The following answer is provided by Larry Chase, dairy nutrition professor at Cornell University. It is excerpted from his presentation at the 2011 4-State Dairy Nutrition and Management Conference in Dubuque, Iowa.
Q: Should you lower dairy ration crude protein levels?
A: You should consider lowering ration crude protein for two primary reasons. The first is to improve profitability by increasing the efficiency of converting feed nitrogen intake to milk nitrogen output while at least maintaining milk production. The second reason is that feeding lower crude protein rations decreases the excretion of nitrogen to the environment and lowers ammonia emissions.
On many farms, there is an opportunity to decrease ration crude protein by 0.5 to 1.5 units with minimal risk to lowering milk production. This can have significant implications on both farm profitability and nutrient management practices.
Even though dairy cow nitrogen metabolism is complex, it can be broken down to a few key points. Nitrogen consumed in feed is either used as a nutrient source to support milk and milk protein production or it is excreted via urine and feces.
A number of research trials have been conducted to explore the ramifications of lower ration crude protein levels on milk production. Keep in mind that many of these have been partial lactation studies rather than full lactation studies.
For example, a Swedish trial concluded that rations with 16 to 17 percent crude protein were adequate for early lactation cows when rations were balanced including RDP and RUP. A Wisconsin study featuring mid-lactation cows averaging about 90 pounds per day reported that diets containing 16.1 percent crude protein had similar milk and milk protein levels to a ration with 18.8 percent crude protein.
Meanwhile, a full lactation study found that cows that consumed higher levels of dietary nitrogen excreted it in manure, rather than using it.
More recently, a New York field trial featuring two dairy herds examined the use of the Cornell Net Carbohydrate and Protein System (CNCPS) model to lower crude protein levels. The initial herd rations were evaluated and ration adjustments suggested that crude protein levels could be lowered, Rations were adjusted a number of times over the eight months of the trial, which took place from September 2008 to April 2009.
Results show that:
- There was an increase in percentage of milk true protein in both herds. This most likely was related to the decrease in ration fat levels and the increase in ration starch levels.
- Milk urea nitrogen values decreased by about two units in these herds. This reflects improved nitrogen utilization by the cow and less nitrogen excretion.
- Ration crude protein levels were lowered about one unit.
- Ration fat levels were lowered and ration starch levels increased as ration crude protein was lowered. This provides an opportunity to increase microbial protein synthesis.
- Total ration metabolizable protein (MP) was decreased in one herd, but total MP changed very little for the other farm even though ration crude protein decreased.
- Manure nitrogen and urinary nitrogen decreased in both herds. This would decrease the ammonia emission potential of these herds.
- Milk nitrogen (as a percentage of nitrogen intake) increased about two to three units. This is an index of improved efficiency of nitrogen use. The ratio of milk nitrogen to urinary nitrogen also increased. This is another indication of improved nitrogen use and less nitrogen excretion.
- Both total and purchase feed cost was reduced in both herds.
- Income over feed cost and income over purchased feed cost increased in both herds.