Protein in Pastures: Can It Be Too High?
The nutrient quality of well-managed pastures is often higher than the same plant material harvested as silage or hay. Well-managed spring and autumn pastures may be 25% crude protein or even higher. On some occasions, we have observed 30% crude protein in the spring. The protein content usually decreases during the warmer months and fiber content tends to decrease.
Is the Protein Content of Pasture Too High?
The topic of high protein in pastures has been discussed at various meetings. With high quality pastures often containing 25% protein or higher, even supplementation with low protein supplements will not reduce the total ration crude protein (CP) down to the suggested 16 to 18%. Not only is the total protein too high, but the protein in pasture is highly degradable in the rumen (70 to 80% of CP) and not efficiently utilized in the rumen.
Grains and supplements that are high in readily fermentable carbohydrates need to be fed to enable rumen microbes to “capture” this protein and synthesize microbial protein, which is ultimately utilized by the dairy cow.
In addition, the rumen undegradable protein (RUP), and the protein and amino acids reaching the small intestine may be inadequate to meet the requirements of early lactation, high producing cows.
The consequences of this excess total and rapidly degradable protein in pasture and inadequate amounts of supplemental fermentable carbohydrates include:
- fast nutrient passage through the rumen
- loose manure
- reduced milk fat percent
- loss of body condition
- less than optimum milk production
When excess protein is fed, the protein is deaminated by rumen microbes to ammonia in the rumen. When inadequate dietary carbohydrates are available to “capture” the ammonia, the ammonia is absorbed from the rumen. High degradability of the protein in pasture can lead to losses of up to 50% of the ammonia-nitrogen from the rumen at high pasture intake. This ammonia is converted to urea in the liver to detoxify the excess ammonia. The metabolic costs associated with absorption of ammonia and detoxifying ammonia to urea require energy. This is commonly referred to as urea cost, or the energy that is used to excrete the excess ammonia from high protein diets. In turn, this energy is not available for milk production, and results in less than optimal animal performance.
Urea Cost – Milk Loss
The energy required for this “urea cost” may result in 3 to 6 lb less milk/cow/day. Pastures with 25% to 30% crude protein instead of 20% can result in this amount of lost milk production. The large milk production loss often seen when cows move to pasture in the spring may be associated with this urea cost. Using nutrition models, up to 9 lb/cow/day lost milk yield may occur in some situations with high producing cows. This process results in inefficient utilization of pasture protein, with perhaps only 16 to 18% of the dietary nitrogen appearing in milk. The excretion of over 80% of the dietary protein is a loss of the most expensive nutrient and is harmful to the environment. Strategic supplementation can increase dietary protein utilization (feed to milk) to about 25%.
Milk Urea Nitrogen
The urea that is produced in the liver from excess rumen ammonia eventually appears in milk as milk urea nitrogen (MUN). Therefore, MUN testing can help monitor excess dietary protein and inadequate dietary carbohydrates. When cows graze pasture as the major forage, the MUN values are generally higher than with a non-grazing feeding program. However, studies at Cornell and Penn State Universities that monitored numerous grazing herds, all of which were feeding supplemental concentrates and forages, found MUN values of 14 to 15 mg/dl. These were not excessive since we consider values above 16 to signal excess dietary protein in relation to dietary carbohydrates.
In a recent study at Penn State, we found somewhat elevated MUN in cows fed only pasture (Table 1). Milk yield was 20 lb/day higher when fed 19 lb of concentrate, indicating a greater than 1 lb milk per 1 lb concentrate fed. Milk fat percent was lower and milk protein percent was higher with supplementation. The MUN was nearly 3 mg/dl lower (14.1 vs. 11.3) with supplementation suggesting improved utilization of dietary protein.
Table 1. Milk production of unsupplemented and supplemented Holstein cows grazing cool season grasses with 20% crude protein during 6-month grazing season.a
|Milk fat, %||3.80||3.30|
|Milk protein, %||2.96||3.10|
After fertilization with nitrogen (N) from urea, the total and rumen degradable protein in pasture often increases. In turn, the MUN increases because of this higher protein intake. In a recent study at Penn State, we found elevated crude protein in pasture (25 to 30%) about two weeks following each of four N fertilizations (Figure 1). In turn, the MUN reached about 18 to 22 mg/dl, a significant increase compared to the MUN prior to the N fertilization. Clearly, these high MUN values, which occurred following application of 50 lb nitrogen fertilizer/A, indicate a high loss of protein in the rumen and a high “energy cost” to excrete the excess urea.
Figure 1. The crude protein % of pasture and the weekly milk urea nitrogen (MUN) content in relationship to four fertilization times (indicated by arrows) with 50 lb N/acre from urea.
Milk production typically declined after each fertilizer application. The use of N fertilizers other than urea may have minimized this situation. These data suggest that N fertilization should occur more than two weeks prior to the cows grazing the pasture. Monitoring of MUN in milk can be a useful tool to monitor the dietary protein and energy with a pasture-based system.
Is Dietary Protein Supplement Needed?
From this discussion, we could conclude that concentrate supplements with proper amounts and types of nonfiber carbohydrates will improve the utilization of the protein in pasture and that the concentrate should contain little additional protein. Grains which supply readily fermentable carbohydrates to “capture” some of this ammonia are most important. We generally recommend the concentrate supplements for primarily pasture-based forage program contain about 12 to 14% total protein. This will provide a total ration of about 16 to 18% CP. This supplement may require the addition of a small amount (0.4 to 0.8 lb) of a protein supplement. This supplement can provide some peptides and amino acids that are required by rumen microorganisms.
Do we need to supply RUP to high producing cows?
The National Research Council nutritional guidelines (2001) indicate that rumen undegradable protein is often needed for cows producing more than above 70 to 80 lb of milk/day. Research studies and field experiences do not report a consistent response to increasing the RUP in supplements. A Penn State study found that small amounts (0.5 to 1.0 lb/day) of RUP increased milk protein yield in multiparous cows averaging 80 lb milk/day and fed pasture as the primary forage. Protein sources such as brewers, distillers, corn gluten meal, and roasted/cooked soybeans are good sources of RUP and 0.5 to 1.5 lb of RUP should be considered with grazing cows producing greater than 70 to 80 lb of milk. Costs need to be considered. However, energy is the first limiting nutrient and must be supplemented to the rumen to optimize the capture of nitrogen from pasture and to optimize rumen microbial production.
To answer the question in the title; yes, the crude protein content of pasture can be too high and be detrimental to cow performance due to the energy required to excrete the N that is not utilized in the rumen. Strategic feeding of concentrate supplements that contain the proper types and amounts of rumen fermentable carbohydrates can improve the utilization of this excess protein and improve milk yield. In addition, improved utilization of the high protein in high quality pasture may improve reproductive efficiency.
Written by Lawrence D. Muller, Professor Emeritus
Published as pages 71-74 in proceedings from "Nutrition of Dairy Cows on Pasture-Based Systems" held March 31, 2003 in Grantville, PA.