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ANIMAL GENETICS |
* Department of Animal and Poultry Sciences and
and
Department of Biomedical Sciences and Pathobiology, Virginia Polytechnic Institute and State University, Blacksburg 24061
Abstract
This study describes responses to artificial infection with Haemonchus contortus in ewes and lambs of 50% Dorset, 25% Rambouillet, and 25% Finnsheep ancestry and provides estimates of genetic parameters for measures of parasite resistance. One hundred ninety-eight ewes out of 64 sires, and 386 lambs out of 25 sires were evaluated in autumn and spring of 2 yr. Ewes were dewormed shortly after weaning their lambs and lambs were dewormed at about 120 d of age. One week after deworming, ewes and lambs were dosed with approximately 10,000 infective larvae of H. contortus. After infection, BW, fecal egg counts (FEC), and packed cell volume (PCV) were measured weekly for 7 wk in lambs kept in drylot and fortnightly for 11 wk in ewes on pasture. Summary traits were defined as initial PCV, mean BW across all times, and means for PCV (MPCV) and log-transformed FEC (MLFEC) at wk 3 to 7 after infection for lambs and wk 3 to 11 after infection for ewes. Ewes and lambs did not lose weight overall in any year or season, but there was no consistent effect of year or lambing season on mean LFEC or mean PCV during infection in either ewes or lambs. Yearling ewes were less resistant to infection than older ewes, with lower PCV (P < 0.05) and higher LFEC (P < 0.05). During infection, PCV was positively correlated with BW and negatively correlated with LFEC in both ewes and lambs. In lambs, heritabilities were 0.39 (P < 0.01) for PCV, 0.10 (P < 0.05) for LFEC across all measurement times, and 0.19 (P < 0.01) for three measures of LFEC taken at the peak of infection. Heritability estimates for ewes were 0.15 (P < 0.05) for PCV and 0.31 (P < 0.01) for LFEC. Repeatabilities for LFEC and PCV across measurement times were moderate in ewes and lambs. Correlations between dam and lamb records for MLFEC were generally low, suggesting different mechanisms of resistance in lambs and nonlactating ewes. Ewes with higher genetic merit for growth as lambs were less resistant to infection as adults, but genetic merit for fertility and prolificacy were not related to parasite resistance. Lambs with higher genetic merit for body weight were more resistant to infection. Selection for resistance to H. contortus is therefore possible and should not adversely affect growth of lambs and fertility of ewes in this production environment.
Key Words: Genetic Parameters • Haemonchus contortus • Health • Internal Parasites • Sheep
Introduction
Infections with Haemonchus contortus are prevalent all over the world and are responsible for economic losses in sheep production (Barger and Cox, 1984
). The increasing prevalence of anthelminthic resistance (Overend et al., 1994) and growing demand for animal products that are produced without the use of chemical substances have prompted a search for alternative methods to control helminthiasis in sheep. Over the past several years, evidence has emerged that suggests a genetic basis for resistance to gastrointestinal nematodes in sheep. There have been reports of genetic differences among breeds (Courtney et al., 1985; Baker et al., 1999) and within-breed variation in resistance to infection by gastrointestinal helminthes. Moderate heritabilities have been reported for resistance to H. contortus in the Australian Merino (Woolaston and Piper, 1996), Trichostrongylus colubriformis in New Zealand Romneys (Bisset et al., 1992), and Ostertagia ostertagi in Scottish Blackface sheep (Bishop et al., 1996). Haemonchus infections are prevalent in several parts of the United States, and early North American studies documented within-breed variation in response to helminth infection (Scrivner, 1967); however, there have not been any published reports of genetic parameter estimates for parasite resistance traits in sheep in the United States.
This experiment was designed to measure responses to an artificial H. contortus infection in lambs and ewes to validate procedures for assessment of parasite resistance in intensively managed sheep and to estimate heritability and repeatability for fecal egg counts (FEC) and packed cell volume (PCV) in lambs and ewes following infection. A further objective was to estimate relationships of FEC and PCV with genetic merit for production traits.
Materials and Methods
Animals, Management, and Experimental Design
Data were collected from lambs and ewes in a flock of 50% Dorset, 25% Rambouillet and 25% Finnsheep breeding maintained at the Virginia Polytechnic Institute and State University Sheep Center at Blacksburg. This flock was established in the early 1980s (Fossceco and Notter, 1995). In 1988, animals were subdivided into an autumn-lambing flock selected for reduced seasonality in breeding, an unselected autumn-lambing environmental control line, and an unselected, spring-lambing genetic control line (Al-Shorepy and Notter, 1996). Selection continued through the autumn 1998 lambing. Lambs for the current study were born in spring (March and April) and autumn (late September to early November) of 1997 and 1998 (Table 1).
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Ewes, aged 1 to 10 yr, were dewormed before the start of lambing, but had not been treated again before weaning. After weaning, ewes were allowed to dry off, dewormed with levamisole, and dosed with approximately 10,000 infective larvae of H. contortus 1 wk later. Spring-lambing ewes in yr 1 accidentally received approximately 5,000 larvae and were dosed with an additional 5,000 larvae 1 mo later. For these ewes, the time of the second dose was treated as the time of infection. Ewes were maintained on pasture and thus may have been continuously exposed to infection. Body weights, FEC, and PCV were determined at the time of infection and 3, 5, 7, 9 and 11 wk after infection. Fortnightly measurements were taken in ewes because the risk of disease is not as severe as in lambs. One spring-lambing ewe died of an unrelated infection, and two autumn-lambing ewes were removed from the study because of low PCV. A total of 276 records from 198 ewes by 64 sires were evaluated over the 2 yr.
Fecal egg counts were determined using the modified McMaster’s technique (Whitlock, 1948), with each egg observed representing 50 eggs/g of feces. Packed cell volumes (%) were determined by the micro-hematocrit centrifuge method.
Data Analysis
Data for lambs and ewes were analyzed separately. Fecal egg counts were not distributed normally. Therefore, a set of logarithmic transformations was applied to FEC, and the resulting transformed variables were tested for normality with the univariate procedure of SAS (SAS Inst., Inc., Cary, NC). Normality of residuals was tested using probability plots, skewness and kurtosis values, and the Shapiro-Wilk statistic. The most appropriate transformations (LFEC) were ln(FEC + 2,000) in lambs and ln(FEC + 25) in ewes. These transformations appeared to best normalize FEC in lambs and ewes and were used in all subsequent analyses. Means for LFEC were back-transformed for reporting; SE were estimated by assuming that SE on the logarithmic scale approximately equal CV on the actual scale. One ewe was removed from the data as an outlier because of extremely high FEC.
The 35 spring-born lambs that received 5,000 infective larvae in 1997 had lower (P < 0.01) mean FEC (1,293 ± 192 vs. 2,275 ± 202 eggs/g) than did lambs that were dosed with 10,000 larvae. Their BW (46.6 ± 0.9 and 47.9 ± 0.9 kg, respectively, for lambs that received 5,000 or 10,000 larvae) and PCV (27.8 ± 0.4 and 27.1 ± 0.4%, respectively) were similar. Lambs that received 10,000 infective larvae thus exhibited no apparent ill effects, and 10,000 larvae were used as the standard dose in future replicates.
Changes in BW, PCV, and LFEC over time were analyzed with a repeated-measures ANOVA using the mixed models procedure of SAS. The model included fixed effects of year, season, sex (for lambs) or age-category (for ewes), week (the repeated factor), all two-way interactions, and the three-way interactions among year, season, and week; an unstructured covariance matrix was assumed for the repeated effect. Age categories separated ewes of 1, 2, 3 to 6, or more than 6 yr. Fecal egg counts before wk 3 were consistently near 0 eggs/g, so only measurements taken at wk 3 or later were included in the analysis of LFEC.
A set of summary traits was defined to study interrelationships among measures of resistance and relationships with production traits. Summary traits included initial (wk 0) packed cell volume (IPCV) and means for PCV (MPCV), BW (MBW), and log-transformed FEC (MLFEC). Body weights were averaged over all measurement times in both lambs and ewes. The MLFEC and MPCV were averaged over the period of infection: wk 3 through 7 in lambs and wk 3 through 11 in ewes. For calculation of summary traits, occasional missing values for BW, LFEC, and PCV were replaced by predicted values derived from a nested analysis of variance including fixed effects of year, season, week, and age-category (for ewes) or sex (for lambs), all two-way interactions, the year x season x week interaction, and a random effect of animal nested within year, season, and sex (for lambs) or age-category (for ewes). Values were missing for PCV in 2% of ewes and 3% of lambs; values were missing for FEC in 4% in ewes and 6% in lambs. A nested analysis was used to predict missing values because the mixed-model repeat-measures procedure did not provide for prediction of missing values.
Interrelationships among summary traits were obtained from a multivariate analysis of variance with year, season, age-category (for ewes) or sex (for lambs), and their two-way interactions in the model. These analyses were also used to obtain correlations between records of lambs and their dams after additive adjustment for the age category of the ewe and sex of the lambs. Records of ewes with more than one lamb were repeated for each lamb.
Genetic parameters for BW, PCV, FEC, and LFEC were estimated in single-trait analyses using restricted maximum likelihood (Boldman et al., 1993). For lambs, the model included fixed effects of year, season, sex, and week, as well as random animal additive, litter, and animal permanent environmental effects. Litter effects quantify maternal and other nongenetic resemblances among full-sibs; animal permanent environmental effects predict the consistency of repeated records on the same lamb. The analysis of LFEC in lambs was also conducted using only observations taken at wk 3, 4, and 5. In ewes, the model included fixed effects of year, season, ewe age category, and week, plus random effects of animal breeding value, between-year ewe permanent environment, and within-year ewe permanent environment (i.e., ewe x year interaction). Ewe and ewe x year variance components were used to estimate repeatabilities of ewe measurements between and within years, respectively. In both lambs and ewes, heritabilities of individual measures of PCV at each measurement time, and of LFEC at each measurement time during infection, were also estimated in single-trait analyses using the same model as before, but excluding fixed effects of week and random within-year animal permanent environmental effects. Tests of significance were done using likelihood ratio tests after deleting each random effect from the model.
Numbers of observations were not adequate to estimate genetic correlations involving resistance traits. However, EBV for birth weight (BRW), maternal birth weight (MBRW), weaning weight (WW), maternal weaning weight (MWW), 120-d postweaning weight (PWW), fertility in autumn lambing (FF), and number of lambs born per 100 ewes lambing (NB) were available for all animals in the study from the associated selection study (Al-Shorepy and Notter, 1996). Associations between EBV for production traits and summary measures of parasite resistance could therefore be estimated by regression analyses. Each analysis included fixed effects of year, season, year x season interaction, and sex (for lambs) or age category (for ewes) and one of the EBV. Additionally, initial BW (IBW) at the start of the trial was fitted as a second covariate in regression analyses involving EBV for WW and PWW of ewes and lambs to attempt to distinguish genetic and nongenetic effects of body size. Residual correlations between IBW and WW EBV were 0.54 (P < 0.01) and 0.47 (P < 0.01) in lambs and ewes, respectively. Residual correlations between IBW and PWW EBV were 0.55 (P < 0.01) and 0.48 (P < 0.01) in lambs and ewes, respectively. Thus, animals with similar EBV for body weights exhibited substantial variation in actual weight at the start of the trial, including variation associated with nongenetic effects of ewe age and type of birth and rearing in lambs and production history in ewes. These moderate correlations indicate opportunity to separate genetic and nongenetic effects of body weight on parasite resistance. Regression coefficients (ß) involving MLFEC indicate that a 1-unit change in EBV was associated with a proportionate change of eß in the back-transformed mean value of (FEC + 2,000) in lambs and (FEC + 25) in ewes.
Results
Responses in Ewes
Year x season x week interactions were significant for all traits in ewes in the repeated-measures analysis (Figure 1). Despite relatively consistent declines in BW between wk 3 and 5 and between wk 9 and 11, ewes did not lose weight overall in any year or season. Autumn-lambing ewes consistently had more rapid initial increases in LFEC than did spring-lambing ewes. In autumn-lambing ewes, LFEC initially increased until wk 5 and then declined in both years. In spring-lambing ewes, LFEC increased gradually until wk 9 in both years and increased dramatically after wk 9 in yr 1. The PCV generally decreased through at least wk 9 in all years and seasons. Initial PCV was consistently higher in autumn-lambing (33.9 ± 0.4%) than in spring-lambing (30.5 ± 0.5%) ewes. Yearling ewes had a more rapid (P < 0.05) initial and overall decline in PCV compared to ewes of other age-categories and had much higher (P < 0.05 to P < 0.01) LFEC at all times (Figure 2).
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Residual correlations among summary traits are shown in Table 2. Initial PCV and MPCV were positively correlated with each other and with MBW; both were negatively correlated with MLFEC, although the correlation between IPCV and MLFEC was not significant. Mean BW was not correlated with MLFEC.
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). Lambs continued to grow during infection. Fecal egg counts were elevated by wk 3, but subsequently varied among years and seasons. Autumn-born lambs in yr 1 and spring-born lambs in yr 2 had much higher egg counts than did spring-born lambs in yr 1 or autumn-born lambs in yr 2. The PCV decreased almost linearly through wk 3 and remained relatively low and stable thereafter. Spring-born lambs in yr 2 had lower PCV than other groups, and initial PCV was higher (P < 0.05) in ewe lambs (33.2 ± 0.2%) than in ram lambs (32.5 ± 0.2%).
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Residual correlations among summary traits in lambs (Table 2
) show that initial PCV was positively correlated with MBW and MPCV, but was not associated with LFEC. Mean PCV was positively correlated with MBW and negatively correlated with LFEC. Mean BW was negatively correlated with LFEC.
No association (correlation of 0.00) was observed between mean FEC of lambs and their dams. However, positive correlations between lambs and dams were observed for MBW (0.16; P < 0.01), IPCV (0.14; P < 0.01), and MPCV (0.12; P < 0.05).
Genetic Parameter Estimates
In lambs, significant additive effects were observed for all measurements (Table 3), with associated heritability estimates of 0.50 for BW, 0.39 for PCV, and 0.10 for LFEC. When calculated separately for each measurement time, heritability estimates were relatively consistent for PCV during infection (ranging from 0.29 to 0.49; all P < 0.01), and the heritability of IPCV (0.28; P < 0.01) was not greatly different from the heritability of PCV during infection. Heritability estimates for LFEC, however, were higher at wk 3 (0.25; P < 0.01), 4 (0.22; P < 0.01) and 5 (0.20; P < 0.05) than at wk 6 (0.07; P = 0.32) and 7 (0.00). When only data from wk 3, 4, and 5 were used in analysis of LFEC (Table 3), the heritability estimate increased to 0.19 (P < 0.01). Significant litter effects were observed for BW and LFEC but not for PCV. Individual animal effects were substantial. Correlations between repeated records taken at weekly intervals were 0.94 for BW, 0.66 for PCV and increased from 0.40 for measures of LFEC across all weeks to 0.53 for measures taken between wk 3 and 5.
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). Heritabilities for PCV measures during infection were similar across weeks, ranging from 0.14 (P < 0.29) to 0.24 (P < 0.10) and were similar to the estimate for IPCV (0.19; P = 0.13). In contrast, heritabilities of LFEC measures at 3, 5, 7, 9, and 11 wk after infection increased over time (0.16, P = 0.27; 0.25, P < 0.10; 0.41, P < 0.01; 0.37, P < 0.01; and 0.26, P < 0.01; respectively). For the 78 ewes with records in both years, significant between-year permanent environmental effects were observed for BW and PCV, but not LFEC (Table 5). Across-year repeatability estimates for BW, PCV, and LFEC were 0.79, 0.33, and 0.32, respectively. Corresponding repeatabilities for measures of BW, PCV, and LFEC taken at 2-wk intervals in the same year were 0.94, 0.51, and 0.46, respectively.
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) indicated that, as expected, MBW was significantly and positively associated with all body weight EBV and was positively associated with MBRW and NB EBV. The IPCV and MPCV increased significantly, and MLFEC correspondingly decreased as EBV for MBRW, WW, and PWW increased. The EBV for FF, NB, BW, and MWW were not associated with parasite resistance in lambs. When IBW was included in regression analyses involving EBV for WW and PWW, the significance of associations between EBV and MLFEC was reduced, but the sign of the relationship remained the same (regression coefficients of –0.11 ± 0.07 kg–1 [P = 0.06] for WW EBV and –0.02 ± 0.01 kg–1 [P = 0.22] for PWW EBV). Associations of EBV for WW and PWW with IPCV and MPCV remained significant and positive, although lower in magnitude (0.97 ± 0.42%/kg, P < 0.05 and 1.29 ± 0.42%/kg, P < 0.01, respectively, for WW; and 0.33 ± 0.13%/kg, P < 0.05 and 0.29 ± 0.15%/kg, P < 0.01; respectively, for PWW). After accounting for effects of WW or PWW EBV, no additional effects of IBW on MLFEC were observed in lambs, but there was a positive association between IBW and both IPCV and MPCV (regression coefficient of 0.07 ± 0.02%/kg; P < 0.05 for both). These results indicate that the favorable overall relationship between BW and LFEC in lambs could not be easily partitioned into genetic and nongenetic components using these data. In contrast, a positive association of body weight with PCV was indicated on both genetic and nongenetic levels.
Results of EBV regressions in ewes (Table 6) indicate that, as in lambs, MBW was significantly positively associated with all body weight EBV and with NB EBV. Initial PCV was positively associated with EBV for WW, FF, and NB, and negatively associated with EBV for MWW. Mean PCV also increased with increases in EBV for FF and NB. Unlike in lambs, MLFEC increased with increases in EBV for BRW, WW, and PWW. There were also trends for MLFEC to be positively associated with EBV for NB (P = 0.09), and for MPCV to decrease with increases in PWW EBV (P = 0.34).
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Discussion
Ewes did not lose weight and lambs continued to grow throughout the measurement period, indicating no major negative effects of infection on BW in this production environment. There was considerable seasonal variation in patterns of FEC over time in ewes. Generally, autumn-lambing ewes showed an early increase in FEC, whereas spring-lambing ewes showed a later increase in FEC, presumably associated with reinfection in spring-lambing ewes grazing contaminated pastures during summer. No clear differences between years or seasons were observed for lambs. Since lambs were kept in drylot after infection, seasonal variations in FEC due to reinfection were not anticipated. Initial PCV was generally higher in both autumn-born lambs and autumn-lambing ewes.
In both ewes and lambs, higher FEC were associated with lower PCV during infection. In lambs, higher FEC were also associated with lighter BW, indicating increased susceptibility to infection in smaller lambs. However, no association between BW and FEC was observed in ewes. Lambs are generally susceptible to infection until about 1 yr of age and become increasingly less susceptible as they grow older (Courtney et al., 1985; Gamble and Zajac, 1992; Kambara et al., 1993). In contrast, adult ewes are relatively resistant to infection except during late pregnancy and lactation (Courtney et al., 1984). In our study, ewes also generally had higher PCV and lower FEC during infection than lambs, in spite of being continually reinfected. Also, yearling ewes were less resistant to infection than older ewes, with higher mean FEC, a steeper decline in PCV, and lower PCV at all post-infection measurement times. In an early study, Gregory et al. (1940) also observed that younger ewes were more susceptible than older ewes, but that after 2 yr of age, resistance to parasitic infection was not affected by age.
Female lambs have been reported to be more resistant to infection and have lower FEC than males after puberty, although there appears to be no differences between sexes before puberty (Courtney et al., 1985; Barger, 1993; Woolaston and Piper, 1996). Our results with these prepubertal lambs are thus consistent with earlier reports.
The lack of a significant association between FEC in lambs and their nonlactating dams is discouraging. Adult, nonlactating ewes are generally resilient to effects of internal parasites and could likely be evaluated for resistance with little risk of adverse effects on performance. However, our results suggest that selection for indicators of parasite resistance in nonlactating ewes would have little correlated effect on resistance in lambs and cannot be recommended. A significant association was observed between PCV in ewes and lambs. However, this association was similar before (0.14 for IPCV) and during infection (0.12 for MPCV), suggesting that it reflects the inheritance of PCV in uninfected animals rather than an association that is diagnostic of response to infection. The economic impact of parasitism is much larger in lambs than in ewes, and selection should focus on reducing parasite susceptibility in lambs. Therefore, genetic improvement of parasite resistance in lambs will likely require direct evaluation of parasite resistance in young animals, even though these animals have a greater risk of morbidity and mortality. Courtney et al. (1986) also reported that the correlation between FEC of dams and their ewe lambs was not significant. They suggested that selection for reduced FEC in periparturient ewes might not improve resistance in progeny and that acquired resistance in young lambs and periparturient ewes may be controlled by different genetic mechanisms. However, other studies indicate that selection of lambs for increased resistance to parasitic infection also confers a degree of resistance in periparturient ewes (Woolaston, 1992; Morris et al., 1998).
In lambs, BW, PCV during infection, and LFEC at 3 to 5 wk after infection were all moderately heritable. Albers et al. (1987) reported heritabilities of 0.45 and 0.35 for individual PCV measurements taken 4 and 5 wk after infection, respectively, in young lambs, which were similar to the heritability estimates for IPCV and individual PCV measures during infection in this study. However, Baker et al. (2003) reported lower estimates for heritability (0.09 to 0.12) and repeatability (0.49) of PCV in 4- to 6-mo-old lambs under conditions of natural infection in Kenya.
In lambs, heritabilities of LFEC were similar to those reported for H. contortus (Albers et al., 1987) and T. colubriformis (Bisset et al., 1992; Woolaston and Windon, 2001) infections. Albers et al. (1987) reported a heritability estimate for square-root-transformed FEC at 4 and 5 wk after infection of 0.30 ± 0.10 in 5- to 6-mo-old Merino lambs infected with H. contortus. Woolaston and Piper (1996) reported a heritability of 0.23 for untransformed FEC in response to an artificial H. contortus infection in 5- to 6-mo-old Merino lambs. Morris et al. (1997) reported that heritabilities for individual FEC ranged from 0.29 to 0.42 in Romneys under natural mixed challenges on pasture, with the highest heritability recorded at 7 to 8 mo of age. Heritability estimates for LFEC in Dorper- and Red Maasai-sired lambs grazing infected pastures in Kenya were generally less than 0.10, but tended to be higher for Dorper-sired lambs after 6 mo of age (Baker et al., 2003). Barger and Dash (1987) reported a repeatability of 0.56 for LFEC during an extended H. contortus infection in lambs. In comparing current results with those of previous studies, most previous studies did not explicitly consider nonadditive litter effects. Higher estimates of heritability would be anticipated when litter effects are ignored, and removal of litter effects from the analysis in Table 4 increased the heritability estimate of LFEC between 3 and 5 wk from 0.19 to 0.25.
In lambs, the higher heritability of LFEC in lambs at 3, 4, and 5 wk postinfection than at 6 and 7 wk postinfection suggests that expression of genetic variation in FEC in drylot was reduced as worms derived from the initial infection became senescent. In ewes, the pattern of change in heritability of FEC differed from that observed in lambs. Heritabilities of FEC in ewes were highest late in the evaluation period, suggesting that the observed genetic variation was primarily in response to infection obtained from grazing contaminated pastures rather than from the initial artificial infection. Expected values for heritability estimates and for changes in heritability over time must therefore consider the pattern of infection.
The accuracy of genetic evaluation can be increased by repeated sampling. Given the observed heritability and repeatability estimates in Table 4, heritabilities for the mean of three or five measurements of PCV would be 0.45 and 0.54, respectively. Similarly, the heritability of the mean of three LFEC measures at the peak of infection would be 0.28. In ewes, heritabilities for the mean of five measurements of PCV or LFEC would be 0.25 and 0.57, respectively. These predictions were consistent with results of genetic analyses of summary traits and indicate that the average of three LFEC measures taken at peak of infection would be a good selection criterion for reducing fecal egg counts in lambs. Initial PCV or the mean of three to five PCV measures could also be used with LFEC in a selection index to improve resistance, although most authors have recommended LFEC as the primary selection criterion. Woolaston and Windon (2001) suggested using one FEC measure taken between 3 and 5 wk after infection for reduction of FEC in T. colubriformis infections in young Merinos, and Pocock et al. (1995) reported use of a single measure of FEC in industry programs to evaluate genetic resistance to H. contortus in Australian Merino sheep. In contrast, Bishop et al. (1996) suggest that the average of three to four FEC measures be used as the selection criterion to improve resistance to Telodorsagia circumcincta infection in young lambs.
Associations between genetic merit for production traits and parasite resistance differed in lambs and ewes. Lambs with higher genetic merit for body weight were less susceptible to infection, with higher PCV before and after infection and lower LFEC. Also, at the same weaning or postweaning weight EBV, lambs that were phenotypically larger at the time of infection had higher PCV, both initially and when infected. Our results are in agreement with those of Albers et al. (1987), who did not find a significant correlation between resistance to H. contortus and productivity in uninfected lambs. Thus, selection for resistance should not have an unfavorable effect on growth potential of lambs. A selection index approach similar to that proposed by Woolaston (1994) could thus be formulated to identify high-performing, parasite resistant sheep. In contrast to our results, Morris et al. (1997, 2001) reported an unfavorable relationship between resistance to T. colubriformis and postweaning gains and yearling fleece weights.
Ewes with higher genetic merit for growth as lambs were less resistant to infection as adults, with higher FEC after infection, although at the same weaning or postweaning weight EBV, ewes that were phenotypically heavier at the time of infection were better able to cope with infection, as indicated by their higher PCV. Ewes with high genetic merit for growth produce lambs that have higher growth potential and thus are more demanding in terms of nutrients. Bishop and Stear (2001) reported a positive genetic correlation between FEC of ewes in early lactation and the weight of their 4-wk-old lambs. Production of heavier lambs may place extra stress on the ewe during lactation, and the ewe may take more time to recover from effects of lactation, making it more susceptible to infection. In this study, fertility and prolificacy EBV were not associated with FEC, but were positively associated with IPCV and MPCV. Morris et al. (2001) reported a favorable relationship between resistance to Trichostrongylus infections and number of lambs born per ewe mated. However, ewes with twins have been reported to show a higher periparturient rise than ewes with singles (Courtney et al., 1986; Bishop and Stear, 2001). Thus both the timing and conditions associated with measures of both performance and resistance, as well as the interplay of genetic and nongenetic factors, will likely influence expression of relationships between production and resistance.
Implications
The testing protocol used in this study did not result in body weight losses, but produced sufficient differences in fecal egg counts and packed cell volumes to allow testing of resistance of sheep to Haemonchus contortus. This protocol can be integrated into normal flock management and is suitable for on-farm applications. Response to H. contortus infection is moderately heritable, and selection to improve parasite resistance in lambs and ewes is possible. Response to infection with H. contortus seems to be mediated by different mechanisms in lambs and nonlactating ewes, so selection should be applied directly in lambs. Selection for increased resistance will not adversely affect growth in lambs and fertility in ewes, but lambs with high growth potential may be more susceptible to infection as adult ewes.
Footnotes
1 This research was supported by a grant from the Virginia Agricultural Council. The authors thank H. R. Gamble and J. McCray of the USDA-ARS, Beltsville, MD, for providing H. contortus larvae, S. King for skilled laboratory assistance, and the many students from the Virginia Tech Sheep Production class who assisted in collection of data. 
2 Correspondence: Litton Reaves Hall (phone: 540-231-5135; fax: 540-231-3010; e-mail: drnotter{at}vt.edu).
Received for publication April 18, 2003. Accepted for publication February 25, 2004.
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