Tropical Grasslands (2006) Volume 40, 65–69

Diet selection of steers grazing Stylosanthes hamata cv. Veranograss pastures in north Queensland and its potential influence on botanical composition

R.J. JONES1 AND F.D. HU1,2

1 CSIRO Tropical Ecosystems, Davies Laboratory, Townsville, Queensland, Australia
2 Present address: CSIRO Plant Industry, Davies Laboratory, Townsville, Queensland, Australia

Abstract

The diets of steers grazing 3 Stylosanthes hamata cv. Verano-grass pastures, each stocked continuously at 0.65, 0.95 and 1.25 steers/ha, were studied over a 3-year period. Dietary C4 (grass) and C3 (legume) components were estimated using δ13C measurements of the faeces and pasture components on 14 occasions. In May each year, pasture yield and botanical composition were also measured. The % legume in the pastures of Verano–Urochloa mosambicensis cv. Nixon (Nixon) and Verano–Bothriochloa pertusa cv. Bowen (Bowen) were then compared with the % legume in the diets on the dates closest to the May pasture sampling to calculate a selection index (SI) for legume.
Associated grass species had a greater effect on dietary legume content than did stocking rate (SR) and there was no species x SR interaction (P > 0.05). Overall, steers on Nixon pastures consumed diets with a lower proportion of legume (25%) than those on Bowen pastures (47%), with marked differences in the first year of the study, smaller but still significant differences in the second year and no differences in the third year. Diets of steers on rhodes pastures were intermediate (37%). Dietary legume percentages of steers at high SRs were generally lower than those at the other SRs, but on only 3 of 14 sampling occasions were the differences significant (P < 0.05).
Dietary legume content varied widely over the year on all treatments, with highest values in the late wet–early dry season (May–July) and lowest in the late dry–early wet season (November–February).
Although, overall, the SI for legume was higher (more legume) with Bowen, only in the final year was the difference signifi cant (P < 0.01). This index appeared to be related to pasture composition and not, as its name suggests, to animal preference.
It is concluded that the associated grass had a greater effect than stocking rate on diet selection, and that the trend for steers on Bowen pastures to select a diet higher in Verano over a longer period of the year than those on Nixon pastures, could lead to botanical changes that favour the grass component.

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