Effects of Sward Height and Season on Herbage Intake of Strip-grazed Dairy Cows

Abstract

Abstract Herbage intake by strip-grazed dairy cows was estimated, using chromic oxide as marker, during experimental periods covering different parts of the grazing season, different sward heights and different years. Herbage mass, herbage composition and the milk yield, live weight, lactation week and age of experimental animals were also registered with the objective of including those variables that were significant in a model for studying the influence of sward height and season on herbage intake. Pastures were grazed when planned target sward height was obtained. Average sward height over all experimental periods was 15.0 cm (range 7.4-21.2). The grazing season was divided into early season (May 17 to June 19), mid-season (June 20 to July 31) and late season (August 1 to August 20). The data comprise herbage intake measurements on 69 dairy cows with an average milk yield and live weight of 15.6 kg/day and 546 kg, respectively. Pasture allowance was 30 kg dry matter/cow and day. The content of metabolizable energy and the digestibility of organic matter in the pasture was 11.2, 10.2 and 10.2 MJ/kg DM and 77.8, 72.4 and 72.4% for early, mid- and late season, respectively. The variables sward height, season, milk yield, live weight and year, and the interaction between sward height and season, were significant in the statistical analysis and were thus included in the model. In early season, herbage intake increased by 0.8 kg organic matter (OM) per cm increase in sward height. In mid-season, intake was significantly lowered at pasture heights below 9 cm, while in late season there were no differences in intake at different sward heights. Least square means for herbage intake during the early, mid- and late season were 12.8, 12.3 and 10.2 kg OM per cow and day, respectively. Late season intake was significantly lower (P < 0.001 and P <0.01) compared with intake during early season and mid-season, respectively. In the model obtained, the influence of milk yield and live weight on herbage OM intake was 0.2 kg OM per kg milk and 0.5 kg OM per 100 kg live weight, respectively.