Establishing a herd-specific MUN target
Use the following strategy to establish a target MUN level for your herd. If you have a one-group TMR, the strategy is quite straightforward. If you feed multiple rations to lactating cows, it is a bit more tedious as the following process will have to be repeated for each group.
- Balance the diet to just meet (NRC, 2001) requirements for energy, rumen degradable protein (RDP), and microbial protein (MP). Feed the diet for 2 weeks and record the herd or group MUN value (1 bulk tank sample).
- As dietary rumen undegradable protein (RUP) is generally more expensive than RDP, start with RUP, although either way works. Reduce RUP content by 0.25 percent units while holding energy and RDP content constant. Feed the diet for 2 weeks and record the ending milk production, DM intake, and MUN concentration.
- Repeat step 2 until the cows decrease milk production or DM intake.
- The step immediately before the cows lose milk production or DM intake is the requirement for RUP for your herd.
- If there is a loss in production on the very first reduction in RUP, it is possible that the cows were already being fed a deficient diet. In this case, try adding 0.25 percent units RUP to the first ration to see if you get an increase in milk production.
- Once a herd-specific RUP level is determined, repeat the same process for RDP content using 0.5 percent unit reductions while holding energy constant and RUP at the threshold level established above until a loss in milk production or DM intake is experienced.
- The last RDP reduction step before a loss in milk production or DM intake was observed is your herd-specific RDP requirement.
- The final values for RDP, RUP, MP, and MUN are your herd’s target levels.
Feeding to meet but not exceed your herd-specific target RDP and RUP levels will result in the maximum achievable nitrogen efficiency under current feeding conditions and knowledge, and herd MUN values can be compared to target MUN concentrations to determine if the feeding program is staying on target. If MUN increases above the target, the cows are being fed more protein than needed, and nitrogen efficiency has declined. If MUN drops below the target, it is likely that a loss in milk production has occurred or will in the near future, and corrective measures should be taken. In either case, MUN does not provide information regarding the source of the problem. It simply indicates the animals are deficient in nitrogen or have an excess of nitrogen, and you will have to determine whether it is a problem with RDP, RUP, other dietary factors, feed formulation, or animal health. It is also important to recognize that all of the safety margin associated with overfeeding protein has been removed, and thus managing the feeding program to maintain consistency is critical to avoid a loss in production.
The target MUN value should be valid for several years unless you dramatically change your facilities or import different cattle. However, keep in mind that the diet required to obtain the target MUN value may change across several years. Eventually, it may drift due to genetic selection in your herd and should probably be reassessed in 5 years. Thus, you can monitor your herd’s MUN to keep a handle on your nitrogen feeding program and improve animal nitrogen efficiency while simultaneously reducing feeding costs and nitrogen excretion to the environment.