Rumen and blood variables in steers fed grass silage or whole-crop fodder beet silage

The objectives of this study were (i) to examine the effects on rumen and blood variables when grass silage (GS) was replaced with whole crop fodder beet silage (FBS) in the diet of beef cattle, and (ii) an abrupt change from GS to FBS. Six rumen-fistulated Friesian steers (initial bodyweight (BW) 416 kg, SD 43) were offered GS ad libitum for four weeks. The dry matter (DM) consumed daily by each animal in that period (14.3 g/kg BW) was used as the daily allowance for that animal for three consecutive two-week periods, during which the animals were offered diets consisting of increasing proportions (420, 880 and 1000 g/kg) of FBS DM in the total DM. FBS was then offered ad libitum for 14 days. Cattle were then offered GS ad libitum for 14 days, after which GS was substituted with FBS for 14 days. The DM content (g/kg) and contents (g/kg DM) of organic matter, crude protein and total volatile fatty acids (VFA) were 220 and 174, 905 and 749, 153 and 115, and 26 and 75 for GS and FBS, respectively. Animals fed the diets of 0, 420, 880 and 1000 g FBS DM/kg DM had rumen pH, concentrations of ammonia (mg/l), l-lactic acid (mmol/l), d-lactic acid (mmol/l) and VFA (mmol/l) of 6.44, 6.18, 6.61 and 6.75 (linear p < 0.001, quadratic p < 0.001), 149, 104, 65 and 50 (linear p < 0.001), 0.76, 1.69, 1.15 and 3.98 (linear p < 0.01, cubic p < 0.01), 1.65, 2.67, 2.83 and 5.93 (linear p < 0.001, cubic p < 0.001) and 82.7, 81.8, 72.8 and 73.7 (linear p < 0.01), respectively. The corresponding plasma concentrations of urea (mmol/l) were 3.74, 2.32, 1.95 and 1.51 (linear p < 0.001), glucose (mmol/l) were 3.81, 3.51, 3.70 and 3.70 (quadratic p < 0.05) and insulin (μIU/ml) were 19.2, 31.0, 17.4 and 20.4 (quadratic; p < 0.05). Animals offered unsupplemented FBS ad libitum had no obvious symptoms of ill-health and rumen fermentation was qualitatively similar to when offered at a restricted level. When animals were abruptly offered FBS, they had adapted in terms of feed consumption and rumen pH after six days. It is concluded that (i) GS and FBS had different patterns of fermentation in the silo and in the rumen, and (ii) cattle adapted quickly to an abrupt change from GS to FBS.