J. Anim. Sci. 2004. 82:2773-2779
© 2004 American Society of Animal Science
Morbidity effects on productivity and profitability of stocker cattle grazing in the Southern Plains
W. E. Pinchak*,1,
D. R. Tolleson*,
M. McCloy*,
L. J. Hunt*,
R. J. Gill
,
R. J. Ansley* and
S. J. Bevers
* Texas Agricultural Experiment Station and
and
Texas Cooperative Extension, Texas Agricultural Research and Extension Center, Vernon 76384
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Abstract
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Effects of bovine respiratory disease (BRD) on stocker cattle systems are unknown under extensive rangeland environments. Three experiments were conducted to test the hypothesis that BRD-based morbidity is a major factor affecting the productivity and profitability of stocker cattle grazing Southern Plains rangelands. In Exp. 1 (658 male calves; average BW = 231 kg), 17% of the cattle were treated for BRD <8 d, 6% for 8 to 14 d, and 8% for >14 d. Morbid cattle had lower ADG than did healthy cattle (P < 0.10). Cattle requiring >14 d of pharmaceutical therapy gained less than cattle having <14 d therapy (P < 0.01). In Exp. 2, (279 steers and bulls; average BW = 216 kg), the ADG by steers (0.74 kg•animal–1•d–1) was greater (P < 0.05) than by bulls castrated after arrival (0.64 kg•animal–1•d–1). Castration after arrival led to a 13.5% loss in daily gain and a 10.3% loss in season-long gain. More (P < 0.05) bulls castrated after arrival (60%) were morbid compared with steers (28%). In Exp. 3, 633 heifers (average BW = 251 kg) were used to test the effects of morbidity on weight gain and reproduction. Heifers with lower initial weights exhibited increased (P < 0.05) morbidity. Heifers requiring two or more antibiotic treatments gained 0.03 kg/d less (P < 0.10) than did healthy heifers and had lower (P < 0.05) conception rates (66 vs. 81%). Conception rate in twice-treated heifers was 19% less than healthy heifers. Morbid heifers conceived 0.6 mo later (P < 0.05) than healthy heifers. Under the conditions of Exp. 1 and Exp. 2, morbidity decreased net returns 9.7 to 21.3% per animal. Adjusted gross returns per animal in Exp. 3 for replacement heifers were 3 to 7.8% less for morbid heifers.
Key Words: Morbidity • Production • Profitability • Shipping Stress
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Introduction
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Shipping stress, primarily bovine respiratory disease (BRD), and associated morbidity are leading nonfeed costs in feeder cattle production. A variety of stressors, including weaning, commingling, and shipment increase BRD morbidity in calves and yearlings (Loerch and Fluharty 1999
). Bovine respiratory disease impacts feed efficiency, veterinary costs, and death loss (Galyean et al., 1999).
Over 10 million stocker cattle graze wheat, introduced pastures, and native range in the Southern Plains. There are limited data on the potential effect of BRD and morbidity on the productivity and profitability of stocker cattle production systems. Research has focused primarily on transit stress-morbidity relationships and associated nutritional or pharmacological mitigation of BRD in short-term (<60 d) receiving trials in feedlots (Cole et al., 1988; Galyean et al., 1999; Loerch and Fluharty, 1999).
Our objective was to quantify the effects of BRD-induced morbidity on the productivity and profitability of stocker cattle grazing native rangeland season-long in the western Rolling Plains of Texas. Three experiments were designed to determine the effect of morbidity, castration, and grazing environment on weight gain, performance, and cost in stocker cattle. Stocker cattle types included were steers and bulls and replacement heifers. This approach facilitated quantifying the effects of BRD-related morbidity alone and with castration in males and morbidity alone in stocker replacement heifers under extensive grazing conditions.
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Materials and Methods
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Research was conducted on the 13,759-ha Y Experimental Ranch (YER), in Cottle, Foard, King, and Knox Counties of Texas from 1992 to 1994. Native range was dominated by a woody overstory of mixed mesquite (Prosopis glandulosa) and red-berry juniper (Juniperus pinchotti). Understory forages included tobosa grass (Hilaria mutica), sideoats grama (Bouteloua curtipendula), buffalo grass (Buchloe dactyloides), Texas wintergrass (Nasella leucotrica), and a mix of warm-season mid- and short grasses. Mean annual precipitation is 559 mm. Precipitation pattern is bimodal with May and September peaks. Precipitation was above average in 1992 (+26%) and 1993 (+17%). Below-average (–15%) precipitation during the 1994 grazing period resulted in greater grazing pressure in 1994 than in the other years.
Cattle were from mixed genetic origins in all years, although phenotypically over 80% were less than 25% Bos indicus. Cattle were managed under approved Texas A&M University Animal Care and Use Guidelines. Cattle were individually weighed without fill adjustment and processed within 24 to 36 h of arrival to promote reestablishment of normal feeding and watering patterns. No fill adjustment was made on final weights because there was insufficient drylot capacity at the YER. All cattle were marketed with a fixed 2% shrink of final weights, per ranch sale agreements. All animal weight data include the fixed 2% shrink.
Experiment 1
Our objective was to determine the effect of BRD morbidity duration on the productivity and profitability of stocker cattle grazing native range. In 1992, 468 male calves (average BW 250 kg) from Florida and Mississippi were used. Animals were mixed bulls and steers with 34% of steers exhibiting evidence of recently being castrated before arrival. Animals were processed within 24 to 36 h of arrival. Processing required individual animal restraint in a squeeze chute and included vaccination with a modified live IBR, BVD, BRSV and PI3, a 7-way clostridial and for Pasturella haemolytica. Cattle were revaccinated with the same vaccines 14 d later. Cattle were also ear implanted with 36 mg of zeranol (Ralgro, Schering Plough Anim. Health, Union, NJ), number ear tagged, and individually weighed. Bulls were castrated via emasculators. Cattle were dehorned with commercial dehorners. Morbidity was diagnosed by an experienced manager based on visual appearance and appetite. Morbidity was subsequently verified by rectal temperature (
40°C). Morbid cattle were treated with a process of antibiotic therapy prescribed by a consulting veterinarian. Therapies included Ceftiofur, Tilimicosin, and Gentamicin at label dosages and via appropriate routes of administration. Health status of each animal was monitored and recorded throughout the preconditioning period. Animals were classified as healthy, morbid for 1 to 7 d, morbid 8 to 14 d, or morbid >14 d. Morbidity classification assigned to each animal was based on morbidity during the preconditioning period. Cattle were preconditioned for 28 d after arrival. Preconditioning included 5 d of feeding moderate-quality grass hay ad libitum (9% CP and 52% IVDOM, DM basis) plus 0.68 kg•animal–1•d–1 (as-fed basis) of a commercial vitamin E-fortified, 32% CP pelleted supplement (Table 1). Hay was fed in round bale feeders. Pelleted supplement was fed in portable feed troughs that provided 25 cm of trough space per animal. Cattle were maintained in four pens (40 m x 50 m) and two lots (100 m x 100 m) for the first 5 d. Water was provided free choice through two 1.8-m x 0.61-m water tanks per pen or lot. Cattle were then moved over 23 d to increasingly larger paddocks (15 to 100 ha) of native forages and fed 0.68 to 0.91 kg of the 32% CP supplement•animal–1•d–1 (as-fed basis) for the remainder of the preconditioning period. Cattle were then commingled and grazed in common on 371- to 1,173-ha native range pastures from April 1992 to August 1992. Cattle had ad libitum access to water from surface impoundments and water troughs on a piped water system. A commercial 12% Ca:12% P loose mineral with 114 g(animal–1•d–1 targeted consumption was provided ad libitum in all-weather mineral feeders. Across all pastures, season-long daily consumption of mineral supplement averaged 129 g•animal–1•d–1 (as-fed basis). Consumption was determined by dividing mineral supplement disappearance in each pasture by the number of animal days per pasture. Stocking rates ranged from 2.4 to 4.1 ha per animal. Precipitation was above average during the grazing period, and available forage (>1,000 kg/ha based on ocular estimates) was sufficient to not limit intake in any pasture. Steers were individually weighed in early July to market heavier (>318 kg) cattle, and in late August at the planned termination of grazing. From July through August, cattle were fed the daily equivalent of 0.45 kg per animal of the 32% CP supplement twice a week.
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Table 1. Nutrient composition (as-fed basis) of 32% crude protein supplement fed to stocker cattle in all experiments
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Experiment 2
Based on the results from 1992, we designed another experiment to specifically characterize the effect of castration on morbidity and performance of mixed steer/bull loads from central, north, and west Texas. All cattle were obtained from auctions before delivery to YER. In December 1992, 143 steers, and 136 bulls (average BW 216 kg) were delivered in five different loads and allowed to eat, drink, and rest before being processed within 24 h. of arrival. Vaccination, processing, and antibiotic therapy protocols were the same as in Exp. 1, except castration was recorded for each bull calf and cattle were preconditioned an average of 35 d. Preconditioning included 7 d of ad libitum feeding of peanut hay (9.7% CP, 58% IVOMD; DM basis) plus 1.14 kg•animal–1•d–1 (as-fed basis) of the vitamin E-fortified 32% CP pelleted supplement used in Exp.1. Cattle were moved over 28 d to increasingly larger paddocks comprised of native forages and fed 1.36 to 1.81 kg•animal–1•d–1 (as-fed basis) of the 32% CP supplement. Supplementation levels during preconditioning were increased over that fed in Exp. 1 because cattle were grazing winter dormant native forages during the preconditioning phase. Cattle were commingled and grazed on native range pastures similar to those in Exp. 1 from January 1992 to July 1993. However, one commingled herd grazed a 379-ha pasture that had been prescribed burned on March 8, 1993. Stocking rates were similar to those in 1992, except that the burned pasture was stocked at 2.37 ha per animal. Steers were individually weighed and marketed in July or August 1993 similar to Exp. 1. The mineral program was the same as in Exp. 1, and season-long consumption of the mineral supplement was 141 g•animal–1•d–1 (as-fed basis).
Experiment 3
Heifers (633 heifers, average BW 251 kg) were purchased at auctions and at ranches in 1994. Heifers arrived from November to December in seven loads. Heifers were processed, preconditioned, and treated for respiratory infection as in Exp. 2, except they were not implanted with zeranol and were given ad libitum access to a sorghum-sudan hay (8.6% CP and 54% IVOMD; DM basis) during the preconditioning period. Cattle were moved to increasingly larger paddocks comprised of native forages and fed 1.36 to 1.81 kg(animal–1•d–1 of the 32% CP supplement. Heifers were commingled and grazed on native range from December 1993 to August 1994. One 743-ha pasture was prescribed burned on March 4, 1994. Stocking rates ranged from 2.01 to 3.36 ha/heifer. Heifers were exposed to bulls from April through August (150-d breeding season) at a 1 bull:20 heifer ratio. Heifers were individually weighed and rectally palpated for pregnancy and months of pregnancy in August 1994.
Economic Measurements
In Exp. 1, cost per kilogram of gain was used as a measure of additional cost per unit of gain attributable to actual pharmaceutical therapy costs for an average animal in a treatment group. These values did not include costs for increased labor, feeding, water, equipment, or interest. In Exp. 2, individual animal records were kept for castration, and a cost of $2.50/animal was used based on costs at area veterinary clinics. Costs for pharmaceutical therapy were determined as the average for a treatment group in each experiment. Cost of gain in Exp. 2 was based on all animals being indexed to a healthy steer. Gross returns were based on the weighted average sale price for the cattle on the ranch. Adjusted gross returns were gross returns less the costs of castration and pharmaceutical therapy. In Exp. 3, measures of gross returns were calculated as in Exp. 2, with one exception. One objective in Exp. 3 was to market bred heifers, which had greater value at the time the experiment was initiated. Therefore, open heifer prices represented market value per kilogram based on the August 1994 monthly average value for that weight feeder heifer at the Amarillo Auction. Bred heifer prices reflect market value ($700) at the time those heifers were marketed.
Statistical Analyses
Chi-square analysis was used to test for the occurrence of morbidity by source and castration. In all cases, percentage data were normalized by arcsine transformation. Each experiment was analyzed separately using the MIXED procedure of SAS (SAS Inst., Inc., Cary, NC). Animal was the experimental unit in all analyses. Pasture effects were minimized by proportionally allocating cattle from different sources among all grazed pastures within a year. The initial model included morbidity duration, marketing date, and source, with the associated interactions in factorial design for Exp. 1. Nonsignificant interactions were pooled with residual model error for further analyses. In Exp. 2, castration was added as a main effect to the factorial. Experiment 3 used source and morbidity duration in the model. Unless otherwise stated, significance was at P < 0.05. A protected (
0.05) least squares means separation procedure was used when appropriate.
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Results
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Experiment 1
Mississippi cattle experienced less (P < 0.05) morbidity than Florida cattle (34 vs. 26% morbidity). Overall, initial weights were greater (P < 0.05) for Mississippi cattle than for Florida cattle, but ADG by period and overall were not different (Table 2). Subsequent results (Table 3)| were based on the pooled Florida and Mississippi sources because there was no (P = 0.37) source difference in duration of morbidity.
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Table 2. Effect of stocker cattle source on morbidity and performance on rangelands of the Southern Plains, 1992 (Exp. 1)
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Table 3. Effect of morbidity duration during preconditioning on performance by stocker cattle marketed in July or August 1992 from rangelands in the Southern Plains (Exp. 1)
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The ADG and total gain of cattle marketed in July was greater (P < 0.05) than for those marketed in August (Table 3
). July marketing was necessary because 42% of the cattle weighed >325 kg and met sale contract specifications. Morbidity <8 d did not affect the ADG or total gain of July-marketed cattle (P = 0.58). There were insufficient observations in morbidity periods >8 d to estimate effects on animal performance in July-marketed cattle.
A morbidity duration x marketing date interaction (P < 0.05) resulted from cattle marketed in August. Cattle experiencing morbidity had lower ADG and total gain than healthy cattle (P < 0.05; Table 3). Cattle requiring >14 d of pharmaceutical therapy gained less than cattle having <14 d therapy (P < 0.01; Table 3).
Based on the cost of the antibiotic ($11.05 per treatment), a single antibiotic treatment increased cost of gain $0.10/kg in July-marketed cattle and $0.14/kg in August-marketed cattle. Two treatment therapies ($16.74) increased cost of gain $0.26/kg for August-marketed cattle. More than two antibiotic therapies averaged $20.18 and increased cost of gain $0.37/kg in August-marketed cattle.
Gain was valued at $1.94/kg. Gain depression from morbidity in August-marketed cattle resulted in losses of $17.46 to $46.56 per animal for animals that were morbid <8 d and >8 d, respectively. The combined cost of antibiotic therapy and loss in gain resulted in a cost for morbidity ranging from $11.05 per head to $66.74 per animal or $0.05/kg to $0.28/kg initial weight of cattle.
Experiment 2
Source of cattle influenced (P < 0.05) ADG. Cattle from Central Texas had lower (P < 0.05) ADG (0.53 kg/d) than cattle from North and West Texas (0.82 kg/d). Across steers and bulls castrated after arrival, Central Texas cattle experienced greater morbidity (68%) than those from West (28%) and North (43%) Texas. All subsequent analyses were conducted with data pooled across sources because there were no (P = 0.23) source related interactions.
Regardless of morbidity, ADG was greater (P < 0.05) in July- than August-marketed cattle, consistent with results from Experiment 1. The overall effect of morbidity on ADG in July- and August-marketed cattle increased as duration of morbidity increased (P < 0.05; Table 4). Cattle morbid >8 d gained less (P < 0.05) than cattle requiring no pharmacologic treatment. The ADG of July-marketed cattle was lower (P < 0.05) for morbid vs. healthy cattle. The magnitude of decrease in ADG from July to August was not affected by morbidity and averaged 0.22 kg/d.
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Table 4. Effect of morbidity duration during preconditioning on ADG by stocker cattle on Southern Plains mixed-prairie rangeland, 1992 to 1993 (Exp. 2)
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There was not a marketing month or month interaction effect (P = 0.18) on gain response to morbidity. Therefore, the weighted average ADG and total gain were used to calculate effects of morbidity and castration on animal productivity and gross returns (Table 5). The ADG of healthy steers was 0.05 to 0.06 kg/d more than morbid steers or healthy bulls castrated after arrival. The ADG of morbid bulls castrated after arrival was 0.19 kg/d less (P < 0.05) than that of healthy steers. Morbid bulls gained 0.13 to 0.14 kg/d less (P < 0.05) than did healthy bulls and morbid steers, respectively. Castration alone caused a 7.9% loss in productivity and the combined effects of castration and morbidity decreased productivity by 24.8%. In contrast, morbidity alone caused a 6.5% loss in steer productivity.
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Table 5. Combined effects of morbidity and castration on performance and gross returns based on gain of stocker steers and bulls grazing rangelands in the Southern Plains, 1992 to 1993 (Exp. 2)
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Total gain responses to morbidity and castration were similar to ADG responses (Table 5). However, the magnitude of differences in total gain and productivity losses were less than that for ADG, reflecting a longer average grazing period for morbid animals. Healthy bulls were the only group in which losses in total gain increased relative to ADG losses.
Gross returns per day ranged from $1.12 to $1.48 for morbid bulls and healthy steers, respectively (Table 5). A $31.15 per animal loss on 102 kg gain in morbid bulls increased the cost of gain by $0.31/kg relative to a healthy steer. Castration alone led to a $19.60 per animal loss in healthy bulls and increased cost of gain $0.18/kg relative to healthy steers. Morbidity cost $8.05/animal in steers and increased cost of gain by $0.07/kg. The value of gain for a healthy steer was $23 more than for a morbid steer, $22 more than for a healthy bull, and $48 more than for a morbid bull. Morbid bulls had the lowest adjusted gross returns ($179/animal) of all types of animals in this experiment.
Experiment 3
Heifers experienced 49% morbidity. There was no difference (P > 0.20) in morbidity among sources. Morbid heifers weighed less initially and at the end of the study than healthy heifers (Table 6). Morbid heifers requiring >8 d antibiotic treatment had lower (P < 0.05) growth and reproductive performance than healthy heifers (Table 6). Heifers morbid <8 d exhibited intermediate levels of performance. Heifers experiencing >8 d morbidity to BRD exhibited a lower (P < 0.05) pregnancy rate (66%) than healthy heifers (81%). The pregnancy rate (76%) for heifers morbid <8 d was lower than healthy heifers, but greater than heifers treated >8 d (P < 0.05). Months of pregnancy was less (P < 0.05) in morbid (0.6 mo) than healthy heifers.
Pregnancy adjusted gross return per heifer, including antibiotic therapy costs, was $19.98 less in heifers treated <8 d and $51.57 less for heifers treated >8 d compared with healthy heifers.
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Discussion
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Morbidity affected the performance and efficiency of stocker steers, bulls, and heifers over three grazing seasons in North Texas. Animal performance consistently decreased in each experiment when animals required more than one antibiotic therapy. Most research from confinement-feeding experiments indicates early weight loss associated with morbidity is usually compensated for after 30 d (Galyean et al., 1999). Shipping stress research has focused on the effects of transit time, water-deprivation period, food-deprivation period, and post-transit response to feeding, antibiotic therapy, feed efficiency, and performance (Cole and Hutcheson, 1985; Van Koevering et al., 1992; Galyean et al., 1999). Morbidity effects on performance of grazing stocker cattle have not been previously reported. Research during preconditioning suggests that morbid cattle experience at least short-term depression in growth rate compared with healthy cohorts (Van Koevering et al., 1992). Cattle transported the greatest distances in Exp. 1 and Exp. 2 exhibited greater morbidity. These results are consistent with reports by Cole et al. (1988), Hutcheson and Cole (1986), and Phillips et al. (1991) on morbidity and transport time relationships.
Previous feedlot and preconditioning research was conducted with increasing levels of dietary nutrient density and nutrient intake (Galyean et al., 1999). In contrast, our research was conducted under extensive nutrient-poor grazing environments that decrease in forage quantity and nutritive value through time (Pinchak et al. 1990). These studies focused on morbidity and its impacts on production efficiency and profitability, not morbidity response of cattle to an antibiotic treatment or receiving diet. Van Koevering et al. (1992) clearly showed that morbidity depressed ADG over a range of supplemental protein and nonprotein sources during a series of 28-d trials. Gardner et al. (1998) reported lower ADG in fed cattle with nonactive or active respiratory lesions vs. healthy cattle.
Late-season depression in the performance of stocker cattle grazing native range is associated with declining forage conditions (Pinchak et al. 1990; Huston and Pinchak, 1991). The effect of a decreasing plane of nutrition on animal performance is compounded in heat-stressed cattle because of lower forage intake and decreased grazing time (Finch, 1986; Forbes et al., 1998; Sprinkle et al., 2000). Heat stress effects on animal performance can be exacerbated in cattle with compromised pulmonary function (Forbes et al., 1998; Sprinkle et al., 2000) as a result of earlier BRD lesions.
Adams and Hinsley (1989) found castration alone over a 106-d grazing period on wheat pasture decreased ADG. In Exp 2, losses in production with castration and morbidity were additive. The additive effects of castration and morbidity were compounded, with castrated animals experiencing morbidity. The increased stress of castration may predispose animals to increased susceptibility to BRD infection.
The low ADG for heifers in Exp. 3 reflects the effect of drier conditions and increased stocking rates. By mid-June, forage availability and quality (<700 kg/ha, 7.34% CP, 58% IVDOM) limited heifer growth rate in all pastures. Heifers weighed 333 to 352 kg at the end of the experiment. The actual mature weight range of these heifers was unknown; however, if we assume it was 525 kg, heifers attained 63% to 67% of their mature size by the end of the breeding season. Pregnancy rates of heifers morbid >8 d were consistent with heifers below 65% of their mature weight at breeding (Patterson et al. 1992).
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Implications
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We found from these experiments consistent negative effects of castration and respiratory infection on the productivity and profitability of 227- to 272-kg stocker cattle grazing native range. Potential losses in productivity of 10% to 25% could be attributed to these factors. The potential effect of morbidity in the stocker cattle industry in North Texas is over $1 million annually. The research establishes an objective basis for calculating values of bull calf and freshly weaned calves sufficient to offset losses in income resulting from castration and morbidity under the native range grazing conditions of the Southern Plains. Further research is required under winter wheat grazing conditions to determine whether the dietary plane of nutrition mitigates effects of morbidity on stocker cattle in those systems.
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Table 6. Effect of morbidity and antibiotic treatment frequency on gain, reproductive performance, and gross returns from replacement heifers grazing rangelands in the Southern Plains, 1994 (Exp. 3)
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1 Correspondence: P. O. Box 1658 (phone: 940-552-9941, ext 242; fax: 940-553-4657; e-mail: bpinchak{at}ag.tamu.edu).
Received for publication September 18, 2002.
Accepted for publication May 28, 2004.
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Literature Cited
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Adams, N. J., and A. Hinsley. 1989. Performance of stocker cattle purchased as bulls vs. steers. Pages 189–192 in Beef Research in Texas. Consolidated Progress Reports.
Cole, N. A., and D. P. Hutcheson. 1986. Influence of pre-fast feed intake on recovery from feed and water deprivation. J. Anim. Sci. 60:772–780.
Cole, N. A., T. H. Camp, L. D. Rowe, D. S. G. Stevens, and D. P. Hutcheson. 1988. Effect of transport on feeder calves. Am. J. Vet. Res. 49:178–183.[Medline]
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Loerch, S. C., and F. L. Fluharty. 1999. Physiological changes and digestive capabilities of newly received feedlot cattle. J. Anim. Sci. 77:1113–1119.[Abstract/Free Full Text]
Patterson, D. J., R. C. Perry, G. H. Kiracofe, R. A. Bellows, R. B. Stagmiller, and L. R. Corah. 1992. Management considerations in heifer development and puberty. J. Anim. Sci. 70:4018–4035.[Abstract]
Pinchak, W. E., S. J. Canon, R. K. Heitschmidt, and S. L. Dowhower. 1990. Effect of long-term, year-long moderate and heavy rates of stocking on diet selection and forage intake dynamics. J. Range Manage. 43:304–309.
Phillips, W. A., P. E. Juniewicz, and D. L. Von Tunglen. 1991. The effect of fasting, transit plus fasting, and administration of adrenocorticotropic hormone on the source and amount of weight loss by feeder cattle of different ages. J. Anim. Sci. 69:2342–2348.[Abstract]
Sprinkle, J. E., J. W. Holloway, B. G. Warrington, W. C. Ellis, J. W. Stuth, T. D. A. Forbes, and L. W. Greene. 2000. Digesta kinetics, energy intake, grazing behavior, and body temperature of grazing cattle differing in adaption to heat. J. Anim. Sci. 78:1608–1624.[Abstract/Free Full Text]
Van Koevering, M. T., D. R. Gill, F. N. Owens, and R. L. Ball. 1992. The effects of types and qualities of protein on health and performance of shipping-stressed calves. Anim. Sci. Res Report. Oklahoma Agric. Exp. Stn. MP-136:326–332.
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Abstract
Introduction
