Evidence from the literature regarding the sward characteristics
which determine ingestive behaviour and herbage intake in the grazing
ruminant is contradictory and inconclusive. Variation in sward height has
usually been confounded with concomitant changes in sward density, and
often digestibility, making objective interpretation difficult.
In the current work, a series of grass and cereal swards was
produced using d ifferen t seed rates at sowing in an attem pt to obtain a
large and independent variation in sward height and density. These
swards were grazed within three months of sowing to minimise
differences in maturity and digestibility. A few established ryegrass
swards were also grazed, and in the second of the two experiments
grazing or cutting pre-treatments were employed to increase further the
range of sward conditions.
Experiment 1, run over two grazing seasons, comprised a series of
trials on 33 large plots which were stocked with sheep and cattle (1983)
or just sheep (1984). Swards were grazed down over eight days whilst
changes in sward canopy structure, ingestive behaviour and herbage
intake were measured.
The quantity of cattle data collected was limited, but results for
the sheep clearly indicated that bite weight had the dominant influence
on herbage intake. Bite rate and grazing tim e tended to increase as
bite weight and intake declined, both during the defoliation of a sward
and when comparing responses between swards. Bite weight was strongly
influenced by bite depth, and the sheep generally grazed deeper, taking
heavier bites, when the sward was taller and more digestible. The bulk
density of the grazed sward stratum had a minor, and unexpectedly
negative, e ffe c t on bite weight. Possible explanations are given.
A substantial proportion of the variance in both bite depth and bite
weight was attributed to undescribed differences between crops. Since
indoor feeding trials did not indicate any intrinsic herbage qualities
which significantly influenced voluntary intake, these differences probably
reflected unmeasured structural variables.
Experiment 2, run in 1984, involved a more controlled approach
than the large plot trials. Sheep w ere confined in cages and allowed to
take only 20 bites from small patches of sward. Measurements of bite
weight, depth, area and volume were related to the characteristics of
seventeen contrasting swards.
Surface height had a strong positive e ffe c t on bite depth, and
consequently bite volume and bite weight both increased on taller
swards. The variables which determined bite area were less obvious, but
within a given grass species bite area appeared to be related positively
to surface height and negatively to the population density of grazed
plant units.
Grazed stratum bulk density, which varied independently from
surface height, also had a positive e ffe c t on bite weight. The relative
importance of these two key sward variabies in determining bite weight
varied with the range of sward heights under consideration. Their
effects, however, were independent and additive, producing a pianar joint
response surface.
The advantages of the new grazing cage technique are discussed
and suggestions made for further studies.
