Lactational performance effects of 3-nitrooxypropanol supplementation to dairy cows: A meta-regression

A meta-regression was conducted to determine the production effects of 3-nitrooxypropanol (3-NOP) and investigate their associations with dose, dietary nutrient composition, and supplementation length in dairy cows. Forty treatment and control mean comparisons extracted from 21 studies conducted or published between 2014 to 2024 were used in the meta-regression. Response variables were DMI, milk yield (MY), ECM yield, ECM feed efficiency, BW, BW change, and concentrations of milk fat, true protein, lactose, and MUN. Treatment and control mean differences (MD) as well as standardized MD were calculated and used for data presentation and statistical analysis, respectively. Dose (± SD; 77.0 ± 33.17 mg 3-NOP/kg DMI), forage-to-concentrate ratio (FC; 58.9 ± 8.83%, expressed as average ± SD% of forage in the diet), dietary concentrations of CP, ether extract, NDF, and starch (16.3 ± 1.84%, 4.3 ± 1.03%, 33.7 ± 4.40%, and 20.9 ± 3.97%, as average ± SD% of DM, respectively), supplementation length (days), and DIM and BW of the cows at the beginning of the study were used as continuous explanatory variables. Mixed-effects models were fitted using the robust variance estimation method. Full models containing uncorrelated explanatory variables (Pearson correlation <0.50) were fitted, and variables were removed from the final model if nonsignificant using the stepwise selection approach. Compared with control, supplementation of 3-NOP decreased DMI and MY by 0.80 ± 0.149 and 0.98 ± 0.250 kg/d, respectively, but only numerically decreased ECM by 0.50 ± 0.298 kg/d. Consequently, feed efficiency was increased by 0.05 ± 0.012 kg ECM/kg DMI with 3-NOP supplementation. Body weight and BW change were not affected by 3-NOP supplementation. Milk fat, true protein, and MUN concentrations were increased by 0.09 ± 0.028%, 0.02 ± 0.006%, and 0.59 ± 0.106 mg/dL, respectively. Increasing 3-NOP dose further decreased DMI and increased milk fat and MUN concentrations. Increasing dietary NDF and FC lessened the negative effect of 3-NOP on DMI and MY, respectively, and decreased its effect on milk fat and MUN (dietary NDF concentration effect only). Similarly, increasing dietary CP lessened the negative effect of 3-NOP on DMI and decreased its effect on MUN. To explain some of the production effects herein observed, ruminal fermentation data from 6 published studies were included in the meta-regression. Supplementation of 3-NOP tended to increase rumen pH and decreased NH₃ concentration. Concentrations of total VFA, acetate, and the acetate-to-propionate ratio were decreased, whereas the concentration of propionate was increased, and that of butyrate was numerically increased by 3-NOP supplementation. Overall, 3-NOP supplementation of dairy cow diets decreased DMI and MY but did not affect ECM because of increased milk fat and true protein contents. Increased milk fat and MUN responses can be explained by shifted ruminal fermentation, and the increased milk true protein response may be explained by increased rumen propionate molar proportion by 3-NOP supplementation.