High-digestible silages allow low concentrate supply without affecting milk production or methane emissions

In this study, we tested a response function comprising responses in milk to changes in organic matter digestibility of silages and concentrate supply. We studied the effect of changes in silage digestibility and concentrate supply on milk yield, feed intake, body weight, and methane production using 60 Norwegian Red cows. The experiment was a complete randomized block design comprising 3 periods. The pre-experimental period lasted 20 d and all the cows were fed a common silage for ad libitum intake and concentrate according to yield. Next, response period 1 lasted 17 d and the cows were divided into 2 treatments, where a low-digestible silage (LDS) was fed to half of the cows, and the other half were fed a high-digestible silage (HDS). Both groups were fed silage for ad libitum silage intake. Concentrate was optimized according to the yield and type of silage offered. In this period, the effect of silage was evaluated using a mixed model, including the results from pre-experimental period, with parity as a covariate and animal as a random effect. In response period 2, which lasted 20 d, the concentrate level was evaluated by dividing the silage digestibility treatments further into 3 subgroups. Concentrate was increased by 2 kg of dry matter (DM) per day, decreased by 2 kg of DM/d, or remained unchanged. In response period 1, silage treatments were optimized to obtain similar yields and resulted in a lower concentrate offer to HDS treatment. However, the HDS treatment showed a 3.0 kg of DM/d higher total feed intake due to a higher than expected silage intake. This resulted in 3.5 kg higher energy-corrected milk (ECM). Methane emissions were similar between silage treatments, but HDS showed lower methane per kilogram of DM due to its higher intake. The effect of concentrate supply level and interaction with silage digestibility was evaluated using mixed models, including the results for response period 1, with parity as a covariate and animal as a random effect. The reduction in concentrate offer by 2 kg/d in response period 2 was compensated for by increased 1.3 kg of DM/d of silage intake for HDS, resulting in similar intake (22.1 kg of DM/d and 21.7 kg of DM/d without and with concentrate reduction, respectively) and ECM yields (29.4 and 29 kg of ECM without and with concentrate reduction, respectively). However, concentrate offer reduction could not be compensated for by increased silage intake for LDS and resulted in lower milk yields (27.5 kg of ECM). Increased concentrate showed a higher marginal ECM response (kg of ECM per kg of additional concentrate intake) for LDS (1.8 vs. 3.3 kg of ECM for HDS and LDS, respectively). Thus, the drop in milk yields could be compensated for by increased concentrate offers if LDS are fed. Total methane production increased with increased concentrate intake, regardless of silage digestibility. Methane emissions per unit of milk were affected by total DM intake rather than by changes in silage digestibility and concentrate level. The results of this study are based on short-term periods and could show differences if study periods were longer; the results should be interpreted accordingly.