Effect of long or short-term supplementation with concentrates on the pasture intake of grazing dairy cows

Abstract

Animal Production in Australia 1998 Vol. 22 EFFECT OF LONG OR SHORT TERM SUPPLEMENTATION WITH CONCENTRATES ON THE PASTURE INTAKE OF GRAZING DAIRY COWS C.R. STOCKDALE Kyabram Dairy Centre, Institute of Sustainable Irrigated Agriculture, Kyabram, Vic 3620 Research at Kyabram with unsupplemented cows has shown that the pasture intake/pasture allowance relationship varies with the length of time under treatment; Stockdale (1985) found that cows in full lactation experiments ate more pasture at any given allowance than they did in experiments of one to two months duration. This suggests there may also be a training, learning or adaptation response that will affect the reduction in pasture intake that occurs when supplements are fed. An experiment was undertaken to test the hypothesis that the extent of this substitution effect would depend on the length of time that grazing cows were under supplementation. Forty cows grazed irrigated perennial pasture between October and March. Some of the cows were under treatment for six months, while others entered the experiment for short periods of only four weeks, during the second, fourth and sixth months. Cows in both cases were either unsupplemented, or were offered 5 kg DM of barley-based concentrates/day; all cows had a daily pasture allowance of about 40 kg DM/cow. There were two replicates, with five cows per treatment group. The amount of pasture eaten by each group of cows was obtained by using a rising plate meter to estimate pre- and post-grazing herbage mass. Between 30 and 100 height measurements were taken daily in each treatment and mean plot heights were then converted to mass, using calibration equations developed during the experiment. Levels of substitution were obtained by comparing pasture intakes of unsupplemented cows with those recorded for the cows offered the supplement. Mean substitution levels for each month of the experiment are presented in Table 1. Substitution was highest in October and tended to decline throughout the rest of the experiment. There was a strong positive correlation between level of substitution and pasture intake of the unsupplemented cows (r = 0.82), although there were no significant differences associated with length of time under treatment. Pregrazing pasture height declined from 14.0 to 9.3 cm from October to March, and it was this that was most likely to have had an influence on the outcome in pasture intake and substitution, rather than stage of lactation. Stockdale (1996) has previously reported a positive correlation between level of substitution and herbage mass. It was hypothesised that cows would adapt to the grazing system in the long term, with the suggestion that the level of substitution in the long term animals would decline relative to that of animals that were under treatment for a shorter period. This certainly was not the case in this experiment, with no significant differences existing between the treatments. Differences in intake are only likely if the grazing treatments change the composition or the density of the sward. Table 1. Mean levels of substitution (kg DM reduction in pasture intake per kg DM of supplement eaten) measured during the experiment (daily pasture intakes of the unsupplemented cows are in parentheses) Substitution Long term feeding Oct Nov Dec Jan Feb Mar 0.61 0.52 0.50 0.38 0.45 0.40 (17.2) (14.8) (16.9) (13.9) (14.8) (14.7) Short term feeding 0.43 (14.2) 0.40 (14.2) 0.32 (14.1) STOCKDALE, C.R. (1985). Grass For. Sci. 40, 31-39. STOCKDALE, C.R. (1996). Proc. Aust. Soc. Anim. Prod. 21, 381. 374