Effect of nitrogen on intake and digestibility of a tropical grass grazed by Creole heifers

So far, little has been done on the effect of nitrogen fertilizer on intake and digestibility in tropical grazing conditions. The effect of two nitrogen levels, 0 (NF) and 50 kg/ha (F) for three successive grazing cycles, on organic matter intake (OMi) and digestibility (OMd) for Creole heifers (208 kg live weight) was determined. Two groups of four tethered heifers, allowed 18 kg of dry matter daily, grazed individual NF or F circular areas at 28 re-growth days. The heifers were moved and watered daily. Sward characteristics (height, herbage mass, morphological and chemical composition and in situ tiller measurements), OMi, OMd and feeding behaviour (biting rate, bite size, intake rate, grazing and ruminating times) were evaluated simultaneously per circular area. Under nitrogen fertilizer, the stems elongated rapidly (by 133%) as did the leaves, but to a lesser extent (by 40%). Consequently, canopy height, leaf and stem masses and crude protein content increased (respectively by 100%, 66%, 186% and 40%), while total bulk density decreased (by 10.5%). Throughout the successive grazing cycles, there was a particular effect on leaf number, both in the F and NF swards, whereas this parameter was not affected by the fertilizer level. Under fertilizer, OMi and OMd were both 9% greater than for the NF sward. The increase in herbage CP content partly explains the rise in OMi, and there is evidence that rumen load has a determining effect on OMi. Leaf mass is the major factor accounting for the 9% increase in OMd under fertilizer regime. On the other hand, whatever the fertilizer regime, OMd was positively influenced by the greater leaf number throughout the grazing cycles. Biting rate, bite size and intake rate were determined respectively by sward height, CP content and stem fraction, whereas grazing and ruminating times were partially influenced by sward characteristics.