Compensatory Relationship Between Variables in

G razing is a complex activity and involves searching for and selecting suitable forage, after which it is prehended and taken into the mouth (Vallentine, 1 990). The amount of time spent on each phase of activity is very variable. G razing tim.e is generally lowest when forage is abundant and of good quality and highest when forage is of low quality or availability is limited (Vallentine, 1 990).

To some extent animals can maintain a constant herbage intake with changing sward conditions. Hodgson (1 985a) showed that sheep adjusted intake per bite, rate of biting and grazing time in relation to the amount of green leaf on offer. Biting rate usually tends to decline as sward height or herbage mass increase, and as intake per bite increases, principally because the ratio of manipulative to biting jaw movements increases as intake per bite and the size of individual plant components prehended increase (Chambers et a/. , 1 981 ; Hodgson, 1 986). Intake per bite varies less than bite dimensions, because of compensatory

effects between bite area, bite depth and density (Laca et al. , 1 992) . Laca et al. ( 1 992) reported that the compensation of bite depth and bite area among animals resulted in no significant differences in bite volume, hence in intake per bite in dallisgrass (Paspalum dilatatum Poir.), and that compensation of bite volume and bulk density of the grazed horizons resulted in no significant differences in intake per bite in the lucerne.

Sward height and structure can have a major influence on the rate of pasture consumption (Hodgson, 1 985b; Forbes, 1 988a; Black, 1 990). Black and Kenney ( 1 984) showed that the rate of intake by sheep grazing a series of artificial pastures increased with both the height and density of the pastures. However, Kenney and Black (1 986) reported that both intake rate and pasture availability at maximum intake rate depended greatly on the distribution of plant material within the pasture horizons. When sheep grazed artificial pasture of vegetative subterranean clover where most plant material was in the lamina at the top of the sward, intake rate was less affected by pasture height than for grass, and maximum intake rate approached 27 g/min dry matter when pasture availability was between 2 and 3 tlha dry matter (Kenney and Black, 1 986).

Penning et al. (1 991 a) stated that intake rate reached a maximum apparently because the rate of total jaw movements (prehension bites and mastications) during grazing was constant across all treatments, and that a greater proportion of jaw movements was required to masticate and manipulate the herbage ingested as bite mass increased, and therefore biting rate fell and intake rate remained constant. Similar evidence for maximum intake rates was reported by Black and Kenney "(1 984) and Allden and Whittaker ( 1 970) .

Penning et al. (1 991 b) showed that ewes increased their intake rate over time on a short sward (30 mm) by increasing their rate of prehension biting, while decreasing total grazing time compared with those on a taller sward (90-1 20 mm). This indicates that animals grazing short swards, in some circumstances,

are able to increase grazing efficiency after they become adapted to the sward conditions.

Reciprocal changes in intake per bite and biting rate may balance to maintain a roughly constant rate of intake on relatively tall swards, but on shorter swards any increase in biting rate is inadequate to balance the decline in intake per bite and rate of intake declines (Hodgson, 1 986). Allden and Whittaker ( 1 970) showed that on shorter swards (less than 7.7 cm) animals were unable to compensate for a reduced size of bite by increasing the rate of biting, and the rate of intake declined sharply. A constant rate of intake was maintained by reducing the rate of biting when the mean tiller length was greater than 7.7 cm (All den and Whittaker, 1 970). There was initially a rapid change in rate of intake with increasing tiller length (up to about 1 5 cm), thereafter there was little change in the rate of intake with increase in tiller length.

Increasing grazing time results in more energy being used for activity and less for production, thus the minimum grazing time to achieve an adequate intake is considered to reflect optimal behaviour. Increase in grazing time does appear to be a compensating response on the part of the animal to a decline in the short­ term rate of intake. The intake of pasture may be limited by the time available for eating and ruminating when the rate of the dry matter intake is low (Black,

1 990).

When intake per bite is limited herbage intake can be maintained for a short time by compensating increase in grazing time and rate of biting (Vallentine, 1 990). However, this compensation is seldom adequate to prevent a fall in daily intake once the short-term rate of intake starts to decline (Hodgson, 1 986; 1 990). Le Du

et al. ( 1 979) stated that under rotational management cows did not compensate

for restrictions in available herbage by grazing longer. On extremely short swards intake per bite, rate of biting and grazing time may all decline together (Hodgson , 1 990). G razing and ruminating time may be interchangeable to some degree.

Penning et al. (1 991 b) showed that there was a concomitant decrease in time spent ruminating, and time spent idling remained relatively constant, as grazing time increased.