It has been known for more than 50 years that rumen bacteria can convert NPN sources into bacterial protein which can then be utilised by the ruminant. The advantage of the conversion is that NPN sources are cheaper than true protein supplements. There are however two issues which should be considered: (1) urea as the major NPN sources may be toxic at high levels, and (2) it has been shown that at high milk yields, microbial protein is limiting in certain amino acids, which then have to be supplemented by protein sources with rumen-bypass characteristics. To protect against urea toxicity, a slow-release product has entered the market, as the toxicity results because of the rapid conversion of urea in the rumen to ammonia. Although these principles are well-established, researchers from time to time revisit the literature, especially as they now have the advantage of robust statistical procedures such as meta-analysis. This then was the intention of the researchers cited.
They conducted a network meta-analysis to determine the effects of feeding feed-grade urea (FGU) or slow-release urea (SRU) as replacements for true protein supplements (Control – CTR) in high-producing dairy cow diets. A total of 44 research papers were fit for analyses from experiments covering the period 1971 to 2021. The cows included in the TMR study produced 32.9 ± 5.7 litres milk per day, 3.46 ± 0.5% fat, and 3.11 ± 0.2% protein, and their DM intake was 22.1 ± 3.45 kg per day. The energy contents of the diets were formulated to enable this production level. The average FGU supplemented was 209 g per cow per day and that of the SRU 204 g per cow per day, which implies about 0.93% in the diet.
With some exceptions, the feeding of FGU and SRU did not affect nutrient intake and digestibility, N utilization, milk yield and milk composition compared to CTR. However, FGU reduced the proportion acetate in volatile fatty acids (VFA) in the rumen and SRU the butyrate proportion compared to CTR. Also, ruminal ammonia-N concentration increased from 8.47 (CTR) to 11.5 (FGU) and 9.3 (SRU) mg per desi litre respectively, and urinary N excretion increased from 171 (CTR) to 198 g per day in the two urea treatments.
In conclusion: Despite the knowledge about the need for bypass protein to support high milk yields, these results show that at least to yields as high as 30 to 35 litres per day (corresponding to most SA cows), urea may be successfully supplemented at about 1% in the diet to cut cost. Since there were no differences between FGU and SRU, there is no reason to use SRU, as it is also more costly than FGU.