preferred component in the pasture decreases, the cost of selective grazing increases and Ž
the proportion in the diet is likely to decline see Parsons et al., 1994b; Thornley et al., .
1994 . The objective of this study was to test the hypothesis that in response to intestinal
parasitism, grazing lambs will increase the proportion of white clover in their diet as a means of increasing protein intake, and that the extent of this selection for clover will
depend on the grass species available as the dietary alternative.
2. Animals, materials and methods
This study was conducted during autumn 1996 at AgResearch Flock House, approxi- Ž
. mately 50 km west of Palmerston North latitude 408S , in a moist, temperate environ-
ment. The trial design was a 2 = 2 split-plot, with pasture species as main plots, parasite infection level as the split-plot, and there were 2 replications.
2.1. Pastures Ž
. In the autumn of 1995 March , two species of grass with differing morphology,
Ž .
Ž browntop
Agrostis capillaris L. cv. Muster , and perennial ryegrass cv. Grasslands .
Ž .
Nui were each established with white clover cv. Grasslands Tahora on plots of 0.8 ha, with two replicates for each grass–clover combination. Browntop and ryegrass differ in
Ž .
tiller morphology, with browntop tending to form densely tillered swards Harris, 1973 . They also differ in aspects of larval ecology, with pure swards of browntop having a
Ž .
higher density of larvae Trichostrongylus colubriformis and Ostertagia circumcincta Ž
. in the upper stratum of the sward, compared with ryegrass Niezen et al., 1998 . The
Ž trial area was situated on a recent alluvial sandy-loam soil Rangitikei series soil type,
. Cowie et al., 1967 . Each plot received a standard annual fertiliser dressing of 150 kg
ha
y1
of sulphur-enriched superphosphate in the spring. The swards were cut and harvested for silage 1 month before the trial started. One
week prior to the start of the trial, the pastures were grazed by ewes with a low Ž
Ž .
y1
. nematode burden mean faecal egg count FEC 300 eggs g
, at a stocking density of 94 ewes ha
y1
for 24 h to provide each paddock with a light, even contamination of Ž
gastro-intestinal nematode eggs. During the trial, a variable stocking rate Mott and .
Lucas, 1952 was used to maintain a sward surface height of 5 cm. Non-trial,
put-and-take lambs, similar in weight and parasite status to trial lambs, were added or removed as necessary to maintain uniform sward conditions across treatments.
Ž .
Pasture height was measured twice weekly, using a sward stick Bircham, 1981 , by taking a minimum of 25 measurements per plot. Herbage mass was measured monthly,
and hand-pluck samples were taken monthly for separation into botanical components. Additional hand-pluck samples were taken weekly by the zig-zag traverse method
Ž .
Taylor, 1939 to determine larval density on pasture. Third-stage nematode larvae were Ž
counted after separation from herbage by a modified Baermann procedure Anderson .
and Walters, 1973 .
2.2. Animals Recently-weaned Romney ram lambs were randomly allocated to treatments based on
Ž
w
. liveweight, drenched with ivermectin Ivomec , Merial and then grazed on the trial
Ž .
swards for 134 days January 31 to June 14 . Lambs were weighed and faeces sampled at 2-weekly intervals. To create the parasite infection treatment for each pasture type,
one-third of the lambs were drenched with ivermectin at 2-weekly intervals to suppress Ž
. parasite infection suppressive drench, SD . Two-thirds were allowed a moderate level
of infection, but were drenched when mean FEC reached 1000 eggs g
y1
of fresh faeces Ž
. Ž
. epg on any treatment, in order to prevent debilitating infection trigger drench, TD .
These TD-lambs required anthelmintic drenching on February 3, March 17, and May 5. A greater number of lambs were allocated to the TD treatment because earlier
experiments had shown greater variability in liveweight gain in parasitised than in non-parasitised lambs. Put-and-take lambs were added or removed in multiples of three
to maintain a 2:1 ratio of TD to SD lambs. All anthelmintic treatments were given at 300
y1
Ž .
mg kg liveweight 1.5 times the recommended dose rate . Faecal egg counts were
Ž .
done using a modified McMaster technique Whitlock, 1943 , in which one egg counted represented 50 eggs g
y1
fresh faeces. To characterise the parasite species being ingested, lambs that had been reared
Ž .
indoors free of nematode infection tracer lambs were grazed on the swards for 3 weeks, and then housed for a further 3 weeks to allow ingested larvae to mature. The
lambs were then slaughtered and the abomasa and intestine removed to determine worm burden. Digesta was removed from these organs by washing, rinsed through a 60 m
sieve, and 5 aliquots were taken from the washings for adult worm identification and counting, and for counting of fourth-stage larvae. The following genera were identified;
Ž Haemonchus, Ostertagia, and Trichostrongylus in the abomasum mean 6600 nema-
y1
. todes lamb
and Trichostrongylus, Nematodirus, and Cooperia in the small intestine Ž
y1
. mean 11300 nematodes lamb
. There were no significant differences between lambs from ryegrass or from browntop in the number of each species or in the total number of
nematodes. 2.3. Diet selection and intake
Sub-groups of 12 lambs grazing each replicate of the ryegrass and the browntop plots were selected to represent the mean liveweight for all lambs on the plot. These
sub-groups were comprised of 4 SD, and 8 TD lambs. These lambs received an Ž
. intra-ruminal controlled-release capsule Captec NZ containing synthetic C32 and C36
Ž .
alkanes, by oral administration on April 12 day 0 . Faecal samples were taken from each dosed lamb, by rectal grab sampling, on days 10, 12, 14, 17, and 19 after dosing.
These samples were freeze-dried, ground to pass a 1 mm sieve, and bulked on an equal dry weight basis across days. This resulted in a single faecal sample for each lamb
which was then analysed for concentration of individual alkanes ranging in chain length
Ž .
from C27 to C36. Pluck samples of grass browntop and ryegrass and clover were taken at the end of this period to represent the parts of plants being eaten by the lambs. These
samples were dried and ground, and sub-samples analysed for alkane concentrations by
Ž .
gas chromatography, as were the faecal samples. The ratios of even chain synthetic Ž
. and odd-chain alkanes naturally occurring in the plant cuticular wax in the faeces and
herbage were used to derive estimates for intake, proportion of grass and clover in the diet and in-vivo digestibility. Intake was calculated using C32 and C33 alkanes, and in
vivo digestibility using C35 and C36. Diet composition was estimated using a least-
Ž .
squares optimisation procedure EatWhat, Dove and Moore, 1995 . Diet composition, intake and digestibility estimates are mean values for the 10-day period over which the
faecal samples were collected. A further sub-sample of the ryegrass and browntop herbage was analysed for total nitrogen concentration.
Liveweight gain was measured over the 2-week period that encompassed the faecal sampling period, in order to relate these estimates to animal performance. At the start of
Ž .
the intake measurement period lambs on browntop Agrostis capillaris L. and white
Ž .
Ž .
clover BWC were heavier than those on perennial ryegrass Lolium perenne L. and Ž
. Ž . Ž
. white clover Trifolium repens L.
RWC 37.6 kg vs. 34.2 kg, P s 0.002 , but SD
Ž .
lambs did not differ significantly from TD 36.8 kg vs. 35.0 kg, P s 0.19 . At the end of the measurement period, the difference in liveweight between BWC and RWC had
Ž .
diminished 39.8 kg vs. 38.2 kg, P s 0.07 . Daily intake was analysed on the basis of per animal, per unit of liveweight, and per unit of metabolic liveweight. Statistically
similar comparisons were obtained in each case, and results are presented only for intake per animal.
2.4. Statistical analysis For animal-related variables, group means for each treatment were compared by
Ž .
Ž .
analysis of variance ANOVA , using a split-plot model 7 df in which grass species Ž
. formed the main-plot and drench regime the split-plot. Grass species effect 1 df was
Ž .
Ž .
tested against replicate x grass species effect 2 df
and the drench 1 df
and Ž
. Ž
. drench = species effect 1 df was tested against residual error 2 df . Main-plot means
for pasture variables and larval density, and the trigger-drenched group mean faecal egg Ž
. Ž
. count after log
transformation were compared by one-way ANOVA 3 df with grass
10
Ž .
species 1 df tested against residual error.
3. Results
