EFFECTS OF SWARD HEIGHT AND CONCENTRATE SUPPLEMENTATION ON THE INGESTIVE BEHAVIOR OF SPRING-CALVING DAIRY-COWS GRAZING GRASS CLOVER SWARDS

Forty-eight, lactating, spring-calving, multiparous, Holstein-Friesian cows, continuously stocked on mixed grass-clover swards, were blocked by calving date, parity and milk yields in week 2 of lactation and randomly allocated to six treatments: 0 (U) or 4 (S) kg concentrate supplementation at sward heights of 40, 60 or 80 mm as measured by a rising plate meter. Jaw movements were recorded automatically for 24 h for a different cow on each treatment on each of 6 days during two periods (15-25 June (P1) and 25 July-6 August (P2)). Swards of 40 mm height were not used in P2. Total time grazing, ruminating or idling, and the number and duration of bouts of these activities, biting rate while grazing and chewing rate, number of boluses and chews per bolus while ruminating were recorded. Data were smoothed to give a minimum bout duration of 5 min. Herbage intakes were estimated using n-alkanes. Grazing time was greater at 40 mm than at 60 or 80 mm when unsupplemented but less when supplemented. Proportionately 0.88 of grazing time and 0.67 of ruminating time by dairy cows occurred during daylight with a large evening meal particularly evident. On the 40 mm sward unsupplemented animals grazed for longer with a higher biting rate but lower bite mass and less mastication and rumination. When supplement was offered at the 40 mm sward height animals appeared to 'give up' harvesting the sward more readily. Durations of ruminating and idling bouts were similar across all treatments with low variability.