Herbage characteristics effects on in vivo digestibility and intake by Creole heifers grazing a tropical sward
Résumé
Little investigation has been done on the effects of sward characteristics on intake and digestibility at pasture and even less in tropical swards. We induced sward characteristics variations by fertilizing (F, 50 UN ha-1 by regrowth) or not (NF) and during 3 successive grazing sessions. The sward was grazed at 28 regrowth-days by 8 tethered Creole heifers with a 18 kg DM daily allowance. The heifers were moved and watered daily. Sward characteristics (height, biomass, morphological and chemical composition, in situ tillers measures) were measured by circle simultaneously as OM intake (OMi), in vivo OM digestibility (OMd), and ingestive behaviour (bite rate, grazing and ruminating times) by the heifers. Fertilization in F sward induced a rise in stem (7.7 vs 3.3 cm) and leaf length (6.4 vs 4.6 cm) compared to NF swards. Consequently canopy height was twice and herbage mass was near twice in F sward compared to NF sward. The grazing sessions had a significant effect only on the number of leaves which did not vary with fertilizer application. Digestibility was the main animal parameter related to some sward characteristics, to leaf mass (OMd=0.57 + 1.19 10-3 X - 0.21 10-5 X², R²=0.63, s.e=3.6) and to number of leaves per tiller in each sward, F (OMd=0.43 + 0.09 X - 0.007X², R²=0.72, s.e=1.8) or NF(OMd=1.02 - 0.23 X + 0.03 X², R²=0.50, s.e=2.9). Intake was not closely linked to sward features, except to herbage crude protein content. Bite rate, was influenced primarily by sward height.