Abstract: Twenty-eight male, weaned Chinese Holstein calves ((156.8±33.4) kg) were used to investigate the effects of dietary forage to concentrate ratio (F:C) and forage length on nutrient digestibility, plasma metabolites, ruminal fermentation, and fecal microflora. Animals were randomly allocated to four treatments in a 2×2 factorial arrangement: whole-length forage (WL) with low F:C (50:50); WL with high F:C (65:35); short-length forage (SL) with high F:C (65:35); and SL with low F:C (50:50).  Chinese wildrye was used as the only forage source in this trial.  The grass in the SL treatments was chopped using a chaff cutter to achieve small particle size (~50% particles >19 mm).  Dry matter intake (DMI) and organic matter (OM) intake was increased by increasing both F:C (P<0.01) and forage length (FL) (P<0.05), while acid detergent fiber (ADF) and neutral detergent fiber (NDF) intakes were only increased by increasing the F:C (P<0.01).  The digestibility of NDF was increased as the FL increased (P<0.01), and it was also affected by interaction between F:C and FL (P<0.05).  Cholesterol (CHO) (P<0.01), leptin (LP) (P<0.05), and growth hormone (GH) (P<0.01) concentrations in plasma were increased as dietary F:C increased.  A significant increase in plasma triglyceride (TG) (P<0.01), insulin (INS) (P<0.05), and GH (P<0.01) levels was observed with decreasing dietary FL.  Ruminal pH values of calves fed with low F:C diets were significantly lower than those in high F:C treatment (P<0.05).  Increasing the F:C enhanced ruminal acetic acid (P<0.05) and acetic acid/propionic acid (P<0.01).  Fecal Lactobacillus content was significantly higher, while Escherichia coli and Salmonella contents were significantly lower in WL and high F:C groups (P<0.05).  Lower fecal scores (higher diarrhea rate) were observed in calves fed with SL hay compared to WL hay (P<0.05).  Denatured gradient gel electrophoresis (DGGE) bands and richness index (S) were significantly affected by the interaction between F:C and FL (P<0.05), under high F:C, band numbers and richness index from WL group were higher than that from SL group (P<0.05), whereas there were no differences between WL and SL groups under low F:C (P>0.05).  Microflora similarity was 50–73% among the different treatments.  It is concluded that the WL with high F:C (65:35) diet is suitable for weaned calves.

Key words: forage to concentrate ratio ,  forage length ,  nutrient digestibility ,  plasma metabolites ,  ruminal fermentation ,  fecal microflora ,  male Holstein calves