Effect on calf growth of accounting for early in vitro degradation kinetics when composing a calf starter ration

Degradation kinetics of concentrates were investigated in vitro and fed to calves to determine if weight gains reflected in vitro results. Five concentrate ingredients (steam dried sugar beet pulp, drum dried sugar beet pulp, toasted faba beans, untreated faba beans and soybean hulls) were fermented using the in vitro gas production technique in two 48 hour fermentations and degradation parameters were established. Two pairs of calf starter rations were formulated based on the in vitro degradation profiles of the individual concentrates and pelleted. The rations were balanced using NorFor – the Nordic feed evaluation system- to provide a similar net energy (MJ/day) but with differing in vitro degradation kinetics during the first 16 h of fermentation. Each pair consisted of rations made with concentrate ingredients that had an expected in vitro rapid (“Fast”) degradation and an expected in vitro slower (“Slow”) degradation relative to each other during the first 16 h of fermentation. The first pair (Fast_1 and Slow_1) consisted mainly of a single fast or slow in vitro degraded concentrate ingredient, either steam dried sugar beet pulp (48.5 % of dry matter) or drum dried sugar beet pulp (49.5 %), respectively. The second pair (Fast_2 and Slow_2) included two expected fast or slow in vitro degraded ingredients. The extra “Fast” concentrate ingredient, in addition to steam dried sugar beet pulp, was untreated faba beans, which brought the total fast concentrate ingredients in the Fast_2 ration to 64 % of DM. The second “Slow” concentrate ingredient was toasted faba beans, in addition to soybean hulls, which replaced the drum dried sugar beet pulp. The total “Slow” ingredients in the Slow_2 ration was 65.8 % of DM. The first pair of concentrate rations were each fed to 18 calves on a commercial dairy farm, while the second pair was fed to 59 calves each on a beef fattening station. The calves were fed for 8 weeks, and each calf was weighed weekly. The mean total weight gain of each calf after the trial was 68.5 kg and 68.6 for Fast_1 and Slow_1, respectively and 64.9 kg and 63.8 kg for Fast_2 and Slow_2, respectively. There was a tendency for Slow_1 to lead to increased growth compared to Fast_1 (P = 0.052), when accounting for the initial weight of the calves, but there were no differences between Fast_2 and Slow_2. There was no significant effect on feed efficiency. The large differences in fermentation kinetics seen in the concentrate ingredients in vitro were not seen in the concentrate rations or the weight gain response of the growing calves.