This experiment was conducted to investigate the effects of different dosages and types of medium‐chain fatty acids (MCFAs) on rumen fermentation in vitro under low‐ and high‐concentrate diets. For this purpose, two in vitro experiments (Exp.) were conducted. In Exp. 1, the concentrate–roughage ratio of the fermentation substrate [total mixed rations (TMR), dry matter (DM) basis] was 30:70 (low‐concentrate diet), while in Exp. 2, it was 70:30 (high‐concentrate diet). Three types of MCFAs with octanoic acid (C8), capric acid (C10), and lauric acid (C12) were added accounting for 1.5%, 6%, 9%, and 15% of the in vitro fermentation substrate weight (200 mg or 1 g, DM basis) based on control group, respectively. The results showed that the addition of MCFAs all could significantly reduce methane (CH4) production and the number of rumen protozoa, methanogens, and methanobrevibacter under the two diets with the dosages increased (p < 0.05). In addition, MCFAs had a certain degree of improvement on rumen fermentation and influenced in vitro digestibility under low‐ and high‐concentrate diets, and their effects were related to the dosages and types of MCFAs. This study provided a theoretical basis for the selection of types and dosages of MCFAs in ruminants production.

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The nutritional value of whole crop wheat hay (WCWH) harvested at different maturation stages are different, and its feeding effects on dairy cows have not been thoroughly evaluated. In this study, the in vitro digestibility of whole wheat (Nongda 22) hay harvested during the flowering, late milk and dough stages were evaluated using batch culture technique. The neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents of whole wheat hay decreased by 35.5% and 40.4%, respectively, whereas the non-fibrous carbohydrates (NFC) content increased by 50.3% in WCWH harvested during the dough stage as compared to the flowering stage (p < 0.01). The pH of the fermentation liquid and acetate to propionate ratio was greatest in the wheat harvested during the flowering stage and lowest during the dough stage (p = 0.03), whereas the volatile fatty acid (VFA) concentration was greatest during the dough stage and lowest during the flowering stage (p < 0.01). The dry matter loss (DML) was 9.6% and 6.2% greater (p < 0.01) during the late milk stage than in the flowering or dough stages, and the NDF loss (NDFL; p = 0.01) and ADF loss (ADFL; p < 0.01) was greater in both the flowering and late milk stages. In conclusion, though the content of NDF was lower in the dough stage, and the starch to NFC ratio was greater, we determined that the optimal harvest stage should be the late milk stage due to the greater dry matter digestibility, the relatively greater NFC content and the shorter planting days.