed efficiency did not differ for d 0 through 28 of the study, but for d 15 through 28, control steers tended to be more efficient than steers in the other treatments. In addition, Zn methionine-supplemented steers tended to be less efficient than ZnO-supplemented steers. Chirase et al. (1991) conducted 2 trials to evaluate the effect of Zn methionine in steers challenged with IBRV. Positive responses to supplemental Zn from Zn methionine (89 to 90 mg of Zn/kg) vs. a nonsupplemented control diet (31 to 35 mg of Zn/kg) were noted for DMI after the challenge, as well as for maintenance of pretrial BW.
Serum Zn concentrations of the heifers in Exp. 2 are shown in Figure 1. A day x treatment interaction (P = 0.04) was observed, but no differences were observed (P 0.21) on the various sampling days in serum Zn concentration among the 4 treatments. Salyer et al. (2004) used fasting-refeeding-fasting stress and ovalbumin injection to assess the humoral immune response of heifers supplemented with 75 mg of Zn/kg from either Zn sulfate or Zn polysaccharide. On d 0 and 14, heifers receiving supplemental Zn sulfate had greater serum Zn concentrations than those receiving Zn polysaccharide, but no differences between Zn treatments were observed on d 7 or 21. Spears et al. (1991) observed no differences in serum Zn concentration obtained on d 14 from steers receiving no supplemental Zn or 25 mg of Zn/kg from Zn methionine or ZnO. Chirase et al. (1991) obtained serum Zn samples on d 0, 4, and 11 after the IBRV challenge and found no differences in serum Zn between control- and Zn methionine-supplemented steers; however, serum Zn concentration decreased with time in both treatments.
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Figure 1. Serum Zn concentrations in heifers fed different sources of supplemental Zn after injections of ovalbumin on d 0 and 14 (Exp. 2). Con = control (no supplemental Zn); and Sul = Zn sulfate, Met = Zn methionine, and Prop = Zn propionate (75 mg of supplemental Zn/kg of DM from each source). Pooled SE were 0.16, 0.12, 0.12, and 0.11 for d 0, 7, 14, and 21, respectively.
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Titers for IgG specific to ovalbumin are shown across sampling days in Figure 2. There were no day x treatment interactions for ovalbumin titers (P = 0.98), and there were no differences among treatments (P 0.51), but data are shown by time to illustrate the increases in titer over the 21-d period. These results are in contrast to those of Salyer et al. (2004), who reported that heifers receiving an organically complexed source of supplemental Zn (Zn polysaccharide) had greater ovalbumin IgG titers on d 14 and 21 than heifers receiving supplemental Zn sulfate. Lack of a response to organically complexed Zn in the current study compared with the results of the Salyer et al. (2004) study could be the result of procedural differences. The fasting-refeeding-fasting stress used by Salyer et al. (2004) might have enhanced treatment differences in the humoral immune response, whereas the immune response of heifers in the current study was the result of injection with ovalbumin in the absence of other stressors. Similarly, Spears et al. (1991) measured antibody titers in newly received steers vaccinated against BHV-1 and PI3 and receiving no supplemental Zn, Zn methionine, or ZnO. Steers also were subjected to the additional stressors of transport, weaning, commingling, and restricted feed and water intake. On d 14, antibody titers to BHV-1 tended to be greater in steers supplemented with Zn, with the greatest response observed in steers fed Zn methionine. No differences were observed in PI3 titers among the 3 treatments, presumably as a result of previous exposure to the PI3 virus.
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Figure 2. Ovalbumin IgG titers in heifers fed different sources of supplemental Zn after ovalbumin injections (Exp. 2). Con = control (no supplemental Zn); and Sul = Zn sulfate, Met = Zn methionine, and Prop = Zn propionate (75 mg of supplemental Zn/kg of DM from each source). Pooled SE were 0.86, 0.85, and 0.83 for d 7, 14, and 21, respectively.
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Source of supplemental Zn did not significantly affect the health of newly received heifers or their ADG and DMI during a subsequent finishing period. Nonetheless, heifers that never received supplemental Zn tended to have lower G:F at the end of the finishing period than heifers in the supplemental Zn treatments. In terms of receiving period performance, our results are contradictory to the NRC (2000) recommendation that lightweight, stressed cattle should be supplemented with 75 mg of Zn/kg of dietary DM and suggest that the heifers used in our experiment likely had adequate body stores of Zn when the trial was initiated. Source of supplemental Zn also did not significantly affect the humoral immune response of heifers injected with ovalbumin, but this response might differ from what could happen when cattle are faced with an infectious disease challenge or other types of stress.
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Footnotes
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1 Supported, in part, by funds from Kemin Industries, Des Moines, IA 50306. The Jessie W. Thornton Chair in Animal Science Endowment at Texas Tech Univ. also provided funding to support this research. We thank Cactus Feeders, Ltd. (Amarillo, TX) for providing cattle used in some experiments, and DSM Nutritional Products (Belvidere, NJ), Elanco Animal Health (Greenfield, IN), Fort Dodge Animal Health (Overland Park, KS), Intervet (Millsboro, DE), and Schering-Plough Animal Health (Union, NJ) for supplying products used in the experiments. We also thank K. Robinson and R. Rocha for technical support and J. Horton (Hereford, TX) for assistance in designing and conducting the experiment. 
2 Current addresses: G. A. Nunnery (MIN-AD Inc., Amarillo, TX); G. B. Salyer (Nutrition Service Associates, Pratt, KS); C. H. Parsons (Cargill Inc., Canyon, TX); and P. J. Defoor (Cactus Feeders Ltd., Amarillo, TX).
3 Corresponding author: judson.vasconcelos{at}ttu.edu
Received for publication March 14, 2007.
Accepted for publication May 15, 2007.
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Abstract
INTRODUCTION
