Sward structure and intake of ruminants

Abstract

Advocates of mixed grazing have argued that grazing habits of different animal species tend to be complementary rather than exclusively competitive. However there were few objective data for variation in pasture intake and diet selection among species grazing the same pasture or on the extent of variation in diet selection between animals of differing age within a species on which to base this assumption.

In the three experiments in this study the objectives were to: compare species intake and diet selection in a range of sward structures; develop techniques that would enable grazing behaviour to be related to prevailing pasture conditions; test the hypothesis that the force required to sever individual bites may determine bite dimensions and resultant bite weight and intake rate, and differences in grazing strategy between animal species.

Diet selection and intake of calves, lambs and kids, simultaneously, but separately, grazing four pasture masses (3000, 3800, 4500 and 5400 kgDM/ha) on a temperate ryegrass white-clover sward, were compared in the first trial. A common pasture allowance for all species was calculated at twice the interspecies mean maintenance requirement (0.5 JME/kg⁰.⁷⁵/d). Animal intake was estimated from faecal recovery using Cr₂O₃ as a marker corrected for diurnal variation. Diet composition and quality were measured in the extrusa (OE) obtained from oesophageal fistulae of four individuals of each species. A further four fistulated older goats and sheep were used to compare diet selection among animals of differing age within a species.

Mean species intake rates were 77.8, 60.0 and 48.7 gDOM/kg⁰.⁷⁵/d for calves, lambs and kids respectively. Lambs were the only species where intake was not affected by pasture mass (PM). All species consumed a diet of similar quality (organic matter digestibility), however botanical composition while similar at low pasture masses differed increasingly as pasture mass increased. Dietary overlap suggested the species were more competitive than complementary when grazing intensively managed pastures. Pasture and animal measures in this trial were too imprecise and/or infrequent to enable species grazing strategies to be characterized.

In the second experiment a technique involving short term grazing of pasture turfs cut from a ryegrass white clover pasture was developed to enable intake and bite variables to be related to sward structure. Two animals of each species progressively defoliated, in 4 grazing periods, 3 turfs of similar mass (4700 kgDM/ha) and composition. Intake rate (IR) of the sheep and goats was greater than that of the calves (124, 100 and 40 mgDOM/kgLW/min, respectively). Intake rate of sheep was insensitive to declining PM or pasture surface height While goat bite weight declined with both PM and surface height it did not effect IR. Goats exhibited an aversion for older vegetative leaf and preferentially consumed young leaf while cattle preferentially consumed the second to oldest leaf. Bite depths of cattle and goats were similar and shallower than sheep (2.9 and 2.7 vs 4.0cm respectively). It was concluded that this technique, with further modifications, was suitable for identifying causal relationships between sward structure, intake and bite variables.

In the first trial of the final series of experiments pure perennial ryegrass turfs (surface area 0.lm² x 0.lm soil depth) were manipulated by clipping to produce 3 pasture heights (5, 10 and 15cm) x 3 pasture structures (varying in leaf to pseudostem ratio and bulk density). Six sheep grazed 4 turfs of each height and density fixed to a force plate. Mean peak bite force, bite weight, bite depth, bite area, bite volume and grazed stratum bulk density were calculated. Peak bite force increased with pasture surface height but significantly so only between 10 and 15cm. Pasture structure had no effect except on the 5cm pasture treatment with the highest grazed stratum bulk density. Bite depth and bite weight increased with pasture height and were not influenced by pasture structure at a constant height. Bite area was similar at all pasture heights, but decreased as grazed stratum bulk density increased. On short pastures (<10cm) proximity to the ground restricted bite depth and bite weight rather than peak bite force. Low bite weights per newton of peak bite force may constrain bite depth and weight rather than absolute peak bite force.

In a further trial the influence of sward and animal factors controlling intake rate were compared on three pasture species (ryegrass white clover, prairie grass and tall fescue) where peak bite forces were similar and not considered to be primarily involved in the control of intake rate. Four sheep grazed 4 turfs of each of the immature pasture species grown from seed in a glass house. IR on the white clover was 50% greater than on prairie grass and 46% greater than on tall fescue. Sheep did not compensate for the lower grazed stratum bulk density of the grasses (0.47, 0.56 vs 1.68 mgDM/cm³ for prairie grass, tall fescue, and white clover respectively) by altering bite dimensions. Prehension rate appeared an important determinant of intake rate. In addition, dry matter content of the grazed stratum accounted for 56 and 29%, respectively, of the variation in intake rate of prairie grass and tall fescue.

Peak bite force and bite rate did not change when 150 mm ryegrass turfs were grazed a second time after all pasture above the mean BD at the first grazing had been removed. However BW (0.07 vs 0.11 gDM) and IR (4.8 vs 7.6 gDM/min) decreased in the second grazing through a reduction in bite area and bite depth. A compensatory increase in bite area (44%) occurred when a grid restricted the preferred bite depth of sheep to two thirds of a paired control turf. Such compensation maintained short term intake rate.

In the final experiment 150 mm turfs were grazed by goats and the peak bite force and intake variables compared with sheep. Peak bite force in goats was 22% of that of sheep. Goat BD was shallower than that of sheep although bite areas were similar. Goats were able to maintain a similar intake rate to goats by virtue of a greater bite rate (42.9 vs 24.6 bites/258 in goats and sheep respectively).