K. Griswold and J. L. Firkins1
The Ohio State University Department of Animal
A continuous culture system was used to set passage rates and other variables to study ruminally degradable protein (RDP) and urea as protein sources for mixed ruminal microbes. Ammonia concentration was found to be critical for adequate growth of bacteria, as has been shown in numerous reports. However, results of this research also showed that ammonia supply is critical for protein degradation and that higher RDP increased hemicellulose digestion.
In the rumen, RDP is degraded to small peptides, amino acids, and ammonia prior to incorporation into microbial protein. Although RDP is the major source of protein for microbes, peptides can be washed out prior to breakdown to ammonia or incorporation into microbial protein. Urea is a much cheaper source of ammonia for microbes than is true protein. The Cornell Net Carbohydrate and Protein System (CNCPS), which is used by many dairy nutrition consultants, assumes that the supply of only peptides (and not ammonia) will limit starch-degrading microbes ability to break down starch and that supply of only ammonia (and not peptides) will limit the ability of fiber digesters to grow and degrade fiber. It does not model the needs of ammonia separate from peptides. Because different cows have different passage rates from the rumen and different proteolytic activity among microbes in their rumens, there is a gap in our ability to refine predictions of protein needs for cattle without overfeeding RDP. Continuous culture allows us to characterize more specific needs for RDP compared with numerous studies done with batch culture of single and mixed species of bacteria or with animals (in vivo). Our goal is to use such knowledge to better predict and, therefore, supply the needs of RDP to dairy cows without overfeeding protein and causing more nitrogen to need to be disposed of in manure.
Diets were balanced to provide RDP at 50 or 70% of total protein. Dairy rations typically have 60 to 65% RDP. To ensure adequate supplies of ammonia (separate from complete breakdown of RDP), urea was infused at the rate of 0.4 g/liter of artificial saliva based on a previous study done to make sure this supplied more than adequate (> 5 mg/dl) concentrations of ammonia-N in the fermenters. The design was a 2 x 2 factorial arrangement of treatments in a 4 x 4 Latin square. Selected data are shown in Table 1.
Table 1. Effects of Ruminally Degraded Protein (RDP) and Urea on Concentrations of Metabolities and on Digestibility of Nutrients in Continuous Culture1.
|Urea||No Urea||P values|
|Total VFA, mM||97.5||100.1||78.5||78.6||10.1||0.49||0.01||0.83|
|Ammonia N, mg/dl||7.36||7.42||0.31||0.20||0.53||0.95||0.01||0.77|
|Organic matter (OM) digestibility, %||45.7||50.0||30.1||39.9||3.4||0.01||0.01||0.19|
|NDF digestibility, %||52.7||52.5||47.5||53.2||1.7||0.03||0.03||0.02|
|ADF digestibility, %||46.0||46.8||38.7||41.1||3.1||0.21||0.01||0.65|
|Nitrogen digestibility, %||56.8||69.3||38.7||51.9||7.8||0.01||0.01||0.91|
|Efficiency of bacterial protein synthesis, g N/kg OM truly digested||44.1||48.8||38.3||40.1||3.7||0.08||0.02||0.50|
|1 SE = standard error, VFA = volatile fatty acids, NDF = neutral detergent fiber, and ADF = acid detergent fiber.|
Providing urea to increase ammonia N concentrations increased volatile fatty acid (VFA) production by increasing digestibility of neutral detergent fiber (NDF), nitrogen (N), and organic matter (OM). The continuously fed fermenters optimized bacterial growth, thereby accentuating the need for ammonia N. Because NDF digestibility was affected most by lack of RDP and urea (interaction among treatments) while acid detergent fiber (ADF) digestibility was affected less, our results show that digestibility of hemicellulose (which is part of NDF but not ADF) was decreased more than digestibility of cellulose by lack of peptides. Digestibility of ADF was affected by urea but not RDP. Hemicellulose is degraded by cellulose-digesting bacteria that require ammonia and cannot use peptides. However, it is also degraded by a bacterium, Butyrivibrio fibrisolvens, which also has a major role in protein and starch digestion. Therefore, our results show that models like the CNCPS that assume that ammonia N, but not peptides, is needed for fiber (hemicellulose plus cellulose) digestion oversimplify the actual situation. Much more work is needed to characterize RDP needs of microbes that digest all fiber sources (not just cellulose) to improve prediction of RDP needs for dairy cows.
Interestingly, N digestion (feed protein but excluding urea) was reduced greatly when urea was not infused. The activity of protein degradation probably is proportional to bacterial concentration. Increasing growth factors (such as ammonia) increases bacterial growth (cell division), which increases the amount of proteolytic enzymes. Although this response in increased proteolysis often has been shown to be proportional to the supply of digestible carbohydrate, the effect of ammonia N supply on N digestion has rarely been shown. The efficiency of conversion of degraded N into microbial protein was decreased when RDP, but especially urea, was limiting.
These results confirm the need for ammonia N separate from peptides for optimal microbial growth in the rumen and also the need for peptides for fiber digestion. Our results may help explain why ammonia N concentrations that have been previously reported to maximize OM digestion or microbial protein synthesis often are higher than ammonia N concentrations needed to saturate enzyme systems used by bacteria to convert ammonia N into amino acids.
The results reported should help dairy nutritionists justify future research on protein and nonprotein N sources to supply peptides and ammonia for optimum fiber digestion in the rumen; that is, RDP is needed to provide ammonia as well as peptides. Our results confirm the need to provide adequate RDP before supplemental undegraded (bypass) protein is added to dairy rations. Ultimately, studies such as this will help us formulate rations to meet RDP without extra safety factors (higher protein concentration than needed), which will decrease the excretion of N into the environment.
1 For more information, contact at: The Ohio State University, 223 Animal Science Building, 2029 Fyffe Road, Columbus, OH 43210; (614) 688-3089, Fax (614) 292-1515; email:firstname.lastname@example.org