This study aimed to investigate the effects of the oat hay feeding method and compound probiotics (CMP) on the growth, health, serum antioxidant and immune indicators, rumen fermentation, and bacteria community of dairy calves from 3 to 5 months of age. Forty-eight female Holstein calves (80 ± 7 days of age, 93.71 ± 5.33 kg BW) were selected and randomly divided into four groups. A 2 × 2 factorial design was adopted for the experiment, with the factors of the oat hay feeding method (fed as free-choice or 16.7% in the diet) and compound probiotics (CMP) inclusion (0.15% or 0%) in the pelleted starter. The results showed that, compared with giving oat hay as free-choice, feeding a diet of 16.7% oat hay increased the pelleted starter intake at 1–84 d (p < 0.05), with an average daily gain (ADG) at 61–84 d (p = 0.02); adding CMP to the pelleted starter did not significantly affect body weight, and reduced the fecal index (p < 0.05). Feeding 16.7% oat hay increased the concentration of IgA, IgG, and IgM (p < 0.01), while adding CMP increased the catalase (p < 0.01) and decreased the concentration of malondialdehyde (p < 0.01) in serum. Feeding 16.7% oat hay increased the ruminal concentration of propionic acid (p < 0.05) and isobutyric acid (p = 0.08), and decreased the ruminal pH (p = 0.08), the concentration of acetic acid (p < 0.05), and the ratio of acetic acid to propionic acid (p < 0.01). Feeding 16.7% oat hay reduced the relative abundance of ruminal Firmicutes, Unidentified-Bacteria, Actinobacteria, Prevotella, NK4A214-group, Olsenella, and Actinobacteriota (p < 0.05); adding CMP increased the relative abundance of ruminal Prevotella, Rikenellaceae-RC9-gut-group, Ruminococcus, NK4A214-group, and Ruminococcus (p < 0.05), and decreased the abundance of Desulfobacterora, Prevotella-7, and Erysipelotricaceae-UCG-002 (p < 0.05). In conclusion, feeding a diet of 16.7% oat hay increased the pelleted starter intake and average daily gain, while slightly reducing the ruminal pH values; adding CMP to the pelleted starter resulted in reduced diarrhea incidence, increased serum antioxidant capacity and immunity, as well as ruminal richness and diversity of microorganisms in dairy calves from 3 to 5 months of age.

In this study, changes in milk performance, nutrient digestibility, hindgut fermentation parameters and microflora were observed by inducing milk fat depression (MFD) in dairy cows fed with a high-starch or a high-fat diet. Eight Holstein cows were paired in a completely randomized cross-over design within two 35 d periods (18 d control period and 17d induction period). During the control period, all cows were fed the low-starch and low-fat diet (CON), and at the induction period, four of the cows were fed a high-starch diet with crushed wheat (IS), and the other cows were fed a high-fat diet with sunflower fat (IO). The results showed that, compared to when the cows were fed the CON diet, when cows were fed the IS or IO diet, they had lower milk fat concentrations, energy corrected milk, 3.5% fat-corrected milk yield, feed efficiency and apparent digestibility of NDF and ADF. However, cows fed the IO diet had a lower apparent digestibility of ether extracts. In addition, we observed that when cows were fed the high-starch (IS) or high-fat (IO) diet, they had a higher fecal concentration of propionate and acetate, and a lower NH₃-N. Compared to when the cows were fed the CON diet, cows fed the IS diet had a lower pH, and cows fed the IO diet had a lower concentration of valerate in feces. In the hindgut microbiota, the relative abundance of Oscillospiraceae_UCG-005 was increased, while the Verrucomicrobiota and Lachnospiraceae_AC2044_group were decreased when cows were fed the IO diet. The relative abundance of Prevotellaceae_UCG-003 was increased, while the Alistipes and Verrucomicrobiota decreased, and the Treponema, Spirochaetota and Lachnospiraceae_AC2044_group showed a decreasing trend when cows were fed the IS diet. In summary, this study suggested that high-starch or high-fat feeding could induce MFD in dairy cows, and the high-fat diet had the greatest effect on milk fat; the high-starch or high-fat diet affected hindgut fermentation and apparent fiber digestibility. The changes in hindgut flora suggested that hindgut microbiota may be associated with MFD in cows.

The objective of this study was to examine the effect of ruminal pulse dose of free linoleic acid (LA) and free docosahexaenoic acid (DHA) on microbial populations in the rumen, duodenal fatty acid (FA) flow, milk composition, and milk FA profiles of Chinese Holstein dairy cows. Four rumen- and duodenal-fistulated Chinese Holstein cows in mid lactation (138.5 ± 10d in milk) were randomly assigned to 3 treatment groups and 1 control group in a 4 × 4 Latin square design over 4 periods (3 wk per period). Diets contained either no LA or 2.7% LA and either no DHA or 0.5% DHA in a 2 × 2 factorial arrangement of treatments. Ruminal pulse dose with DHA increased counts of Megasphaera elsdenii, decreased Fibrobacter succinogenes, but did not affect Butyrivibrio fibrisolvens or Ruminococcus flavefaciens. The pulse dose of LA at 2.7% dry matter had no effect on the population sizes of the 3 major cellulolytic bacterial species or M. elsdenii, and no interaction was observed between LA and DHA. The pulse dose of LA or DHA, either alone or in combination, increased the duodenal flow of vaccenic acid (VA). The milk VA and cis-9,trans-11 conjugated linoleic acid (CLA) contents also increased in response to the fatty acid pulse dose, and the pulse dose of both LA and DHA together had the most profound stimulatory effect. This study indicated that ruminal pulse dose of LA or DHA could be used to increase duodenal flow of VA and the milk contents of potentially health-promoting FA, such as VA and cis-9,trans-11 CLA. These results might be useful in formulating dietary interventions to improve milk cis-9,trans-11 CLA contents.