Biuret is available exclusively through ADM Alliance Nutrition^® and is approved for use in dairy lactation diets.  Biuret is formed by the controlled decomposition of urea; condensing two molecules of urea into a single molecule of biuret, which retains three of the nitrogen atoms (see Figure 1).  Feed-grade biuret is a mixture containing biuret and urea (maximum of 14%) with small amounts of triuret and cyanuric acid.  Typically, biuret contains a minimum of 35% nitrogen with a crude protein value of 246% (see Table 1). 

Table 1  Nutrient Composition of Biuret
Item	% AF	% DM
DM, %	99.0	-
Protein, %	246	248.5
RDP % CP	100	100
SIP1, % CP	100	100
NPN, % SIP	100	100
NRC 2001 A protein, % CP	100	100
1 For use in formulation, biuret is slowly soluble

Biuret is less soluble than urea.  The combination of structural and physical characteristics slows the rumen digestion of biuret.  The in vitro ammonia release for biuret was similar to that of SBM; whereas, ammonia production from urea was markedly more rapid (see Figure 2; Bartle, et al.,1998).  Similar results were observed for blood serum ammonia, when biuret and urea were compared with SBM in beef feedlot diets (Bartle, et al., 2000).

Research on biuret as a feed ingredient was performed in the late 1950s through the 1970s, mostly in beef cattle diets, where biuret has been used for many years.  The slower degradation and physical properties assist in forming a high-quality, self-fed supplement.  Biuret is at least 7.3 times less toxic than urea, making it ideal when using higher levels of NPN and in self-fed applications.  A small amount of work involved biuret in dairy feeds where biuret was substituted for true protein.  This was before the concepts of RDP and RUP were in practice, and at lower production than is common today, but milk production was unaffected by use of biuret (Fonnesbeck et al., 1975). 

ADM Alliance Nutrition research has evaluated the use of biuret in lactating diets (see Figure 3).  Approximately 0.5 and 1 lb/head/day of soybean meal replaced about 0.1 and 0.2 lb/head/day of biuret.  Diets were equal in crude protein and predicted RUP.  Feeding biuret did not affect milk production or milk composition of the higher production animals.  Milk urea nitrogen (MUN) was similar for each treatment.  In the lower producing cows, milk production was unaffected by biuret use, but when receiving 0.2 lb/head/day biuret, MUN was elevated and fat content was reduced.  The connection between the high MUN and depressed fat is unclear, but is sporadically reported in the literature in studies focused on RDP feeding.  With appropriate balancing of RDP and RUP, biuret was an effective substitute for degradable true protein in these lactation diets.

 

Recommendations:

Nutritional Models:  Biuret is unique as a slowly soluble source of NPN.  Biuret has a slightly greater ammonia release rate than SBM.  Using biuret in NRC 2001 CPM or CNCPS models presents descriptive challenges.  As a source of NPN, it fits biologically into the definition of the “A fraction” as a source of ammonia.  However, the assumption is made that NPN is immediately available to the rumen, which is incorrect for biuret.  The “B fractions” of the models characterize a true protein that supports rumen peptide effects in the CPM and CNCPS models.  The “B fractions” also predict the dietary RUP and, thus, metabolizable protein available to the animal.  While ammonia release rates of biuret are similar to SBM, the NPN conversion to urea and ammonia does not fit the assumptions of true protein within the models.

 
At the present, biuret is best incorporated in models as an NPN ingredient.  In the case of NRC, this means inclusion into the vitamin-mineral category to avoid affecting energy equations.  Within the CPM and CNCPS models, the nitrogen would be 100% SIP and 100% NPN to avoid affecting bacterial efficiency via the peptide equations. 

Rumen Balance:  Because of the broad spectrum of forages and ingredients available, recommendations for degradable protein (RDP) are general guidelines.  In general, RDP should be from 10.5% to 12% of diet DM to maximize rumen bacterial yield.  To meet the demands beyond ruminal supply of MP, the remainder of the protein delivered will be RUP (6-7% of DM).  Below 10.2%, RDP may limit rumen efficiency.  Soluble protein will typically range from 5.5 to 6.5% of DM.  Protein fractions may also be expressed as a percentage of total dietary CP, in which case, RUP will range from 32-40% of CP and SIP from 20-30% of CP.  Practically, RUP balance will be most beneficial from 34-38% of CP.  Biuret should be included in the SIP fraction and the SIP values can float towards the upper end of the range.   Historical recommendations for dietary NPN levels are 1% of diet DM to avoid intake concerns.

Intake:  Biuret should be treated as a source of NPN relative to intake.  There is little data to support improved palatability of biuret compared with urea.  Levels of NPN should not exceed about 1% of the diet to avoid intake concerns.  In practice, this will be affected by the balance of RDP in the diet and management.  Within concentrates, the recommendation is typically 1.5% NPN to avoid palatability concerns. 

Carbohydrates:  Biuret will be less sensitive to inclusion of rapidly fermented carbohydrates than urea.  Use of NPN is typically most effective when balanced against rapidly fermenting carbohydrates to capture the nitrogen in bacterial protein.  Molasses and sugar supplements and refined starches (bakery waste, ground high-moisture corn, etc.) have typically been increased with urea feeding.  With a slower release, biuret will be effective with ground corn and non-forage fiber sources as well.

With Urea:  Combinations of biuret and urea will likely be desired to maximize NPN in diets to reduce protein cost when faced with combinations of sugar, finely ground HMC, and the more rapidly fermented, processed corn silages.  In field practice, for diets having more fermentable energy sources, a 50:50 ratio of urea and biuret may be reasonable, with the amount of biuret rising as the level of NPN is increased and SIP raised beyond 33% CP (see Table 2).

Minerals:  The level of sulfur needs to be reviewed in rations with added NPN.  A dietary ratio of between 10:1 and 12:1 of nitrogen:sulfur is commonly recommended (target 11:1), which would equate to about 0.23-0.25% sulfur.  With a longer time for rumen availability, there is the potential to impact digestion of non-forage fiber sources, such as soyhulls as well, so supplemental cobalt should be considered to maximize the efficiency of the cellulolytic bacteria.

 

Table 2  Maximum Levels of Added Urea and Biuret in Lactation Diets
Ration SIP, % CP	lb/Cow/Day
Before NPN Addition	Urea	Biuret
> 38	0.00	0.00
36 to 38	0.00	0.05 to 0.15
33 to 36	0.05	0.10 to 0.20
30 to 33	0.10	0.10 to 0.25
27 to 30	0.15	0.15 to 0.25
< 27	0.20	0.15 to 0.25