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ANIMAL PRODUCTION |
* USDA-ARS, Forage-Animal Production Research Unit, Lexington, KY 40546;
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USDA-ARS, Dale Bumpers Small Farms Research Center, Booneville, AR 72927; and
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Animal Science Department, University of Tennessee, Knoxville, TN 37996–4574
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Abstract |
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 Top Abstract INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Key Words: beef cattle • Festuca arundinacea • forage • Neotyphodium coenophialum • tall fescue • toxicosis
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INTRODUCTION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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). An endophytic fungus (Neotyphodium coenophialum) infects tall fescue plants, inhabits greater than 90% of the tall fescue pastures (Sleper and West, 1996), and produces toxins that induce toxicosis (Sleper and West, 1996). This malady has symptoms that include retention of rough hair coat, elevated body temperature, labored respiration, and decreased serum prolactin and BW gain (Schmidt and Osborn, 1993).
Heat stress and poor BW gain from toxicosis are most severe at the onset of high ambient temperature and humidity during the spring and summer (Hemken et al., 1981). Body weight gains of steers have been improved during this period by feeding concentrates (Aiken et al., 1998) or by moving cattle to a warm-season grass pasture in the late spring (Aiken and Piper, 1999). Use of steroidal implants is another approach to improve calf performance on endophyte-infected (E+) tall fescue (Bransby et al., 1994). Coffey et al. (2001) reported that inplanting with trenbolone acetate and estradiol resulted in a 14% increase in ADG for steers grazing tall fescue. In an experiment with alfalfa diets, Dinius et al. (1978) reported that ADG increased 30% with a progesterone-estradiol implant. Rumsey and Hammond (1990) concluded that DMI must be 1.4 times the maintenance rate to achieve a 10% increase in BW gain by cattle implanted with estrogenic growth promoters. Thus, benefit from implanting pastured cattle is dependent on forage quality and availability.
Implantation of cattle grazing E+ tall fescue could enhance BW gain, but implantation with steroid hormones could affect health and stress levels from toxicosis. A grazing experiment was conducted to evaluate the interaction of stocking rate and implantation with BW gain on E+ tall fescue and to monitor heat stress related to fescue toxicosis after placement on a fescuefree diet.
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MATERIALS AND METHODS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Grazing Experiment
In each of 2 yr, 36 crossbred yearling steers (Bos taurus) were randomly allotted to eight 1-ha pastures of Kentucky-31 E+ tall fescue. Treatments were arranged as a 4 x 2 factorial. Stocking rate (3.0, 4.0, 5.0, and 6.0 steers/ha) and ear implantation (with or without Synovex-S; 200 mg of progesterone and 20 mg of estradiol benzoate; Fort Dodge Animal Health, Fort Dodge, IA) combinations were assigned to pastures in a completely randomized design without replication (Riewe, 1961; Bransby et al., 1988). Treatment combinations were rerandomized in 2004.
On the initial days of grazing (19 March 2003 and 24 March 2004), all steers were dewormed with ivermectin (Ivermec, Merial Limited, Duluth, GA) and implanted if assigned to the implant treatment. After 6 d of adjustment to pastures, steers were weighed unshrunk to obtain initial BW [288 ± 24 (SD) kg in 2003 and 270 ± 34 kg in 2004]. Unshrunk BW also was taken on d 56 and 84, and implanted cattle were reimplanted on d 56.
On the initial day of the adjustment period (d 6) and on d 84, blood was collected (approximately 10 mL) from the median caudal vein at the base of the tail. Within 5 h of collection, blood samples were centrifuged (3,000 x g for 30 min) to obtain serum, which was stored frozen (0° C). Serum was assayed for prolactin by RIA, following procedures described by Bernard et al. (1993). Intra-and interassay CV were 2.0 and 5.0%, respectively, for 2003 samples and 8.8 and 16.5%, respectively, for 2004 samples. Hair coat conditions were rated as sleek, rough, or transitional (i.e., some roughness of hair over the rump and shoulder regions).
Pastures were grazed continuously for 84 d; grazing was terminated on 17 June 2003 and on 23 June 2004. A mineral and vitamin supplement (Nutrena Feeds Division, Cargill Inc., Kansas City, KS) was offered ad libitum and contained Ca (minimum 12.0%, maximum 14.0), P (minimum 12.0%), Na (minimum 4.6%, maximum 5.9%), Mg (minimum 1.25%), K (minimum 1.25%), Cu (minimum 21 ppm), Se (minimum 18 ppm), Zn (minimum 300 ppm), vitamin A (minimum 440,000 IU/kg), vitamin D3 (99,000 IU/kg), and vitamin E (33 IU/kg).
Pastures were fertilized with N on 13 March 2003 at a rate of 67 kg/ha, with N, P, and K on 30 January 2004 at rates of 67, 29, and 56 kg/ha, respectively, and with N on 12 March 2004 at a rate of 67 kg/ha. Forage availability was monitored with a disk meter that was similar in design to one described by Bransby et al. (1977), with the exception that the falling plate was 45 cm in diameter and weighed 1.9 kg. Disk meter height was recorded for 50 random locations within each pasture at 14-d intervals by taking 50 disk meter readings per pasture. In the second year, when disk meter heights were collected, forage was clipped to ground level beneath the disk meter plate for 3 locations in each pasture. These samples were dried at 60° C in a forced-air oven for 48 h and weighed; DM per unit of land area was subsequently regressed on disk meter height.
Postgraze Monitoring
In the second year, after termination of grazing, the 36 steers were maintained as a single group on a pasture (approximately 0.4 ha) of bermudagrass and offered free-choice bermudagrass hay and water. Rectal temperatures were determined, and blood samples were collected at 0800 on trial d 84, 85, 86, 87, 90, 92, and 94, which represent 24 (d 85), 48, 72, 144, 192, and 240 h after placement on nontoxic forage. Blood samples were processed and assayed for prolactin, as described earlier.
Heat indices for maximum and minimum temperatures and associated relative humidities were calculated for 22 June (d 0) through 2 July (d 10) with an equation described by Tarazon-Herrera et al. (1999). Weather data was collected from a weather station that was within 1 km of the experimental site.
Statistical Analyses
Average daily gain, forage availability, and serum prolactin were analyzed using Proc Mixed of SAS (SAS Inst. Inc., Cary, NC) to assess effects of stocking rate, year, implantation, and interactions among the 3 experimental variables. Pasture was used as the experimental unit in the analyses. Implantation and year were evaluated as dummy variables, and stocking rate was analyzed as a regression variable, with deviations from the regression used as the error term (Riewe, 1961; Bransby et al., 1988). Quadratic equations for the relationship between BW gain per hectare and stocking rate were derived from calculated linear equations for the ADG stocking rate relationships (Jones and Sandland, 1974; Bransby et al., 1988; Aiken and Bransby, 1992b) for implanted and nonimplanted steers. Estimated BW gains per day, determined from the quadratic equations, were multiplied by the number of days that the pastures were grazed to estimate total BW gain over the experimental period.
Distributions of hair coat ratings were analyzed for year and implantation effects using Chi-Square tests with Proc Freq. Steer numbers per stocking rate were too low to provide reliable analysis of a stocking rate effect.
Models for analyzing rectal temperature and serum prolactin during postgraze recovery evaluated implantation as a discrete variable, stocking rate and hours on fescue-free diet (HFFD) as continuous variables, and interactions of all combinations of the discrete and continuous variables (linear, quadratic, and cubic regression coefficients). Although steers were kept as a single group during the postgraze phase, pasture was used as the experimental unit to evaluate carryover effects of stocking rate and implantation. The HFFD variable was evaluated as a repeated measure using the spatial power covariance structure of Proc Mixed. Insignificant (P >0.05) variables were removed from the models using backwards-stepwise regression.
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RESULTS AND DISCUSSION |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Pastures used in the experiment were reported by Aiken et al. (1998) to have greater than 75% infection levels. Therefore, infection levels for the present experiment were either similar or greater (Shelby and Dalrymple, 1993).
Forage availability declined linearly (P <0.001) as stocking rate increased, and the regression intercept was greater (P <0.001) in 2004 than in 2003 (Figure 1). Implantation did not affect (P = 0.49) forage availability and did not interact (P >0.10) with stocking rate or year. Greater forage growth in 2004 compared with 2003 was likely because of greater rainfall observed in that year and the additional application of N fertilizer.
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There was no year effect (P = 0.94) on ADG; however, stocking rate interacted (P <0.05) with implantation in affecting BW gain (Figure 2). Average daily gain of implanted steers declined (P <0.01) as stocking rate increased; the slope for the nonimplanted steers did not differ from 0 (P >0.10). Aiken and Piper (1999) reported a similar weak ADG response to stocking rate for nonimplanted steers grazing E+ tall fescue with the same stocking rates as in the present experiment. However, declines in available forage with increases in stocking rate beyond those in the experiment would likely reduce ADG.
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reported a strong synergistic effect on ADG of steers with the combined use of steroidal implants and invermectin dewormer. Average daily gain at 2 locations with highly infected tall fescue increased 13% with steroidal implants and no dewormer, 21% with dewormer and no implantation, and 76% with both in combination.
Average daily gain without implantation was greater compared with previous grazing experiments with E+ fescue. Paterson et al. (1995) reported a range in ADG of 0.20 to 0.62 kg/d for experiments conducted with steers grazing highly infected tall fescue. Milder temperatures recorded in the late months of both grazing seasons may have improved ADG in the present experiment. The 2 regression lines intersected at 5.3 steer/ ha, indicating that implantation had no effect on ADG with the heaviest grazing intensity. Positive response to implantation with lower stocking rates suggests that implantation increases forage consumption if forage availability is not limited. Although an implantation effect on forage availability was not detected, methodologies used to estimate forage availability generally lack the sensitivity to detect small differences in availability among treatments presumably affected by DM intake (Aiken and Bransby, 1992a).
Total BW gain per hectare for implanted steers showed a quadratic trend as stocking rate increased (Figure 3). Although a quadratic relationship was derived for nonimplanted steers, lack of an ADG response to stocking rate resulted in a linear trend of total BW gain per hectare as stocking rate increased. Trends in BW gain per hectare indicated that the 6.0 steer/ha rate, with or without implantation, provided approximately a 75% increase in BW gain per hectare over the 3.0 steer/ha rate.
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At the conclusion of grazing there was a difference (P <0.05) in hair coat ratings between years. In 2003, more steers had transitional (58%) hair coats, whereas 66% of steers had rough hair coats in 2004. Transitional hair coats are associated with shedding of winter hair coats or by genetics (Olson et al., 2003). Greater incidence of transitional hair coats in 2003 suggests these steers were less affected by fescue toxicosis than those in 2004; however, this did not correspond with greater steer ADG in 2003 than in 2004. Hair coat distributions did not differ (P >0.10) between implanted and nonimplanted steers, which indicated the steroid hormones did not influence shedding of winter hair coats. This supports results of Coffey et al. (2001), who concluded with steers grazing high and low E+ tall fescue that implantation with trenbolone acetate had no effect on the shedding of winter hair coats.
Postgraze Monitoring
Rectal Temperature.
Mean rectal temperature at termination of grazing was 40.6° C for implanted steers and 40.2° C for nonimplanted steers. There was no carryover effect of stocking rate (P = 0.16), but the regression intercept was greater (P <0.05) for implanted than for nonimplanted steers, and magnitude of the differences did not change (P = 0.87) during the monitoring phase (Figure 4). Rectal temperatures showed a quadratic decrease (P <0.001) as HFFD increased. Rectal temperature increased during the first 24 HFFD, stabilized between 24 and 144 HFFD, and decreased between 144 and 240 HFFD.
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). There was a sudden drop in the indices with maximum ambient temperature and humidity for d 8 when maximum air temperature declined from the previous day (29.4 to 25.1° C). This corresponded with a decline in rectal temperature between 144 and 192 HFFD; however, rectal temperature was recorded early at 0800 when ambient temperature was closer to the daily minimum than the maximum. Furthermore, rectal temperature decreased between 192 and 240 HFFD, which occurred during a period when the indices increased relative to those measured on d 8 (144 HFFD).
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as indicative of a healthy bovine. Ambient temperature affects rectal temperature (Cole, 1993), and the last measures could have been partially influenced by the warmer temperatures during the monitoring phase of this experiment. Final rectal temperatures, however, were considerably lower than those at termination of grazing and had declined to temperatures that indicate that severe heat stress was alleviated. Aiken et al. (2001) reported rectal temperatures of 39.9° C at 10 h after removal from E+ fescue pastures to decline linearly to less than 39.2° C at 82 h, which is less than the 144 to 196 h needed to reduce heat stress in the present experiment.
Research is lacking on effects of estrogen and progesterone on thermoregulation in cattle; however, cattle implanted with progesterone-estradiol benzoate growth promotants have shown slight increases in plasma thyroxine (T4) and reductions in plasma triiodothyronine (T3), resulting in an increase in plasma T4:T3 ratio (Kahl et al., 1978; Rumsey et al., 1992). Rumsey et al. (1997) reported an increase in T4 and thyroid-stimulating hormone in cattle implanted with progesterone-estradiol growth promoter. Al-Haidary et al. (2001) reported T4 to decrease in nonimplanted heifers when they were moved from a thermoneutral environment (21° C) to a heat challenge (31° C). Elevated rectal temperature for implanted steers in the present experiment could have been due to greater basal metabolism stimulated by a shift in thyroid status mediated by the estrogenic growth promoter.
Serum Prolactin.
Baseline measures of serum prolactin at initiation of grazing averaged 138.5 ± 24.0 ng/ mL in 2003 and 146.6 ± 23.5 ng/mL in 2004, and at termination of grazing it was 20.2 ± 3.4 ng/mL in 2003 and 28.9 ± 6.0 ng/mL in 2004. Serum prolactin has not been directly related to the incidence of toxicosis (Strickland, et al., 1993) but is typically used as a marker of toxicosis (Sleper and West, 1996). Prolactin production in the anterior pituitary is inhibited by dopamine (MacLeod and Lehmeyer, 1974; Ben-Jonathan and Hnasko, 2001). Sibley and Creese (1983) reported that ergopeptines serve as dopaminergic agonists with strong affinity with D2 dopamine receptors of the anterior pituitary. Therefore, prolactin serves as an indicator of dysfunction in the endocrine system related to circulating ergopeptines.
There was a curvilinear increase in serum prolactin as HFFD increased (Figure 6), and the cubic (P <0.0001) trend was similar between implanted and nonimplanted steers. General trend was for a substantial increase in prolactin between 0 and 72 HFFD, followed by slight increases between 72 and 192 HFFD and a more substantial increase between 192 and 244 HFFD. Therefore, serum prolactin did not reach a stabilized concentration by 244 HFFD. Stuedemann et al. (1998) reported for steers switched from E+ to endophytenon-infected tall fescue pasture that urinary alkaloid concentrations declined 67% within 24 h. After being switched from endophyte-noninfected to E+ tall fescue, these steers had urinary alkaloid concentrations after 24 h that were similar to those continually grazed on E+ tall fescue. Results of the present experiment support a rapid excretion of alkaloids; however, gradual increases in serum prolactin after 72 HFFD indicate that small concentrations of alkaloids remain in circulation.
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); however, it is possible that greater consumption with greater stocking rates resulted in greater intake of alkaloids relative to low stocking rates. For nonimplanted steers, greater serum prolactin concentrations with low stocking rates were likely caused by the negative influence that the mature, stemmy available forage had on DMI and, hence, ergot alkaloids. |
IMPLICATIONS |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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Footnotes |
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2 Corresponding author: geaiken{at}ars.usda.gov
Received for publication December 21, 2005. Accepted for publication January 18, 2006.
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LITERATURE CITED |
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TopAbstractINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONSLITERATURE CITED |
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