Nematode control in spring-born suckler beef calves using targeted selective anthelmintic treatments

Veterinary Parasitology 205 (2014) 150–157

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Nematode control in spring-born suckler beef calves using targeted selective anthelmintic treatments J. O’Shaughnessy a,b,∗ , B. Earley a , J.F. Mee c , M.L. Doherty b , P. Crosson d , D. Barrett e , M. Macrelli f , T. de Waal b a Animal and Bioscience Research Department, Animal & Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, Co. Meath, Ireland b School of Veterinary Medicine, University College Dublin, Belfield, Dublin 4, Ireland c Animal and Bioscience Research Department, Animal & Grassland Research and Innovation Centre, Teagasc, Moorepark, Fermoy, Co. Cork, Ireland d Livestock Systems Department, Animal & Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, Co. Meath, Ireland e DAFM, Sligo Regional Veterinary Laboratory, Doonally, Co. Sligo, Ireland f Faculty of Veterinary Medicine, University of Milan, Via Celoria 10, 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 22 April 2014 Received in revised form 3 July 2014 Accepted 8 July 2014 Keywords: Cattle TST Anthelmintics Suckler Gastrointestinal nematodes Beef

a b s t r a c t As anthelmintic resistance is increasingly being reported in cattle worldwide, there is a need to explore alternative approaches to gastrointestinal nematode control in cattle. A novel approach is the use of targeted selective treatments (TST) where only individual animals are treated instead of the entire group. The study objective was to determine if anthelmintic usage could be reduced using a TST-based approach in rotationally grazed first-grazing season suckler beef calves without affecting calf performance. Eighty-eight spring-born suckler beef calves, naïve to anthelmintics, with an initial mean (s.d.) age and live weight of 159 (22.4) days and 221 (42.4) kg, respectively, were used. All calves were vaccinated at pasture against dictyocaulosis at 8 and 12 weeks old. On August 9th 2013 (Week 0), when the trial began, calves were randomised by age, weight, sex, dam breed and sire breed to one of two treatments: (1) standard treatment (positive control) (n = 44) and (2) TST (n = 44). Samples collected one week prior to the start of the study were used as baseline covariates. Each treatment group was replicated once. All calves in the control groups were treated subcutaneously with levamisole on Week 0 and on Week 6. Individual calves in the TST groups were only eligible for treatment at pasture with the same product if predetermined thresholds were reached [plasma pepsinogen ≥2.0 international units of tyrosine/litre and faecal egg count ≥200 eggs per gram of faeces]. The trial concluded at housing on Week 13. Data were analysed using repeated measures mixed models ANOVA (PROC MIXED) (SAS 9.3). No calves in the TST groups were treated for gastrointestinal nematodes during the study period as they did not reach pre-determined treatment thresholds. Mean (sem) calf daily live weight gain for control and TST groups was 0.90 (±0.04) and 0.92 (±0.03) kg, respectively (P = 0.68). Using an ELISA to detect antibodies to Dictyocaulus viviparus at Week 11, 81% of calves were seropositive. Gastrointestinal nematode challenge

∗ Corresponding author at: Animal and Bioscience Research Department, Animal & Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, Co., Meath, Ireland. Tel.: +353 0879233209. E-mail address: [email protected] (J. O’Shaughnessy). http://dx.doi.org/10.1016/j.vetpar.2014.07.009 0304-4017/© 2014 Elsevier B.V. All rights reserved.

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in spring-born suckler beef calves under these conditions can potentially be controlled with minimal anthelmintic treatments whilst not significantly impairing calf performance, provided appropriate control measures are taken to prevent dictyocaulosis from occurring. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In Ireland, suckler beef calf production is predominantly a grass-based system (Drennan and McGee, 2009), where the majority of calves are spring-born (DAFM, 2012) and are grazed at pasture with their dams until weaning at approximately 7–9 months of age in the autumn. In such a system, calves are potentially exposed to long periods of gastrointestinal nematode (GIN) challenge throughout their first year at pasture. In contrast to other European countries, Bennema et al. (2010) reported that cattle in an Irish pasture-based production system may encounter higher levels of GIN challenge as a result of greater time spent at pasture, coupled with a climate conducive to parasite development and transmission. Despite the nematode challenge encountered, studies on GIN in spring-born suckler beef calves internationally observed that clinical disease in this category of suckler calf was unlikely to occur (Agneessens et al., 1997; Couvillion et al., 1996; Michel et al., 1972; Owen et al., 1989). Although clinical parasitic disease may occur post weaning (Sargison et al., 2010), parasitic gastroenteritis mostly occurs subclinically in spring-born suckler beef calves, with the effects of parasitic challenge being manifested through impaired live weight gain (Forbes et al., 2002). At present, there is no information on the optimal management of, or of the challenge due to GIN in first-grazing season (FGS) suckler beef calves at pasture, or on the relative importance of Dictyocaulus viviparus as a pathogen, under Irish conditions. In a first study of herd health management practices in Irish suckler beef herds, we reported that the majority of farmers treated calves 3 or more times in their FGS for GIN and had similar parasite control practices for autumn- and spring-born calves (O’Shaughnessy et al., 2013). This frequency of anthelmintic treatment of FGS calves was considerably higher than that reported in the UK (Barton et al., 2006) where less than 25% of beef calves were treated 3 or more times in their first grazing season. Subclinical nematode infections in suckler beef calves may significantly affect calf performance (Forbes et al., 2002; Stuedemann et al., 1989) and strategic anthelmintic treatments can significantly improve performance (Hersom et al., 2011; Stromberg et al., 1997). However, as anthelmintic resistance (AR) is increasingly being reported in cattle worldwide (Sutherland and Leathwick, 2011), there is a need to explore alternative approaches to GIN control in cattle. A novel approach is the use of targeted selective treatments (TST) (van Wyk et al., 2006) where only individual animals are treated with anthelmintics as opposed to treatment of an entire group. The aim of the TST approach is to reduce the use of anthelmintics and thus minimise the selection of resistance nematode alleles, thus increasing the effective lifespan of anthelmintics.

At present, there is little published information on the use of TST-based approaches to GIN control in cattle, with the only studies published using performance-based indicators in dairy-bred cattle (Greer et al., 2010; Höglund et al., 2013a). There is a need to examine TST-based approaches to GIN control in suckler beef calves under Irish conditions given the high frequency of anthelmintic usage here (O’Shaughnessy et al., 2013). The present study reports the findings of a TST-based approach to GIN control in rotationally grazed FGS suckler beef calves in Ireland. The aim of the study was to establish (1) the GIN challenge experienced by suckler calves under Irish grazing conditions and (2) the number of anthelmintic interventions required to prevent clinical parasitic disease and maintain calf performance using a TST-based approach to GIN control. The study hypothesis was that spring-born suckler beef calves using a TSTbased approach to GIN control would require minimal anthelmintic treatments in their FGS and would achieve similar levels of live weight gain to calves receiving two anthelmintic treatments, provided calves were vaccinated to prevent D. viviparus infection from occurring. 2. Materials and methods All animal procedures performed in this study were conducted under experimental licence (B100/2869) from the Irish Department of Health and Children in accordance with the Cruelty to Animals Act 1876 and the European Communities (Amendment of Cruelty to Animals Act 1876) Regulation 2002 and 2005. 2.1. Study location and experimental design The study was conducted on a 70 ha farmlet at the Animal & Grassland Research and Innovation Centre, Teagasc, Grange, Dunsany, Co. Meath, Ireland (longitude 6◦ 40 W; latitude 53◦ 30 W; latitude 53◦ 30 N; elevation 92 m above sea level). Study animals consisted of 69 Charolais and 19 Blonde d’Aquitaine-sired spring-born calves and their dams. There were 40 female and 48 male calves. Dams were of predominantly Limousin (n = 43), Charolais (n = 42), Simmental (n = 2) and Aberdeen Angus (n = 1) genotypes. The trial began on August 9th 2013 (Week 0), when calves, naïve to anthelmintics, were randomised by age, weight, sex, dam breed and sire breed and then allocated to one of two treatments: (1) standard treatment (positive control) (n = 44) ((males n = 24; females n = 20)) and 2), TST (n = 44) ((males n = 24; females n = 20)). Each treatment group was replicated once. Mean (s.d.) calf age and live weight on Week 0 was 159 (22.4) days and 221 (42.4) kg, respectively. All calves in the control groups were

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treated subcutaneously with levamisole (1.0 ml per 10 kg bodyweight, Levacide Injection 75 mg/ml, Norbrook Laboratories Limited) on Week 0 and on Week 6. Individual calves in the TST groups were only eligible for treatment at pasture with the same product at the same dosage rate if predetermined thresholds were reached [plasma pepsinogen (PP) ≥2.0 international units of tyrosine/litre (Utyr) and faecal egg count (FEC) ≥200 eggs per gram of faeces (epg)]. All calves were vaccinated at pasture against dictyocaulosis (Bovilis Huskvac, Intervet Ireland Limited) at 8 and 12 weeks of age. The trial concluded at housing on November 5th 2013 (Week 13) when all calves were treated orally with fenbendazole (7.5 ml per 100 kg bodyweight, Panacur 10% w/v oral suspension, Intervet Ireland Limited). 2.2. Sample collection Cows and calves were weighed and parasite burden was monitored according to the sampling timeline in Table 1. Calves were blood and faecal sampled over 2 days every 3 weeks with 44 calves sampled on each sampling day (Table 1) except for Week 11 when all calves were sampled. Individual calf FEC were determined from rectal faecal samples using the McMaster method with a sensitivity of 50 epg (Urquhart et al., 1996). Rectal faecal samples from cows were also collected on Weeks −10, −4 and 2 to determine their FEC using the same method. Composite faecal cultures were performed per replicate for the 2 treatment groups on each sampling occasion (2 g of faeces per calf (n = 22)). Cultures were incubated at 27 ◦ C for 8 days and 100 L3 larvae per culture were identified to genus level using standard identification keys (van Wyk and Mayhew, 2013) on recovery. All L3 larvae were identified when counts were less than 100. The results of the 2 replicates in each treatment were combined and a percentage contribution of each nematode genus was determined for each treatment on each sampling occasion. Composite faecal cultures of cows were performed for each replicate in each treatment group at Weeks −10, −4 and 2 (2 g of faeces per cow (n = 22)). The results of all 4 groups were combined and a percentage contribution of each nematode genus was determined on each sampling occasion. Blood was collected via direct jugular venipuncture. Blood samples were transported to the laboratory, processed and stored −20 ◦ C until analysis. Cows were also blood sampled on Weeks −10, −4 and 2 to determine their PP concentrations. PP concentrations were determined using the method described by Ross et al. (1967). Blood samples collected from calves at Week 11 were tested for the presence of antibodies to D. viviparus at UCD Veterinary Diagnostic Laboratories with an enzyme-linked immunosorbent assay (ELISA) using recombinant major sperm protein as antigen (von Holtum et al., 2008). Grass samples were collected to determine pasture L3 larval burdens using a standard method of collection (Taylor, 1939) except that one collector was used per plot and grass samples were taken using scissors. A 100 g sub-sample for dry matter estimation was removed. The remaining sample was weighed and then soaked overnight in a 20 l bucket containing warm tap-water (25 ◦ C) and

1 ml of Tween 20 detergent. On the following morning, herbage was removed in small handfuls ensuring that as much water as possible was wrung out of the grass. The sample was allowed to sediment for a further 4 h at 4 ◦ C after which the volume in the bucket was reduced to 2 l using a vacuum line. The sample was allowed to sediment again at 4 ◦ C for another 4 h and the volume was then reduced to 90 ml. The sediment was then re-suspended and then poured into 6 centrifuge tubes (Beckman polyallomer 17 ml centrifuge tubes, Beckman Coulter, USA). After centrifugation at 1000 × g for 2 min, the supernatant was removed using a vacuum line. The sediment was resuspended with saturated sodium chloride (specific gravity 1.2) and centrifuged at 1000 × g for 2 min. The centrifuge tubes were then clamped just below the meniscus using an artery forceps and the contents of the upper chamber (2–3 ml) of each centrifuge tube were poured into a cuvette (spectrophotometer 4 ml polystyrene cuvettes with stopper, Sigma–Aldrich Germany). Saturated potassium iodide was added to the cuvette until a positive meniscus was achieved. The cuvette was then sealed with a cuvette lid. The cuvette was placed horizontally on a compound microscope stage and the number of infective larvae was determined using a magnification 100×. Using the estimated dry matter content obtained from the sub-sample, the pasture larval burdens were expressed as number of L3 larvae per kilogram of dry herbage. 2.3. Animal health Beginning at 9 days old, calves were vaccinated against bovine respiratory syncytial virus (BRSV), parainfluenza type 3 (PI3), bovine virus diarrhoea virus and infectious bovine rhinotracheitis (Rispoval RS + PI3 Intranasal, Rispoval 3 BRSV Pi3 BVD and Rispoval IBR-Marker Live, Zoetis Animal Health). Further boosters were given as per manufacturer’s recommendations with a final booster vaccine administered 3 weeks pre-weaning. Calves were vaccinated at pasture against dictyocaulosis (Bovilis Huskvac, Intervet Ireland Limited) at 8 and 12 weeks old. 2.4. General animal and pasture management Calves with their dams were rotationally grazed together on a predominantly perennial ryegrass-based (Lolium perenne) pasture on a 26.2 ha block within the 70 ha farmlet. The cow/calf grazing block comprised of 28 paddocks, 8 of which were reseeded in 2009, with a further 8 reseeded in 2011. Mean paddock area was 0.94 ha (range 0.84–1.2 ha). The four cow/calf groups were moved approximately every 3–4 days into new paddocks. Paddocks were not assigned to particular treatment groups. Target preand post-grazing sward heights were 10–12 cm and 4 cm, respectively. Cows and calves were only moved to other areas of the farmlet when target pre-grazing sward heights were not reached or to graze the headlands of grass silage fields immediately post-harvest. Cows with their calves were turned out to pasture in batches after calving from Week −23 to Week −12 (mean turnout date Week −19). Concentrates were introduced to calves at pasture approximately 2 weeks pre-weaning.

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Table 1 Sampling timeline for faecal egg counts (FEC) and plasma pepsinogen concentrations (PP) in suckler cows and their calves. Timeline

Study week

Date

FEC

PP

Pre-trial

−10 −7 −4

May 29th and May 30th June 19th and June 20th July 10th and July 11th

Cows and calves Calves Cows and calves

Cows and calves Calves Cows and calves

Baseline

−1

July 31st and August 1st

Calves

Calves

Experimental perioda

2 5 8 11

August 21st and August 22nd September 12th and September 13th October 2nd and October 3rd October 24th

Cows and calves Calves Calves Calves

Cows and calves Calves Calves Calves

a

Sample type and animals sampled

August 9th–November 5th 2013.

Gradual weaning of calves was used whereby calves were batch weaned over three days on Weeks 10 and 11. On each occasion approximately, one third of cows were removed from groups and housed on the same day. The calves remained in the paddocks with the remaining cow-calf pairs until such time as all of the calves were separated from their dams. All calves were then subsequently housed on Week 13. The mean age (s.d.) of calves at weaning was 231 (20) days old. 2.5. Animal performance Calf daily live weight gain for the trial period (Week 0–Week 13) was estimated as follows: calves were weighed twice over 2 days pre-trial (Week −1) and a mean live weight was determined for Week −1. Live weight for the start of the trial (Week 0) was determined by estimating the mean daily live weight gain from Week −1 to Week 0. This mean daily live weight gain was multiplied by a factor of 8 and added to the previously recorded mean live weight on Week −1. Calves were weighed on the day before and on the day of housing (Week 13). The mean of these live weights was used as the housing (end of trial) live weight. The daily live weight gain was calculated as total live weight gain for the trial period divided by the number of trial days (n = 88).

interpretation of results. The MIXED procedure of SAS (9.3) was used to examine the effect of treatment on calf daily live weight gain, FEC and PP. The statistical model included the fixed effect of time, gender, dam breed, dam parity, sire breed and their interactions. If the interaction term was not statistically significant (P > 0.05), it was subsequently excluded from the final model. Calf was the experimental unit and was also included in the models as a random effect. Model effects were considered statistically significant when Type I error rate was less than 5%. Variables having multiple observations such as calf FEC and PP were analysed using repeated measures ANOVA (MIXED procedure of SAS 9.3) with terms for treatment group, time, gender, dam breed, dam parity, sire breed, pasture type and their interactions included in the model. Pasture type was categorised as either permanent pasture or previously reseeded (reseeded in either 2009 or 2011) in the model. The pasture type entered into model was the pasture type grazed by calves three weeks prior to each blood and faecal sampling time point. Differences were determined by F-tests using Type III sums of squares. The PDIFF option and the Tukey test were applied as appropriate to evaluate pair-wise comparisons between the group means. Samples collected at Week −1 were used as baseline covariates (FEC and PP). Any calves removed from the study were excluded from all data analysis.

2.6. Meteorological data 3. Results Meteorological data were recorded at an automated weather station at Teagasc, Grange research centre. Monthly cumulative rainfall (mm) and mean daily temperature per month (◦ C) were calculated and compared to the 25 year average (1988–2012). 2.7. Statistical analyses Normality of data distribution was tested using the PROC UNIVARIATE procedure of SAS 9.3. Data that were not normally distributed were transformed by raising the variable to the power of lambda. The required lambda value was calculated by conducting a Box-Cox transformation analysis using TRANSREG procedure of SAS. Data subjected to transformations were used for P-values. However, the corresponding least squares means (Lsmeans) and standard error of the mean (sem) are presented to facilitate

3.1. Anthelmintic usage, calf performance & clinical data No calves in the TST groups were treated for GIN during the study period as they did not reach the pre-determined treatment thresholds. Calf daily live weight gain for the study period for control and TST groups was 0.90 (±0.04) and 0.92 (±0.03) kg day−1 , respectively (P = 0.68). Calf daily live weight gain from birth to weaning for control and TST groups was 1.10 (±0.02) and 1.08 (±0.02) kg day−1 , respectively (P = 0.58). The mean live weight of male and female calves at weaning was 310 (6.7) and 272 (8.0) kg, respectively (both groups). There was no effect of treatment on calf live weight at weaning (P > 0.05). Clinical signs of parasitic gastroenteritis were not evident in calves during the study. Eight calves showed clinical

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signs of respiratory disease during the period from Weeks 5–7 (4 control and 4 TST calves). All calves received antibiotics and non-steroidal anti-inflammatory medication and responded to treatment. Five calves in total were removed from the study at Weeks −10, 5 and 8, respectively. These calves were removed from the study for the following reasons: (1) 2 calves (twin calves) were removed as their dam had lost considerable body condition and required extra feeding, (2) 1 calf was removed because of chronic ill thrift, (3) 1 calf was removed as it had developed an umbilical abscess and required repeated antibiotic treatments, and (4) 1 calf required repeated antibiotic treatments for chronic pneumonia. 3.2. Calf parasite, haematology and trace element status The mean (arithmetic) FEC (s.d.) for all calves at Weeks −10, −7, −4 and −1 were 84 (126), 81 (130), 90 (121) and 109 (122) epg, respectively. The overall Lsmeans (sem) for control and TST group cows was 16 (±3.22) and 6 (±2.75) epg, respectively. Treatment group did affect cow FEC (P < 0.05) whereby FEC in control cows was significantly higher (P < 0.05) than in TST cows at Week 2. There was no effect of parity or time (P > 0.05) for cow FEC. There was a significant effect of treatment, time and a treatment × time interaction on FEC (P < 0.001) whereby FEC increased from baseline (Week 2) in control calves at Week 5 (P < 0.05) and at Week 11 (P < 0.05) while FEC increased (P < 0.05) in TST calves at Week 5 and on Week 8 (Table 2). Pasture type had a significant effect on FEC (P = 0.006). The mean (arithmetic) PP (s.d.) for all calves at Week −10, −7, −4 and −1 were 0.2 (0.20), 0.33 (0.16), 0.38 (0.21) and 0.19 (0.17) Utyr, respectively. Cow PP concentration was affected by time (P < 0.001) but not by treatment (P > 0.05). The overall Lsmeans (sem) increased from 0.6 (0.04) at Week −10 to 1.3 (0.05) Utyr at Week 2. PP concentrations were affected (P < 0.001) by both time and the treatment × time interaction but not by treatment (P > 0.05). PP was increased (P < 0.05) at Weeks 5, 8 and 11 in both control and TST calves relative to baseline. 3.3. Meteorological data The mean daily temperature was lower than the 25 year average from March to June whereas temperatures were higher than the 25 year average from July to October. Monthly cumulative rainfall amounts from April to June were lower than the 25 year average. Thereafter, there were large fluctuations in monthly cumulative rainfall. July was a considerably wetter month than the average. This was followed by two months (August and September) where cumulative rainfall amounts were considerably below the 25 year average. In contrast, October had a large amount of rainfall relative to the 25 year average. 3.4. Pasture burdens and faecal cultures Pasture larval counts were similar for control and TST calves throughout the study period. Mean pasture larval

counts were 97, 176, 167 and 147 L3 /kg DM at Weeks 2, 5, 8 and 11, respectively. Individual paddock burdens ranged from 64 to 249 L3 /kg DM. Cooperia and Ostertagia species were the two main genera recovered in calf faecal cultures conducted at each sampling time point (Table 3). The percentage of Cooperia spp. identified in faecal cultures increased in both treatment groups from Week 2 to Week 11. Ostertagia was the main genus identified in cow faecal cultures on each sampling occasion. 3.5. D. viviparus ELISA At Week 11, using an optical density ratio (ODR) ≥0.5 indicating patent D. viviparus infection, 67 out of the 83 calves were serologically positive (animal-level seroprevalence 81%), 39 of which were calves in the TST group. There was a significant effect of treatment on ODR determined at Week 11 with Lsmeans (sem) for control and TST calves of 0.49 (0.06) and 0.65 (0.07), respectively (P = 0.005). 4. Discussion To the authors’ knowledge, this is the first study to examine the use of a TST-based approach to GIN control in a suckler beef herd. The approach used in this study resulted in a 100% reduction in anthelmintic use at pasture while calf daily live weight gain during the trial period was similar (P = 0.68) in TST and positive control groups. Given the previously reported frequent use of anthelmintics in Irish suckler beef herds (O’Shaughnessy et al., 2013) and the increased reports of anthelmintic resistance in cattle globally (Sutherland and Leathwick, 2011), it was necessary to determine the likely number of anthelmintic interventions that spring-born rotationally grazed suckler beef calves would require during a grazing season to maintain average daily gain whilst not actively encouraging the selection of anthelmintic resistance. Given the absence of published information on the use of TST-based approaches to GIN control in suckler beef herds, standard indicators of parasitic disease were used as TST measures. To this end, both individual calf FEC and PP were used in combination as indicators of parasitic infection due to Ostertagia ostertagi (O. ostertagi) as this is the main pathogenic GIN affecting calves in temperate climates (Urquhart et al., 1996). Based on previously conducted research (Shaw et al., 1997; Shaw et al., 1998), Vercruysse and Claerebout (2001), who recognised the inherent difficulties of identifying a suitable FEC that could be used as an indicator of subclinical parasitic disease, proposed a FEC greater than 200 epg as a basis for the treatment of an individual calf. The PP threshold used in the present study was based on a study by Nansen et al. (1987) who used the same method to quantify PP as described in the present study. The authors observed that when mean PP in a group of calves at pasture exceeded 2 Utyr, calves began showing clinical signs of parasitism. Thus, an individual PP ≥ 2 Utyr was chosen as our treatment threshold. Given the potential for small differences in live weight gain in calves in this study, it was necessary to use a product

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Table 2 Faecal egg counts (FEC) and plasma pepsinogen (PP) concentrations in control and targeted selective treatment (TST) suckler beef calves. Variable

FEC (epg) PP (Utyr)

Treatment (TRT)

Control TST Control TST

Date

P-values

Week 2 (21–22 Aug)

Week 5 (12–13 Sept)

Week 8 (2–3 Oct)

Week 11 (24 Oct)

TRT

Time

TRT × Time

23a,x (11.7) 79a,y (11.3) 0.20a,x (0.03) 0.20a,x (0.03)

116b,x (29.0) 248b,x (28.7) 0.60b,x (0.04) 0.60b,x (0.04)

14a,x (20.8) 244b,y (19.6) 0.40b,x (0.05) 0.60b,x (0.05)

100b,x (24.4) 156a,x (24.2) 0.60b,x (0.06) 0.50b,x (0.06)

<0.001

<0.001

<0.001

NS

<0.001

<0.001

The values are expressed as Lsmeans (± s.e.m.). a,b means within row and within measured variable not having a common superscript differ (P < 0.05) from Week 2. NS; not significant. x,y means within columns and within measured variable not having a common superscript differ (P < 0.05).

where a high treatment efficacy could be expected, so as not to act as a confound for the results produced. With the majority of reports of AR in cattle involving ML-based products (Sutherland and Leathwick, 2011) and considering there have been few reports to-date of AR to levamisolebased products, levamisole was the anthelmintic of choice for the treatment of cattle in this study. The efficacy of levamisole was confirmed during the study using a Faecal Egg Count Reduction Test (data not shown). The FEC in control calves remained very low throughout the study period with FEC only increasing above 100 epg at Week 5. Although FEC in TST calves showed a significant increase at Weeks 5 and 8, values decreased at Week 11. The increase in FEC in TST calves may have been due to the highly fecund Cooperia spp. (Stromberg and Gasbarre, 2006) as the percentage contribution of this genus increased in faecal cultures from Week 2 to Week 11. The PP concentrations remained low in TST calves from Week 2 to Week 11 and the percentage of Ostertagia spp. identified in faecal cultures never increased above 50%, thus further substantiating the finding that the rise in FEC was due to challenge with Cooperia. This challenge with Cooperia was too low to have any effect on calf performance. Although at high levels of challenge, Cooperia infection in calves can result in weight loss (Herlich, 1965; Stromberg

et al., 2012), such a situation is unlikely to exist given the epidemiology of GIN disease in suckler beef calves. Pasture larval burdens remained low throughout the experimental period and were lower than previously reported studies of spring-calving suckler beef herds in temperate climates (Agneessens et al., 1997; Michel et al., 1972). The highest pasture burden of 249 L3 /kg DM was recorded at Week 11 and is well below a threshold of 5000 L3 /kg DM where clinical parasitism may occur (Urquhart et al., 1996). The lower than normal temperatures from March to June coupled with reduced levels of rainfall from April to June relative to the 25 year average may have influenced pasture larval burdens as both temperature and rainfall are important factors that determine the development and distribution of nematode larvae on herbage (Urquhart et al., 1996). Although the reseeding of some paddocks in the previous year might have reduced pasture larval burdens in those paddocks as pasture infectivity in reseeded pastures is low (Echevarria et al., 1993), pasture burdens in non-reseeded paddocks were similar to reseeded paddocks in this study (data not shown). FEC values in cows remained low throughout the study. This finding is in agreement with other studies performed in suckler beef herds (Forbes et al., 2002; Höglund et al., 2013b; Owen et al., 1989; Stromberg et al., 1991;

Table 3 Faecal larval culture results for cows and calves. Study week

Nematode genus Cooperia spp.

Ostertagia spp.

Trichostrongylus spp.

% contribution of each genus −10 −4 2

Cowsa

40 14 17

60 86 78

0 0 5

Control TST

0 45

100 45

0 10

5

Control TST

20 45

80 50

0 5

8

Control TST

25 89

75 9

0 2

11

Control TST

56 89

44 11

0 0

2

Calvesb

a The faecal culture results of all four groups were combined and a percentage contribution of each nematode genus was determined at each sampling time point. b The faecal culture results of each replicate in each treatment were combined and a percentage contribution of each nematode genus was determined for each treatment at each sampling time point.

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Stuedemann et al., 1989). Cow FEC remained relatively constant over time, a finding which has previously been reported in suckler beef cows (Höglund et al., 2013b). Previously, it was suggested that the suckler cow acted to reduce the influence of the calf on pasture larval burdens by consuming the bulk of herbage whilst also producing low numbers of nematode eggs in their faeces (Michel et al., 1972). Stromberg (1997) observed that in a suckler beef herd, the dam has a larger influence on pasture contamination than her calf as strongyle egg output is dependent on animal age, live weight and animal faecal egg count. In a Belgian study on spring-calving suckler beef herds, Agneessens et al. (1997) reported that the dam is the main pasture contaminant in the early part of the grazing season and that the level of GIN parasitism experienced by a suckler calf was determined by a combination of the age of calf going to pasture and the faecal egg output of their dam. Höglund et al. (2013b) concluded that the month of birth of a suckler calf was a more important factor in influencing the resulting level of GIN exposure than the level of overwintered larvae on pasture. This is due to older calves consuming greater volumes of herbage, coupled with their low strongyle egg producing dams voiding large volumes of faeces regularly onto pasture. This may have the effect of minimising the influence of initial levels of pasture burden. Thus, the finding that pasture type (permanent versus reseeded) had a small but significant effect on subsequent calf FEC may be of more significance to dairy calves. As the diet of spring-born suckler calves is heavily reliant on milk for the first 3 months of life (Boggs et al., 1980), calves may only start consuming appreciable volumes of herbage when the overwintered burden approaches negligible levels and thus their dam is likely to be a more important pasture contaminant. Thus, the influence of reseeded paddocks in this study on subsequent levels of GIN parasitism in calves may not be as important as expected. Furthermore, differences between studies in spring-calving suckler beef herds are only likely to reflect the level of gastrointestinal burdens in suckler beef dams. A majority of calves developed patent D. viviparus infections as indicated by the ELISA results at Week 11. The percentage of seropositive calves (81%) is considerably higher than previous reports on the seroprevalence of D. viviparus infection in FGS calves. Höglund et al. (2004) reported an animal-level seroprevalence of 11.8% in a study of FGS dairy and beef calves when calves were sampled at or shortly after housing. Calves in the present study were tested for the presence of D. viviparus antibodies using blood samples collected 12 days pre-housing (Week 11) and it is possible that seroprevalence would have been higher if calves were sampled post-housing. Although there were fewer ELISA-positive calves in the control group, the 2 doses of levamisole coupled with vaccination against D. viviparus infection did not prevent patent infections developing in the majority of control calves. Although, the D. viviparus vaccine is regarded as highly efficacious at preventing clinical disease, it does not prevent patent infections from occurring (Downey, 1965) as evidenced in this study by the number of seropositive calves (81%).

However, as faecal larval counts are lower in vaccinated calves compared to unvaccinated calves (Downey, 1965), there should be less build-up of D. viviparus larvae on pasture due to vaccination. Despite the potential of the D. viviparus vaccine to reduce the level of D. viviparus larvae on pasture, the fact that the majority of study calves, whether treated with anthelmintics or not, developed patent infection highlights the challenge posed by D. viviparus infection under these grazing conditions. The vaccination schedule used here is in contrast to what is recommended by manufacturers. Nonetheless, FGS calves can still be vaccinated at pasture to prevent dictyocaulosis from occurring provided the schedule is completed early in the grazing season (Downey, 1968, 1973). There have been few previous studies on TST-based approaches to GIN nematode control in cattle, with all studies being conducted in dairy-bred cattle. Live weight gain was investigated as a potential TST measure in FGS dairy calves (Greer et al., 2010; Höglund et al., 2013a). Both studies reported large reductions in anthelmintic usage although significant differences in live weight gain between control and TST groups were reported. Thus, live weight gain may potentially be regarded as a relatively crude indicator of parasitic challenge in FGS calves. Many factors such as genetic, nutritional, management and infectious factors may influence live weight gain in suckler beef calves over the course of a grazing season. Differences occurring in live weight gain in spring-born suckler beef calves due to GIN infection may be so small that the use of live weight gains as a TST measure in these calves is a relatively insensitive metric. This is evidenced by the similar live weight gains achieved in control and TST calves despite significant differences in FEC values. 5. Conclusions Our TST approach resulted in a 100% reduction in anthelmintic use in calves at pasture with similar levels of calf performance among treatments. This result was due to the particularly low GIN challenge experienced by calves although this finding is in broad agreement with other studies. As this was a baseline study into the use of TSTs in suckler beef calves, our treatment thresholds are not intended to be a concept of proof for their use. Nonetheless, they do potentially highlight that live weight gains may not be suitable as a TST marker in spring-born suckler beef calves as differences in live weight gain due to GIN infection may be so small such that the use of live weight gain as a TST measure in these calves is without merit. This warrants further investigation. Furthermore, any TST approach to GIN control under these grazing conditions must consider the potential challenge posed by D. viviparus infection as evidenced by high animal-level seroprevalence. This factor alone may render live weight gain redundant as a TST measure under these grazing conditions if used in the absence of a measure to control D. viviparus infection. Conflict of interest The authors declare there are no conflicts of interest.

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