J. Anim Sci. 2006. 84:2406-2409. doi:10.2527/jas.2005-761
© 2006 American Society of Animal Science
Chlortetracycline supplementation of yearling dairy heifers1,2
E. D. Reid3,
P. S. Erickson4,
S. Hodgdon,
E. Lennon and
P. C. W. Tsang
Department of Animal and Nutritional Sciences, University of New Hampshire, Durham 03824
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Abstract
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Chlortetracycline is an antibiotic that is used to increase weight gain, efficiency of gain, carcass grade, and conception rates. The objective of this experiment was to evaluate the effects of supplementation of 350 mg/d of chlortetracycline on ADG, G:F, BCS, thyroxine, and systemic glucose concentrations in yearling dairy heifers. Forty 12-mo-old Holstein heifers (initial BW = 363 ± 21 kg) were housed in a free-stall barn with ad libitum access to feed and water for 104 d. A transition period was begun 14 d before the age of 12 mo to acclimate the heifers to the diet. The chlortetracycline-fed group (n = 20) consumed 328 ± 8.2 mg of chlortetracycline/heifer daily. Measurements for BW, withers and hip heights, BCS, and health score were recorded weekly. Dry matter intake was measured daily. Blood was sampled every 4 d to determine plasma thyroxine and glucose concentrations and every 2 d to determine progesterone concentrations. Heifers were artificially inseminated on the first observed standing heat after 13 mo of age. There were no effects of chlortetracycline on ADG, G:F, withers and hip heights, BCS, blood glucose concentrations, peak progesterone concentrations, health, or conception rate. There was an interaction between treatment and time for chlortetracycline on serum thyroxine concentration. In the beginning of the experiment, serum thyroxine concentration was lower in heifers supplemented with chlortetracycline. There was no difference between treatments in thyroxine concentration at the end of the experiment. Chlortetracycline supplementation was not beneficial for yearling dairy heifers.
Key Words: dairy heifer • chlortetracycline • thyroxine • growth
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INTRODUCTION
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Chlortetracycline (CTC) is a broad-spectrum antibiotic that is commonly fed to a variety of livestock to control disease and improve growth and G:F. However, the mode of action of CTC has not been determined. A hypothesis is that CTC reduces intestinal mass (Visek, 1978
) and decreases intestinal turnover (Johnson, 1987), which increases ME supply and the availability of essential AA for growth (Baldwin et al., 2000). Baldwin et al. (2000) observed decreased mass in tissue components of the intestinal tract when CTC was fed, and villus heights tended to be greater in the jejunum of CTC-supplemented steers.
A reduction in GH and thyroid stimulating hormone (TSH) release due to a reduction in pituitary gland sensitivity after a challenge by their respective releasing hormones was observed in steers supplemented with CTC (Rumsey et al., 1999). The authors suggested that this could be a viable mode of action for CTC affecting tissue deposition.
An increase in reproductive efficiency has been observed in cattle supplemented with CTC in various experiments (Rae et al., 1993; Saltman et al., 1998; Rae et al., 2002). The increase in conception rates could not be directly linked to a reduction of vaginal bacterial infections. Rae et al. (2002) observed an increase in conception rate when heifers were supplemented with CTC before and during the breeding period compared with controls, a reduction in conception rate in animals supplemented with CTC only during breeding compared with controls, and no difference in conception rate when animals were fed CTC only before breeding.
Improving G:F, growth, and conception rates are all important goals of the dairy industry. The objective of our study was to determine if CTC fed at maximum amounts to dairy heifers (350 mg/d) improves growth rates and alters concentrations of circulating metabolic and reproductive hormones in yearling dairy heifers.
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MATERIALS AND METHODS
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Animal Care
The protocol for this research was approved by the University of New Hampshire Institutional Animal Care and Use Committee.
Experimental Protocol
Forty 12-mo-old Holstein heifers (initial BW = 363 ± 21 kg) were housed in a naturally ventilated free-stall barn with mattress-covered stalls that were lightly covered with kiln-dried sawdust. Water was provided ad libitum at a stocking rate of less than 16 heifers per fountain. Heifers were assigned randomly to 1 of 2 treatment groups and were blocked in pairs according to date of birth, with treatments fed for 90 d. A 14-mo period was necessary to acquire 40 heifers for the experiment.
Heifers were fed the experimental diet and trained to use Calan feeding doors (American Calan, Northwood, NH) for 14 d before the treatment period. All feed ingredients were sampled before the beginning of the experiment and analyzed for CP (AOAC, 2000), NDF (using sodium sulfite and heat-stable amylase; Van Soest et al., 1991), and ADF using an Ankom-Fiber Analyzer (Ankom Technology, Macedon, NY). Calcium and P were analyzed using a Thermo Jarrell Ash IRIS Advantage Inductively Coupled Plasma Radial Spectrometer (Thermoelectron Corp., Waltham, MA). All feed analyses were conducted at Dairy One, DHI Forage Testing Laboratory, Ithaca, NY.
A diet meeting NRC (2001) requirements for 0.80 kg/d of ADG (diet 1, Table 1) was fed for the 104-d (14-d adaptation, 90-d treatment) period (8 heifers per treatment). Because of a change in grain mix composition and a management concern for abnormally rapid weight gain, a second diet (diet 2, Table 1) was fed for 104 d (14-d adaptation, 90-d treatment; 12 heifers per treatment) to maintain an ADG of 0.78 kg/d (Table 1). Diet 1 was fed for the first 7 mo of the experiment.
Treatments were 10 g of corn meal carrier or carrier plus CTC (350 mg of CTC/d; Aureomycin 90, Alpharma Inc., Fort Lee, NJ). Heifers were fed at 0900 daily, and treatments were top-dressed at feeding. The diet was fed to obtain 10% orts, and feed amounts were adjusted weekly. Weekly orts samples were stored (–20°C) for later analysis. Frozen orts samples were thawed, dried to approximately 90% DM in a forced-air oven at 60°C for 24 h, ground through a 1-mm screen in a Wiley mill (Thomas-Wiley, Swedesboro, NJ), and combined to make monthly composite samples, which were analyzed for residual CTC (Alpharma Inc.).
Withers and hip heights (measured with a sliding-scale height stick with a bubble level), BW, health scores (scored subjectively by primary author; based on a 5-point scale using coughing, diarrhea, coat condition, and eyeball recession into the orbit as indicators of poor health; 1 = poor health and 5 = excellent health), and BCS (Edmonson et al., 1989; scored subjectively by primary author and an evaluator blind to treatment) were recorded at the beginning of the treatment period, weekly during the treatment period, and at the end of the treatment period.
Heifers were bred by AI approximately 12 h after observed standing heat, on the first heat after 13 mo of age (30 d of treatment) by 1 of 4 inseminators (averaged 52.5% first breeding conception for the duration of the study). Heifers were bred at this time for an expected calving age of 22 to 24 mo. Inseminator did not affect conception (P = 0.78). Pregnancy was determined by ultrasonography by the attending veterinarian at least 30 d after breeding. Days of age at first breeding, first breeding conception rate, and conception rate while on study were determined.
Blood samples from all heifers were collected every 4 d at 1500 (6 h after feeding) by coccygeal venipuncture into Vacutainer tubes (Becton Dickinson, Franklin Lakes, NJ). Serum samples were aspirated after centrifugation (3,000 x g at 20° C) and frozen (–20° C) for later analysis. Serum glucose concentrations were measured in duplicate using the Wako Glucose C2 Kit (Wako Chemical USA Inc., Richmond, VA). The procedure was modified by using 0.03 mL instead of 0.02 mL of serum and samples, which were incubated for 10 min instead of 5 min. Serum thyroxine (T4) was measured using a commercially available coated-tube RIA developed for use in human serum (Diagnostic Systems Laboratories Inc., Webster, TX). The assay was validated for use in bovine serum. When T4 was added to samples of bovine serum, T4 measured in the assay averaged 95.3% of the expected value. The assay called for evaluation of 25 µL of serum. When additional volumes as small as 10 µL were assayed and corrected for dilution, the concentration estimated in the assay averaged 103.6% of the expected concentration. The assay was sensitive to 7.89 ng/mL. The intra-and interassay CV averaged 3.03 and 3.95%, respectively.
Vacutainer tubes coated with EDTA (Becton Dickinson) were used to collect blood samples by coccygeal venipuncture for analysis of plasma progesterone (P4). Blood samples were collected every other day, centrifuged (3,000 x g at 4° C), aspirated, and frozen (–20° C) for later analysis. Plasma samples were analyzed for P4 concentrations by RIA (Goldberg et al., 1996).
Statistical Analyses
The experimental design was a randomized complete block. Statistical analysis was performed using the MIXED procedure of SAS (version 8.2, SAS Inst. Inc., Cary, NC). Blood hormone concentrations and weekly measurements were analyzed using the repeated measures function. Covariates for models were determined by best-fit statistics. Initial DMI (final 7 d before treatment application) was used as a covariate for analysis of DMI. Beginning BCS was used as a covariate for BCS. Beginning BW was used as a covariate for ADG, G:F, age at first breeding, and conception rate while on study. All data are reported as least squares means ± SEM. Significance was at the 0.05 level, and trends were designated at the 0.10 level.
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RESULTS
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Nutrient analyses of the 2 diets are presented in Table 1
. There was no effect of supplemental CTC on DMI (Table 2), ADG, G:F, BCS, or health score (Table 2). There was an effect of week for BCS (P < 0.001); control and CTC-fed heifers gained body condition over the 90-d treatment period (3.79 at beginning vs. 3.90 at finish). Heifers supplemented with 350 mg/d of CTC consumed 328 (SD ± 8.2) mg/d of CTC. Efficiency of gain averaged 0.11 (Table 2).
Supplementing dairy heifers with CTC did not alter mean blood glucose concentrations (Table 3). Heifers supplemented with CTC had circulating T4 concentrations similar to control heifers (P = 0.13, Table 3). There was a CTC x time interaction (P = 0.03) for circulating T4 concentrations (Figure 1). Circulating T4 concentrations tended to be lower on d 24 (P < 0.10) and were lower on d 32 and 44 (P < 0.05) of treatment for heifers supplemented with CTC compared with controls (Figure 1).
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Figure 1. Thyroxine concentrations for control and chlortetracycline (CTC) -supplemented heifers during the 90-d treatment period. Samples were collected every 4 d during the study. Treated heifers were supplemented with 350 mg/d of CTC. *P < 0.10; **P < 0.05.
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Although we did not expect to observe reproductive differences with the number of heifers used, reproductive data were collected. Age at first breeding averaged 427 d (428 d for CTC-fed heifers vs. 425 d for controls, P = 0.8). First breeding conception rate for CTC-supplemented heifers was 45% compared with 60% for control heifers. Conception rates for treated and control heifers were similar over the duration of the study; both groups had 12 of 20 (60%) heifers conceive. The length of the estrous cycle did not differ between heifers supplemented with CTC and controls, 20.3 d for each (P = 0.98). There was no difference in peak circulating P4 concentration between treatments (10.8 ng/mL in heifers supplemented with CTC and 9.7 ng/mL in control heifers, P = 0.65).
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DISCUSSION
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These findings are similar to the results of Rumsey et al. (2000) for DMI, ADG, and G:F, but these data are in contrast to the observations of Perry et al. (1986), wherein CTC increased ADG and G:F.
It has been shown that DMI, ADG, G:F, and serum IGF-I concentrations increased in pigs when CTC was supplemented in their diet, suggesting that CTC stimulates IGF-I, which has been shown to increase growth in many species (Hathaway et al., 1996). Rumsey et al. (1999) reported that GH and TSH were lowered when CTC-fed steers were challenged with GH-releasing hormone and thyrotropin-releasing hormone. This led them to conclude that attenuation of GH, TSH, and T4 in CTC-supplemented steers was a viable explanation for increased tissue deposition. In a companion study, these researchers observed increased longissimus fat cover in response to CTC, which is consistent with reduced thyroid status (Rumsey et al., 2000). Our study did not show a significant difference in mean sera T4 concentrations, supporting the lack of a difference in growth. However, for a period of time (20 to 44 d), circulating T4 concentrations were lower in animals supplemented with CTC compared with controls. The reduction in T4 concentration could affect efficiency, but the reason for the return to concentrations similar to those of controls after 44 d in our experiment remains unknown. In addition, there were no significant differences in serum glucose concentrations, which suggests that CTC had little effect on metabolic state with regard to the insulin axis.
Although feeding low levels of CTC (125 mg/d) 30 d before the breeding period increased conception rate by nearly 15% compared with animals receiving no CTC (Rae et al., 1993), data from our study suggest that CTC has little effect on conception rate. In addition, the current study did not show a difference in peak P4 concentrations or length of the estrous cycle. Rae et al. (2002) also noted an increase in conception rate in heifers supplemented with CTC before and during the breeding period compared with heifers receiving CTC only before the breeding period. In comparison to the study of Rae et al. (2002), we used AI instead of bulls, Holstein heifers instead of Brahman-cross heifers, as well as free-stall housing, a greater amount of CTC (328 vs. 125 mg/d), and ad libitum feed vs. grazing conditions. There seems to be a management or breed factor involved in the response to CTC in reproductive programs. Also, we might not have used enough animals to adequately assess the variables.
The results of the current study suggest that CTC supplementation is not beneficial to dairy heifers. In part, this may be attributed to the fact that we used an intensely managed group of animals with ad libitum access to feed and water as well as low competition for shelter and bedding.
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Footnotes
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1 This is Scientific Contribution Number 2264 from the New Hampshire Agricultural Experiment Station. 
2 Support for this project was through a grant from Alpharma, Ft. Lee, NJ.
3 Current address: Dep. Anim. Sci., Univ. of Illinois, Urbana 61801.
4 Corresponding author: peter.erickson{at}unh.edu
Received for publication December 23, 2005.
Accepted for publication April 24, 2006.
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Abstract
INTRODUCTION
