BACKGROUND
Excess dietary protein in ruminants increases nitrogen losses in urine and feces, reducing nitrogen use efficiency and contributing to environmental pollution. The authors studied whether lowering dietary crude protein from `13%` to `11%` while supplementing rumen-protected amino acids could preserve growth and improve nitrogen metabolism in Holstein bulls. Lysine and methionine were identified as the first limiting amino acids for ruminants, and the study specifically tested rumen-protected lysine (RPLys) and rumen-protected methionine (RPMet) as a strategy to offset amino acid deficiencies caused by protein restriction.
METHODS
Thirty-six healthy Holstein bulls with body weight `424 ± 15 kg` and age reported as `13 months old` in the abstract and `14 months old` in Methods were randomly assigned to `3` groups of `12` animals each in a completely randomized design. The control group (`D1`) received a high-protein basal diet with `CP13%`. Two low-protein groups received `11%` crude protein diets supplemented with either lower-dose rumen-protected amino acids (`T2`: `RPLys 34 g/d·head + RPMet 2 g/d·head`) or higher-dose supplementation (`T3`: `RPLys 55 g/d·head + RPMet 9 g/d·head`). The rumen protection rate of the amino acid product was `60.0%`. The experiment included a `14-day` adaptation period, a `2-month` feeding period, and a `7-day` sample collection period. Growth performance was assessed with average daily gain (ADG), average daily dry matter intake (ADMI), and feed/gain ratio (F/G). Nitrogen balance was measured from feces and urine collected over `3` days after `5-day` adaptation in metabolic cages. Blood, rumen fluid, liver tissue, rumen microbiota, and hepatic gene expression were also evaluated. Statistical analysis used one-way ANOVA, with significance at `p < 0.05` and tendency at `0.05 < p ≤ 0.10`.
KEY RESULTS
Growth performance did not differ significantly among groups by the tabulated analysis, although there was a trend for higher ADG in `T3`. ADG was `1.37 ± 0.22 kg` in `D1`, `1.42 ± 0.06 kg` in `T2`, and `1.55 ± 0.07 kg` in `T3`, with `p = 0.055`. ADMI was similar across groups: `10.09 ± 1.20 kg`, `10.17 ± 0.83 kg`, and `10.09 ± 0.81 kg` for `D1`, `T2`, and `T3`, respectively (`p = 0.967`). F/G was `7.34 ± 1.05`, `7.20 ± 0.80`, and `6.72 ± 0.75`, respectively (`p = 0.157`).
Nitrogen metabolism showed the clearest treatment effect. Nitrogen intake fell from `216.76 ± 3.49 g/d/·head` in `D1` to `181.81 ± 0.40` in `T2` and `179.69 ± 1.16` in `T3` (`p < 0.001`). Fecal nitrogen decreased from `71.10 ± 0.42 g/d·head` to `59.98 ± 0.76` and `57.50 ± 0.23` (`p < 0.001`). Urinary nitrogen decreased from `62.76 ± 1.20 g/d·head` to `45.74 ± 2.89` and `43.31 ± 0.38` (`p < 0.001`). The fecal nitrogen/intake ratio was `32.81 ± 0.48%` in `D1`, `32.99 ± 0.39%` in `T2`, and `32.00 ± 0.17%` in `T3` (`p = 0.037`). The urinary nitrogen/intake ratio was `28.95 ± 0.33%`, `25.16 ± 1.59%`, and `24.10 ± 0.17%` (`p = 0.002`). Nitrogen utilization rate improved from `38.24 ± 0.70%` in `D1` to `41.85 ± 1.97%` in `T2` and `43.90 ± 0.31%` in `T3` (`p = 0.004`). Nitrogen metabolic rate also increased from `56.91 ± 0.69%` to `62.45 ± 0.31%` and `64.55 ± 0.31%` (`p = 0.002`).
Serum biochemistry showed no significant differences in `ALT`, `AST`, `ALB`, `TP`, `GLU`, or `GH`. However, blood urea nitrogen decreased from `2.80 ± 0.50 mmol/L` in `D1` to `2.31 ± 0.24` in `T2` and `1.81 ± 0.09` in `T3` (`p = 0.026`). Serum IGF-1 increased from `142.91 ± 1.74 μg/L` in `D1` to `144.47 ± 3.36` in `T2` and `149.32 ± 2.14` in `T3` (`p = 0.047`).
Rumen fermentation was largely unchanged except for acetate. Rumen pH was `6.50 ± 0.21`, `6.39 ± 0.05`, and `6.44 ± 0.13` (`p = 0.598`). NH3-N was `12.67 ± 1.34 mg/dL`, `12.29 ± 3.03`, and `12.23 ± 3.35` (`p = 0.978`). MCP was `4.59 ± 1.37 mg/mL`, `4.91 ± 0.41`, and `4.95 ± 0.77` (`p = 0.843`). Acetate increased to `95.54 ± 1.30 mmol/L` in `T3` compared with `86.35 ± 6.65` in `D1` and `87.56 ± 0.94` in `T2` (`p = 0.026`). Propionate, butyrate, and acetate/propionate did not differ significantly.
Rumen microbiota alpha diversity did not differ significantly: Shannon `8.37 ± 0.57`, `8.53 ± 0.37`, and `8.09 ± 0.49` (`p = 0.462`); ACE `2178.57 ± 281.67`, `2063.77 ± 81.19`, and `1942.29 ± 90.40` (`p = 0.224`); PD-whole-tree `94.83 ± 5.33`, `95.79 ± 5.48`, and `88.50 ± 6.73` (`p = 0.218`). At the genus level, `T3` had lower `Ruminococcaceae_NK4A214_group` (`1.40 ± 0.52` vs `2.61 ± 0.42`, `p = 0.030`) and lower `Christensenellaceae_R-7_group` (`1.02 ± 0.41` vs `2.04 ± 0.36`, `p = 0.022`), but higher `Prevotellaceae_YAB2003_group` (`0.59 ± 0.05` vs `0.27 ± 0.02`, `p = 0.011`) and higher `Succinivibrio` (`0.53 ± 0.08` vs `0.20 ± 0.04`, `p = 0.001`).
The liver gene expression results suggested altered nitrogen handling. The paper states that `CPS-1`, `ARG`, and `N-AGS` were significantly upregulated in `T3` (`p < 0.05`), while `ASS` and `OTC` were upregulated in both `T2` and `T3` compared with `D1` (`p < 0.05`). It also states that `IRS1`, `PDK`, `S6K1`, and `eIF4B` increased significantly (`p < 0.05`), `mTORC1` increased with `p = 0.09`, and `TSC1` decreased significantly (`p < 0.05`). Exact fold-change values for most genes were not clearly reported.
CLINICAL IMPLICATIONS
This is not a human clinical study, but it has practical veterinary and livestock implications. The findings support that a lower-protein diet (`11%` CP) supplemented with higher-dose rumen-protected lysine and methionine can reduce fecal and urinary nitrogen losses while improving nitrogen utilization and biochemical markers linked to protein metabolism. Importantly, the strongest evidence is for improved nitrogen efficiency rather than definitively improved growth, because the ADG difference had `p = 0.055`. The study also notes limited sample sizes for several endpoints and some inconsistencies between the abstract and tabulated microbiota results, so the conclusions are promising but should be interpreted cautiously.