A network meta-analysis of the impact of feed-grade urea and slow-release urea on lactating dairy cattle metabolism and production.

A network meta-analysis was conducted to determine the effects of feeding feed-grade urea (FGU) or slow-release urea (SRU) as replacement for plant protein (PP) in high-producing dairy cattle diets. Research papers were selected (n = 16) from experiments published between 2006 and 2019 based on the following criteria: dairy breed, detailed description of the isonitrogenous diets fed, provision of FGU or SRU, in vivo trials involving high yielding cows and results that included at least milk yield and composition; data on nutrient intake, digestibility, ruminal fermentation profile, and N utilization were also considered. Most studies compared 2 treatments only, and a network meta-analysis approach was adopted to compare the effects among PP, FGU and SRU. Data were analyzed through a generalized linear mixed model network meta-analysis with SAS/STAT (SAS Institute Inc., Cary, NC, USA), using treatment as a fixed effect and the study as a random effect. Differences were declared at P < 0.05. Average diet contained (DM basis) 1.64 ± 0.09 Mcal NEL, 16.8 ± 1.05% CP, 31.9 ± 3.85% NDF, and 22.1 ± 3.76% starch. The average supply of urea was 167 g/cow/d. The FGU and SRU did not affect nutrient intake and N utilization. The SRU reduced total-tract apparent digestibility of DM (69.6 vs 69.7%, respectively) and increased the proportion of acetate (62.3 vs 62.2 mol/100mol, respectively) in the rumen compared with FGU; and increased ruminal ammonia concentration compared with PP (7.53 vs 6.63 mg/dL, respectively). However, all these differences were numerically small. The DMI (23.1 ± 3.00 kg), milk yield (34.4 ± 6.0 L/d), and milk fat (3.59 ± 4.3%) and protein (3.10 ± 2.2%) content were not affected by treatments. However, cows supplemented with FGU and SRU had lower lactose concentration compared with PP (average of 47.1 vs 47.4 g/kg, respectively). Despite the frequent use of urea in dairy diets, only a limited number of papers met the criteria adopted in the present meta-analysis. Neither the use of FGU or SRU modified milk yield nor composition. Thus, feeding FGU to high-producing dairy cows can be justified by its lower cost in comparison to SRU and PP