In order to assess the diet of lactating ewes fed at pasture and the relationship of diet quality to pasture characteristics and milk production (MP), 12 lactating ewes were monitored during five lactation cycles (LCs). Individual faecal samples were collected three times per LC and scanned using a Foss NIRSystem 6500 monochromator. The organic matter digestibility (OMD), organic matter intake (OMI), digestible OMI (DOMI) and the chemical composition of the herbage ingested, i.e. the crude protein (CP) content of the herbage ingested (CPi) and the acid detergent fibre and lignin contents of the herbage ingested (ADFi and ADLi), were determined. The MP of the ewes and the pasture characteristics were measured and their relationships with diet quality were analysed.
OMI and DOMI were higher during the first and the fourth LC (P<0·01). OMD and CPi were higher during the second and the fifth LC (P<0·01). MP varied with OMI (P<0·01) and with DOMI (MP=0·1362×DOMI0·53, R2=0·40, P<0·001), whereas it was negatively correlated with OMD (r=−0·46, P<0·001). OMD and CPi were both positively correlated with the CP content of the pasture (r=0·25, P<0·03; r=0·50, P<0·001), whereas the ADFi was negatively correlated with CP content (r=−0·48, P<0·01). The variation in OMI, DOMI and MP, on the one hand, and OMD and CPi, on the other, throughout the different LCs illustrates the importance of measuring several parameters when evaluating diets at pasture.