SUBSTITUTION OF ROUGHAGE BY CONCENTRATES FOR DAIRY-COWS

Three main variation factors of the substitution rate (SR) between roughage and concentrates have been studied over a course of eight trails: (1) the concentrate supplementation level (low = L, medium = M, high = H, on average 2 kg of dry matter between two consecutive levels); (2) the type of roughage (maize silage, grass silage or hay); (3) the type of concentrate (a high proportion of starch = ST, of fibre of high digestibility = FH, or fibre of low digestibility = FL). In each trial, the three levels of concentrates were compared using a 3 x 3 Latin square design, repeated with three or four groups of Holstein-Friesian crossbred cows in mid-lactation, each period lasting 1 month. The eight trials corresponded to eight different diets according to the type of roughage and the type of concentrate used. The SRs increase systematically with the concentrate supplementation level, on average 0.47 between L and M and 0.67 between M and H. For a given roughage, the increase in the energy concentration of the concentrate corresponds to an increase in the SR values, mainly between L and M. The average SR values observed when using maize silage (0.7) are higher than those observed with grass silage (0.53) and also for the hay (0.44). These trials show, looking at the three variation factors in the study, that in mid-lactation, the higher the energy balance is, the more important the SR values become.