Proc. 57th Southern Pasture and Forage Crop Improvement Conference, Athens, GA April 23-25, 2002
The Salem Road Study: Restoration of Degraded Land with Pasture
-The Concept of Parasite-Free Pastures
Ray M.Kaplan1, John A. Stuedemann2, Alan J. Franzluebbers2, and Dwight H. Seman2
1College of Veterinary Medicine, University of Georgia, Athens 30602
2USDA-Agricultural Research Service, 1420 Experiment Station Road, Watkinsville, GA 30677
Summary
The objective of this report was to determine if parasite-free pastures could be maintained by anthelmintic treatment of animals prior to placing them on pastures. Yearling Angus steers (Bos taurus) were managed in a put-and-take grazing system with three “tester” steers assigned to each paddock and “grazer” steers added or removed at 28-day intervals. From 1994-1998, steers grazed the paddocks for a 140-day period from mid-May until early October each year. Anthelmintic treatment included pour-on ivermectin on day –21, albendazole on day –7, and injectable ivermectin 48 hours prior to stocking of pastures, with the cattle remaining in drylot during the 48-hour period prior to being placed on the experimental paddocks. All steers received only one series of treatments during any given year. Rectal fecal samples for worm egg counts were obtained on day 0 and at 28-day intervals thereafter except in years one, four and five when egg counts were also performed on day –21. On all sampling days after day 0, samples were obtained only from tester animals.
Over the 5-year period, the mean eggs per gram of feces (epg) gradually increased from 0 (following treatment) to a mean of 2.2 (range from 0.7 to 3.0) by the end of the grazing season (the last sampling date) in October. Although there were statistical treatment differences in epg among both the fertilization and forage mass treatments, the very low average epg (0.7 to 3.0) indicated that there were no biological differences.
Although the epg were not zero, they were below threshold levels that would allow development of a parasite burden in cattle. The threshold levels that could allow for development of a parasite burden in the steers are unknown for conditions of this experiment, but because epg did not increase we can say that they are below the threshold levels. Consequently, we can say that under the conditions of this experiment, pastures were maintained in a parasite-free condition for at least five years by simply therapeutically treating animals prior to placing them on the parasite-free pastures. The therapeutic treatment prevented transport of larvae to the pastures, thus preventing pasture contamination and reinfection.
Conceptually, using the current grazing system, it should be possible to maintain these pastures in a parasite-free status indefinitely. By removing the effect of parasites, cattle can grow without the physiological constraints that gastrointestinal parasites place on appetite, digestion, nutrient utilization, and general well being. In traditional management systems, cattle graze parasite-contaminated pastures; therefore, parasites negatively impact growth and productivity throughout the entire grazing period. Periodic anthelmintic treatments simply give a temporary reprieve to those parasitic infections.
Rationale
Gastrointestinal parasites are a major constraint to health and productivity in grazing livestock production systems (Fox, 1997). It has generally been assumed that parasite infections are inevitable in grazing animals; therefore treatment and control programs have been aimed simply at reducing rather than eliminating the effect of parasites. However, this assumption may not be true. Based on a number of reviews on epidemiology of gastrointestinal nematodes (Thomas 1982; Williams, 1986a), cropland with no exposure to cattle for several years would have relatively few infective nematode larvae. If these croplands were converted to pasture, and cattle were treated with anthelmintics prior to placing animals on these pastures, theoretically, it should be possible to maintain the pastures in a parasite-free state or below threshold levels indefinitely. This concept is similar to that of the “safe pasture” preventative control system that combines anthelmintic treatment with management (Michael, 1976; Brunsdon, 1980; Morley and Donald, 1980; Williams, 1986b). In the Southern Piedmont region there has been a trend of conversion of cropland to managed pasture (Census of Agriculture 1992, 1997). This trend provides an excellent opportunity to integrate anthelmintic treatment with management to provide and maintain parasite-free pastures.
The broad objective of the experiment was to identify sustainable cattle production systems that are highly productive, that minimize negative environmental impacts, and that improve soil quality in degraded cropland. The specific objective of this report was to determine if parasite-free pastures could be maintained by judicious anthelmintic treatment of animals prior to placing them on pastures.
Experimental design and parasite methodology
Detailed description of site characteristics and experimental design are presented in the paper by J. A. Stuedemann (in this proceedings).
Cattle
Yearling Angus steers (Bos taurus) were managed in a put-and-take grazing system with three “tester” steers assigned to each paddock and “grazer” steers added or removed at 28-day intervals except during periods of rapid forage mass changes when steers were added or removed at 14-day intervals. Except on rare occasions, three tester steers grazed each of the paddocks. Mean initial steer weights of testers for each year are presented in Table 1.
Table 1. Initial Tester Steer Weights
|
Year |
N |
Weight, kg |
SD |
Minimum |
Maximum |
|
1994 |
51 |
261 |
14.6 |
227 |
286 |
|
1995 |
54 |
273 |
15.8 |
243 |
313 |
|
1996 |
54 |
258 |
13.8 |
227 |
286 |
|
1997 |
54 |
245 |
18.7 |
197 |
279 |
|
1998 |
54 |
247 |
12.9 |
218 |
272 |
The stocking density for a given paddock was computed by assuming that each steer would consume 1 kg of dry matter per 45.45 kg of body weight. The number of steers that could be supported on a given paddock was calculated by dividing the forage mass (kg ha-1) by the estimated average daily intake and then dividing by 28 (the number of days in the grazing period). Estimates of forage mass were made prior to stocking the paddocks and at 28-day intervals except during those times when adjustments were made at 14-day intervals. Estimates of forage mass were made by clipping a .25 m2 area to ground level at 30 m grid sites. Average above ground forage masses (Mg ha-1), during the experimental period, for each fertilization and grazing intensity treatment for each of the five years are presented in Table 2.
Table 2. Average Above-Ground Forage Mass (Mg ha-1) by Nitrogen Treatment and Grazing Intensity
|
Nitrogen Treatment |
Grazing Intensity |
| Year |
Clover |
Litter |
Mineral |
High |
Low |
| 1994 |
5.3 |
5.7 |
6.2 |
4.3 |
7.2 |
| 1995 |
3.8 |
3.5 |
3.9 |
2.6 |
4.8 |
| 1996 |
3.4 |
3.3 |
3.3 |
2.2 |
4.5 |
| 1997 |
3.7 |
3.4 |
4.0 |
2.3 |
5.1 |
| 1998 |
2.2 |
2.4 |
2.6 |
1.5 |
3.3 |
| Mean |
3.7a |
3.7a |
4.0b |
2.6c |
5.0d |
a,b Values with different superscripts across nitrogen treatments differ, P<.05. Year*nitrogen treatment interaction was significant, P<.0001.
c,d Values with different superscripts across grazing intensity differ, P<.05. Year*grazing
intensity interaction was significant, P<.0001.
Steers grazed the paddocks for a 140-day period from mid-May until early October each year except during the first year of treatment implementation (1994) when grazing began in mid-July due to repairs to infrastructure following a tornado. Shrunk weights (16 h off water) were obtained initially and at 28-day intervals except when stocking rate adjustment were made at 14-day intervals.
Anthelmintic treatment
Anthelmintic treatment included pour-on ivermectin on day –21, albendazole on day –7, and injectable ivermectin 48 hours prior to stocking of pastures, with the cattle remaining in drylot during the 48-hour period. This treatment regimen would be expected to remove greater than 99.9 % of all gastrointestinal nematodes infecting cattle. All steers received only one series of treatments during any given year.
Fecal sampling and analyses
Rectal fecal samples for worm egg counts were obtained on day 0 and at 28-day intervals thereafter, except in years one, four and five when egg counts were also performed on day -21. On all sampling days after day 0, samples were obtained only from tester animals. A modification of Stoll’s flotation-centrifugation technique was used to determine the number of nematode eggs per gram of feces (epg) (Stoll, 1930).
Statistical analyses
Because fecal worm egg counts would presumably increase during the grazing season and were greatest at the last sampling date of each grazing season, only data from the October sampling were statistically analyzed for treatment differences. Fecal epg showed a heterogeneity of variance among treatments, so epg values were transformed by calculating base 10 logarithm(total epg +1). Paddock was considered the experimental unit so paddock epg averages were used. Data were analyzed as a randomized complete block design that included random and fixed effects with repeated measures (years). Block was considered as a random effect while year, nitrogen, and grazing intensity effects were fixed effects. Statistical analyses were calculated using the Mixed Procedure of SAS (Littell, et al., 1996). The best fitting model, as determined by Schwarz’s Bayesian Criterion was with block specified as random, with year repeated, and using the AR (1) (first order autoregressive) error structure on the residuals. The AR (1) error structure allows for equal variances on the main diagonal (year). Covariances on the off-diagonal is the variance multiplied by the repeated measures correlation coefficient raised to increasing powers as the measures become farther separated in time. These covariances become smaller as powers increase. The model included nitrogen and grazing intensity effects and their interaction. These were tested with the block*nitrogen*grazing intensity term. Year, and its interactions with nitrogen and grazing intensity were included and tested with residual error. The effect of block was not significant (P>.05) and was dropped from the analysis.
Results and discussion
Steer performance
Average daily gain (ADG) of tester steers for each of the five years and overall are presented in Table 3. Among the pasture fertilization treatments, steers grazing the clover plus nitrogen treatment had higher (P<.05) ADG than those on either of the other treatments. There is no obvious explanation for these differences in ADG. There was very little clover available for consumption, other than some residue in the early part of the grazing season, because the clover was mowed approximately one month before grazing began each year. Steers grazing at the low forage mass (high intensity) had lower (P<.05) ADG than those grazing at the high forage mass (low intensity). This difference could be attributed to greater selection potential for steers grazing the high forage mass. The steer ADG during this five-year period was excellent across all treatments. These results could partly by attributed to the fact that these pastures were free of nematode parasites.
Table 3. Average Daily Gain (kg day-1) of Tester Steers by Nitrogen Treatment and Grazing Intensity
|
Nitrogen Treatment |
Grazing Intensity |
|
Litter |
Mineral |
High |
Low |
||
| 1994 |
1.10 |
.86 |
.84 |
.74 |
1.13 |
| 1995 |
.67 |
.62 |
.57 |
.56 |
.68 |
| 1996 |
.85 |
.65 |
.77 |
.72 |
.79 |
| 1997 |
.75 |
.67 |
.77 |
.58 |
.88 |
| 1998 |
.84 |
.80 |
.85 |
.75 |
.91 |
| Mean |
.84a |
.72b |
.76b |
.67c |
.88d |
a,b Values with different superscripts across nitrogen treatments differ, P<.05.
Year*nitrogen treatment interaction was significant, P=.0203.
c,d Values with different superscripts across grazing intensity differ, P<.05. Year*grazing
intensity interaction was significant, P<.0001.
Fecal egg counts
Fecal egg counts for the last sampling date in each year and a mean for the last sampling date across the five years are presented in Table 4. In each year, the epg were highest at the end of the grazing season (the last sampling date) in October. Although there were statistical treatment differences, the very low average epg (0.7 to 3.0) indicate that there were no biological differences among both the fertilization and forage mass treatments.
Table 4. Average Fecal Egg Counts (eggs/gram) for October
|
Nitrogen Treatment |
Grazing Intensity |
| Year |
Clover |
Litter |
Mineral |
High |
Low |
| 1994 |
.7 |
4.1 |
6.1 |
2.6 |
4.7 |
| 1995 |
0 |
.7 |
.4 |
.7 |
.1 |
| 1996 |
.9 |
4.2 |
4.0 |
3.1 |
3.0 |
| 1997 |
.4 |
3.6 |
2.4 |
.6 |
.7 |
| 1998 |
1.3 |
2.6 |
1.7 |
.8 |
2.9 |
| Mean |
.7a |
3.0b |
2.9b |
1.6c |
2.9d |
a,b Values with different superscripts across nitrogen treatments differ, P<.05.
c,d Values with different superscripts across grazing intensity differ, P<.05. Year*grazing
intensity interaction was significant, P<.0251.
Although the epg were not zero, they were below threshold levels that would allow development of a parasite burden in cattle. The threshold levels that could allow for development of a parasite burden in the steers are unknown for conditions of this experiment, but because epg did not increase we can say that they are below the threshold levels. A variety of factors including forage mass, cattle numbers and animal class could influence the threshold levels necessary to allow development of a parasite burden in the cattle. Consequently, it can be said that under the conditions of this experiment, pastures were maintained in a parasite-free condition for at least five years by simply therapeutically treating animals prior to placing them on the parasite-free pastures. The therapeutic treatment prevented transport of larvae to the pastures, thus preventing contamination.
The sensitivity of detection used for performing fecal egg counts in this study was much greater than that used for normal diagnostics. If the more usual methods had been used, every animal in this study over the 5-year period would have tested fecal-negative for parasites on every test date. Over the 5-year period, mean epg at the end of the grazing period was 2.2; this level is approximately 2 orders of magnitude less than mean epg levels that one would expect under a more traditional management scheme. The treatment regimen used in this study was selected to ensure that cattle would enter the paddocks each spring in an essentially parasite-free state. Our more recent studies have demonstrated that this same effect can be achieved by a single dosing with 2 different anthelmintics given simultaneously on day –3.
Implications and conclusions
Conceptually, using the current grazing, it should be possible to maintain these pastures in a parasite-free status indefinitely. By removing the effect of parasites, cattle can grow without the physiological constraints that gastrointestinal parasites place on appetite, digestion, nutrient utilization, and general well being. In traditional management systems, cattle graze parasite-contaminated pastures; therefore, parasites negatively impact growth and productivity throughout the entire grazing period. Periodic anthelmintic treatments simply give a temporary reprieve to those parasitic infections. By using anthelmintic treatments in a prophylactic manner in combination with parasite-free pastures, we have demonstrated that the goal of parasite-free grazing of cattle is an achievable goal. This approach reduces the number of anthelmintic treatments that are required, while enhancing animal productivity.
Literature cited
Brunsdon, R. V., 1980. Principles of helminth control. Vet. Parasitol. 6, 185-215.
Fox, M., 1997. Pathophysiology of infection with gastrointestinal nematodes in domestic ruminants: recent developments. Vet. Parasitol. 72, 285-297.
Littell, R. C., Miliken, G. A., Stroup, W. W., Wolfinger, R. D., 1996. SAS System for Mixed Models. SAS Institute Inc., Cary, NC.
Michael, J. F., 1976. The epidemiology and control of some nematode infections in grazing animals. Adv. Parasitol. 14, 355-397.
Morley, F. H. W., Donald, A. D., 1980. Farm management and systems of helminth control. Vet. Parasitol. 6, 105-134. Stoll, N. R., 1930. On methods of counting nematode ova in sheep dung. Parasitology 22, 116-136.
Thomas, R. J., 1982. The ecological basis of parasite control: Nematodes. Vet. Parasitol. 11, 9-24.
Williams, J. C., 1986a. Epidemiologic patterns of nematodiasis in cattle. In: Gibbs, H. C., Herd, R. P., Murrell, K. D. (eds.), Parasites: Epidemiology and Control, The Veterinary Clinics of North America Food Animal Practice 2 (2), pp. 235-246.
Williams, J. C., 1986b. Control strategies for nematodiasis in cattle. In: Gibbs, H. C., Herd, R. P., Murrell, K. D. (eds.), Parasites: Epidemiology and Control, The Veterinary Clinics of North America Food Animal Practice 2 (2), pp. 247-260.