Variation in composition of pre-grazed pasture herbage in the United Kingdom, 2006–2012

• 8814 samples of pre-grazed pasture herbage were analysed.
• Mean annual crude protein and nitrate concentrations tended to decrease over the period 2006–2012.
• There was a large range in all components within months.
• High concentrations of crude protein give cause for concern.
• Pre-grazed herbage samples should be taken for analysis at weekly intervals.

A total of 8814 samples of pre-grazed herbage samples was analysed by near infrared spectroscopy calibrated against wet chemistry between March 2006 and October 2012. There were decreasing trends over the seven-year period in annual mean crude protein (CP) and nitrates (NO3). Within years, median values for dry matter (DM), water-soluble carbohydrates (WSC) and metabolisable energy (ME) tended to decrease whilst neutral and acid detergent fibre (NDF and ADF) tended to increase from March to October. There was little change in median monthly NO3 during the season. Median CP tended to decrease from March to May and to increase thereafter. Within months, the range in concentration of most components tended to be greatest in mid-season, with the exception of DM and ADF which had relatively large ranges in March. The range in CP was large in all months except October. Mean DM was 183 (±39.4) g/kg fresh weight. Mean ME was 11.7 (±0.75) MJ/kg DM. Mean concentrations (g/kg DM) were, 214 (±52.0) for CP, 442 (±61.3) for NDF, 233 (±45.0) for ADF, 90 (±24.7) for WSC and 36.5 (±8.04) for oil. Mean NO3 was 425 (±781) mg/kg fresh weight. In view of the large range in values within months it is recommended that pre-grazed herbage samples should be taken at weekly intervals to increase accuracy of grazing management and diet formulation for grazing ruminants.