For dairy farmers a good estimate of the current grass yield is essential for grassland planning. To see to what extent grass yield and nitrogen content can be measured with remote sensing, spectral reflection measurements were compared in a field trial with dry matter yields and nitrogen content of mown grass (‘ground truth’ data). These yields and contents were also used to validate a model-based estimate of the grass yield and the nitrogen content or crude protein content. A practical comparison has also been made with grass height measurements. Reflection measurements gave a rough estimate of the dry matter yield. By making a distinction between location and by including basic growth data, the estimate improved considerably. A model-based growth prediction gave a better estimate of the dry matter yield and improved even further in combination with reflection measurements or grass height. With the current techniques, nitrogen content of grass appears to be difficult to measure and predict.