SU MMARYRelationships between sward height and short-term ingestive behaviour of cattle were examined for two tropical stoloniferous grasses with contrasting growth forms : centipede grass (Eremochloa ophiuroides (Munro) Hack.; (CG) ; strongly prostrate) and bahia grass (Paspalum notatum Flu¨gge; (BG) ; more erect). Turves (500r500 mm) were extracted from field monoculture swards of each grass after the varying duration of re-growth, and presented to animals for a short period (10 bites) to measure bite dimensions (area, depth and volume), bite mass, time per bite and intake rate. In the same re-growth period, CG was always shorter and denser than BG. Bite dimensions, the bite mass and the intake rate of animals increased at a declining rate as the sward height increased for both grasses, showing a tendency for a steeper initial increase, an earlier plateau and a lower maximum in CG than in BG. Due to the difference in the shape of the intake rate response, animals on BG were estimated to require a longer grazing time than those on CG to attain the same daily herbage intake, when the sward is shorter than about 200 mm. The sward height below which the daily intake of animals may be restricted was lower for CG (61-70 mm) than for BG (71-92 mm). The results indicate an advantage of strongly prostrated, highly dense grasses (e.g. CG) over more erect, less dense grasses (e.g. BG) when grazed at a relatively low height (<200 mm). In relation to increasing bite mass, the time per bite pooled over CG and BG was constant until the bite mass reached a critical value (0 . 55 g dry matter (DM)) and thereafter increased linearly with the bite mass, confirming that cattle are able to perform compound jaw movements that gather herbage into the mouth (manipulative jaw movement) and chew herbage already in the mouth (chewing jaw movement) within one cycle of opening and closing of the jaws.
A herd of 28-31 Japanese Black (Bos taurus) cows were allowed to graze an experimental plot comprising monoculture swards of centipedegrass (Eremochloa ophiuroides; CG; preferred grass; 0.30 ha) and bahiagrass (Paspalum notatum; BG; less preferred grass; 0.22 ha) for five consecutive days (09:00-16:00 h) in two seasons (summer and autumn), under a substantial travel cost between the two grasses and a trade-off between the forage preference and some abiotic factors. The two swards were apart from each other and connected by an alley of about 80 m. The CG comprised low (CGL) and high (CGH) availability areas (0.15 ha each). The animals grazed mainly the CGH and CGL, which were in the distance and lacked water and shade, in the morning when the air temperature was relatively low. Then, in the afternoon, they shifted the major grazing area to the BG with water, shade, and close proximity to the barn where they camped. On a daily basis, the animals consistently showed preference for the CGH except one case, switching 6-14 times between the BG and CG. The results demonstrate that animals, even when the preferred species is of lower accessibility and in a less favorable abiotic environment, switch between the preferred and less preferred species and eat mixed diets to show a partial preference. They cope with increased travel costs and trade-offs between forage preference and abiotic environment by reducing switching frequency between the species and shifting target species over the course of the day. Further studies are warranted to test how these mechanisms are successful in maintaining the preference under increased distances and trade-offs.
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