A network meta-analysis of the impact of feed-grade and slow-release ureas on lactating dairy cattle

A network meta-analysis was conducted to determine the effects of feeding feed-grade urea (FGU) or slow -release urea (SRU) as a replacement for true protein supplements (control; CTR) in high-producing dairy cattle diets. Research papers were selected (n = 44) from experiments published between 1971 and 2021 based on the following criteria: dairy breed, detailed description of the isonitrogenous diets fed, provision of FGU or SRU (or both), high-yielding cows (>25 kg/cow per day), and results that included at least milk yield and composition, but data on nutrient intake, digestibility, ruminal fermentation profile, and N utilization were also considered. Most studies compared only 2 treatments, and a network meta-analysis approach was adopted to compare the effects among CTR, FGU, and SRU. Data were analyzed using a generalized linear mixed model network meta-analysis. Forest plots of milk yield were used to visualize the estimated effect size of treatments. Cows included in the study produced 32.9 +/- 5.7 L/d of milk, 3.46 +/- 5.0% of fat, and 3.11 +/- 0.2% of protein with an intake of 22.1 +/- 3.45 kg of dry matter. Average diet composition was 1.65 +/- 0.07 Mcal of net energy for lactation, 16.4 +/- 1.45% CP, 30.8 +/- 5.91% neutral detergent fiber, and 23.0 +/- 4.62% starch. Average supply of FGU was 209 g/cow per day, whereas the average supply of SRU was 204 g/cow per day. With some exceptions, feeding FGU and SRU did not affect nutrient intake and digestibility, N utilization, and milk yield and composition. However, the FGU reduced the acetate proportion (61.6 vs. 59.7 mol/100 mol) and the SRU reduced the butyrate proportion (12.4 vs. 11.9 mol/100 mol) compared with CTR. Ruminal ammonia -N concentration increased from 8.47 to 11.5 and 9.3 mg/dL in CTR, FGU, and SRU, respectively. Urinary nitrogen excretion increased from 171 to 198 g/d in CTR versus the 2 urea treatments, respectively. The use of moderate doses of FGU in high-producing dairy cows may be justified based on its lower cost.

Automated summary

A network meta-analysis was conducted to compare the effects of feeding feed-grade urea (FGU) or slow-release urea (SRU) as a replacement for true protein supplements (control; CTR) in high-producing dairy cattle diets. The study analyzed 44 research papers published between 1971 and 2021, and found that FGU and SRU did not have significant effects on nutrient intake, digestibility, N utilization, milk yield, and composition in most cases. However, FGU reduced the acetate proportion and SRU reduced the butyrate proportion compared to CTR. The study suggests that moderate doses of FGU may be justified in high-producing dairy cows due to its lower cost.