A study of herbage production and its utilisation by dairy cattle from continuously grazed swards

1. The literature is reviewed on the effect of frequency and severity of defoliation on herbage production and quality; on factors affecting herbage utilisation; and on the estimation of herbage mass from measurements of herbage height. 2. Three experiments were carried out; in Experiment 1 a study was made with dairy cows of the effect of stocking rate in the early season on herbage parameters; Experiment 2 examined the growth of herbage within protected areas of the sward; and in Experiment 3, three methods for measuring herbage height were evaluated. 3. In Experiment 1, a high stocking rate (H) in the early season produced a sward of short dense herbage with a greater crude protein, P and K content than medium (M) and low (L) stocking rates although D-values varied little between treatments. Herbage production and utilisation were increased. Selective grazing by the cows kept at a low stocking rate in the early season resulted in patches of long rank rejected herbage. These under-grazed areas were characterised by increased aerial tillering and accumulation of litter in the base of the sward. 4. Tiller population increased throughout the grazing season in contrast to the decline which is often reported under cutting managements and rotational grazing. The difference between treatments in tiller population was small and non-significant. 5. Difficulties were experienced in the calibration of the grass disc to establish a relationship between herbage height and herbage mass, but a better fit and lower S.E. of the estimate was given by a quadratic regression through the origin than the 'best-fit' linear regression. The results suggest that it is essential for the calibrations to cover an adequate range of heights to avoid extrapolation from limited data. The overestimation of herbage production was thought to be due to high estimates of growth recorded in protected areas of the sward. 6. Many of the differences in the results of sward parameters between the day and the night field could be attributed to a difference in grazing pressure rather than to the different sward composition in the two fields. Results for the two fields may have been similar had the grazing area been divided more equally rather than on the 60:40 ratio in favour of the day field. 7. Cows were able to overcome constraints imposed on them at high stocking rates in spring by increasing their frequency of defoliation. They utilised more herbage (kg DM ha[-1]) and produced on average 1 kg more milk per day over the season than cows at low stocking rates in spring. 8. Results in Experiment 2 tended to confirm that the high estimates of herbage production in Experiment 1 were due to overestimation of growth in the protected areas. This suggests that protected areas should be moved sufficiently often for growth within them to be representative of growth on the grazed area - leaving them in one position for 2 weeks when conditions were suitable for rapid regrowth gave an overestimation of growth compared with that on the grazed area. 9. Results in Experiment 3 suggest that estimates of herbage height vary according to the technique used to measure them and that there is a need to define the technique used in experiments involving the measurement of herbage height. The ratio of herbage height measured by a graduated rule, grass disc and grass meter was 1:1.63:1.53