Relationship Between Ruminal Ammonia and Nonprotein Nitrogen Utilization by Ruminants. I. Development of a Model for Predicting Nonprotein Nitrogen Utilization by Cattle 1 R. E. R O F F L E R and L. D. S A T T E R Department of Dairy Science University of Wisconsin Madison 53706 ABSTRACT
INTRODUCTION
The influence of ration composition on mean ruminai ammonia concentration was studied by collecting samples of ruminal ingesta from cattle fed rations varying in crude protein and total digestible nutrient content. A minimum of four sampling times distributed throughout the day permitted calculation of mean ruminal ammonia concentrations. Mean ruminal ammonia concentration was positively related to dietary crude protein concentration and negatively related to total digestible nutrient concentration. It is postulated that mean ruminal ammonia concentration may be a useful criterion for predicting efficacy of nonprotein nitrogen supplementation. A quantitative approach for evaluating nonprotein nitrogen supplementation based upon determination of the point at which ruminal ammonia exceeds the requirement (5 mg ammonia nitrogen/100 ml rumen fluid) of the ammonia-utilizing bacteria is proposed. Dietary conditions expected to result in excessive concentrations of ruminal ammonia are defined and recommended upper limits for nonprotein nitrogen supplementation are presented. Theoretical relationships between composition of the unsupplemented ration, amount of nonprotein nitrogen added, and efficiency of nonprotein nitrogen utilization are discussed. The practice of adding nonprotein nitrogen so as to exceed 12 to 13% crude protein in typical dairy or feedlot rations needs to be reevaluated.
Qualitative aspects of nonprotein nitrogen (NPN) utilization are well understood. For example, NPN is used most efficiently when small amounts are added to rations low in protein and relatively high in fermentable energy. Quantitative information regarding NPN utilization is lacking, however. Dietary protein concentrations above which NPN supplementation ceases to improve animal performance is not known. Understanding the limits of NPN utilization is necessary for profitable use of NPN supplements in ruminant rations. It is reasonable that NPN will be of little benefit to ruminants unless it is converted first into ammonia and subsequently utilized for microbial protein synthesis in the rumen. This assumption rests on the observation that swine and poultry fed rations of natural feedstuffs do not benefit from urea supplementation. Maintenance of ruminal ammonia concentrations in excess of the bacterial requirement would result in wastage of nitrogen, adding cost but no benefit. Satter and Slyter (20), using continuous culture fermentors, demonstrated that rumen bacteria can scavenge adequate amounts of ammonia from dilute solutions. An ammonia concentration of 5 mg NH3-N/100 ml rumen fluid was sufficient to support maximal rates of microbial growth. Increasing the ammonia concentration above this amount resulted in no further increase in microbial protein production. Knowledge of dietary conditions which result in ruminal ammonia concentrations in excess of bacterial requirements is essential for understanding and predicting NPN utilization. The purpose of our research was to determine the influence of ration composition on mean ruminal ammonia concentration. This information was used to develop a model which quantitatively estimates NPN utilization for various types of cattle rations. Theoretical considerations to develop the model are in this
Received September 26, 1974. i Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by the Soybean Research Council of the National Soybean Processors Association, Washington, DC.
1880
RUMINAL AMMONIA AND NITROGEN UTILIZATION paper. Supporting evidence, taken from published studies, is reviewed in a companion paper (18). E X P E R I M E N T A L PROCEDURE
The influence of ration composition on mean ruminal ammonia concentration was studied by collecting ruminal ingesta samples from 211 cows maintained under a variety of feeding programs. All rations in the 35 trials were formulated from natural protein sources. Ration crude protein (CP) and total digestible nutrient (TDN) concentrations were varied by changing ration ingredients and by altering their relative proportions. Following minor ration changes (e.g., proportion of soybean meal in concentrate), animals were adapted to a ration for a minimum of 1 wk before sampling. Longer adaptation periods were used following major ration changes (e.g., t y p e of roughage). Proportions of ration ingredients as well as dietary CP and TDN concentrations are in Table 1. Crude protein (N x 6.25) was determined by the Kjeldahl procedure. Ration TDN was calculated from National Research Council (NRC) tabular values (16). All CP and TDN values are on a dry matter basis. Most of the 1033 ruminal ingesta samples were collected via stomach tube; a few samples were obtained via rumen fistula. Direct comparison of method of sampling showed close agreement in ruminal ammonia values. Animals were sampled at least four times during the day, with mean ruminal ammonia concentrations obtained by averaging results from the individual samples. When ration ingredients were fed separately, sampling times were spaced to allow all ingredients to influence the average. Ammonia was determined in the supernatant fraction of centrifuged rumen fluid by steam distillation over magnesium oxide (1). Simple and multiple regression equations relating mean ruminal ammonia concentration to ration composition were computed. Because the number of observations varied among trials, regression analyses (24) were weighted. RESULTS A N D DISCUSSION
Mean ruminal ammonia concentrations ranged from .8 to 56.1 mg NH3-N/100 ml rumen fluid (Table 1) and increased as the percent CP in the ration dry matter increased
1881
60
"E 50
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~=10.57-2.50 X+ .15gX "~ Y=RUMINAL NH3-N (mcj/100 m,D X=°~ CP IN DM J
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12
14
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16 18 a.~ CP IN DM
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01
20
22
I 24
26
FIG. 1. Relationship between dietary crude protein (CP) and ruminal ammonia. (Fig. 1). The positive relationship between mean ruminat ammonia concentration and percent dietary CP is described by the equation: Ruminal NH3-N (rag/100 ml) -- 1 0 . 5 7 -2.5 %CP + .159 %CP2; r 2 = .88.. Both linear and quadratic effects were highly significant (P
INTRODUCTION
The influence of ration composition on mean ruminai ammonia concentration was studied by collecting samples of ruminal ingesta from cattle fed rations varying in crude protein and total digestible nutrient content. A minimum of four sampling times distributed throughout the day permitted calculation of mean ruminal ammonia concentrations. Mean ruminal ammonia concentration was positively related to dietary crude protein concentration and negatively related to total digestible nutrient concentration. It is postulated that mean ruminal ammonia concentration may be a useful criterion for predicting efficacy of nonprotein nitrogen supplementation. A quantitative approach for evaluating nonprotein nitrogen supplementation based upon determination of the point at which ruminal ammonia exceeds the requirement (5 mg ammonia nitrogen/100 ml rumen fluid) of the ammonia-utilizing bacteria is proposed. Dietary conditions expected to result in excessive concentrations of ruminal ammonia are defined and recommended upper limits for nonprotein nitrogen supplementation are presented. Theoretical relationships between composition of the unsupplemented ration, amount of nonprotein nitrogen added, and efficiency of nonprotein nitrogen utilization are discussed. The practice of adding nonprotein nitrogen so as to exceed 12 to 13% crude protein in typical dairy or feedlot rations needs to be reevaluated.
Qualitative aspects of nonprotein nitrogen (NPN) utilization are well understood. For example, NPN is used most efficiently when small amounts are added to rations low in protein and relatively high in fermentable energy. Quantitative information regarding NPN utilization is lacking, however. Dietary protein concentrations above which NPN supplementation ceases to improve animal performance is not known. Understanding the limits of NPN utilization is necessary for profitable use of NPN supplements in ruminant rations. It is reasonable that NPN will be of little benefit to ruminants unless it is converted first into ammonia and subsequently utilized for microbial protein synthesis in the rumen. This assumption rests on the observation that swine and poultry fed rations of natural feedstuffs do not benefit from urea supplementation. Maintenance of ruminal ammonia concentrations in excess of the bacterial requirement would result in wastage of nitrogen, adding cost but no benefit. Satter and Slyter (20), using continuous culture fermentors, demonstrated that rumen bacteria can scavenge adequate amounts of ammonia from dilute solutions. An ammonia concentration of 5 mg NH3-N/100 ml rumen fluid was sufficient to support maximal rates of microbial growth. Increasing the ammonia concentration above this amount resulted in no further increase in microbial protein production. Knowledge of dietary conditions which result in ruminal ammonia concentrations in excess of bacterial requirements is essential for understanding and predicting NPN utilization. The purpose of our research was to determine the influence of ration composition on mean ruminal ammonia concentration. This information was used to develop a model which quantitatively estimates NPN utilization for various types of cattle rations. Theoretical considerations to develop the model are in this
Received September 26, 1974. i Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, and by the Soybean Research Council of the National Soybean Processors Association, Washington, DC.
1880
RUMINAL AMMONIA AND NITROGEN UTILIZATION paper. Supporting evidence, taken from published studies, is reviewed in a companion paper (18). E X P E R I M E N T A L PROCEDURE
The influence of ration composition on mean ruminal ammonia concentration was studied by collecting ruminal ingesta samples from 211 cows maintained under a variety of feeding programs. All rations in the 35 trials were formulated from natural protein sources. Ration crude protein (CP) and total digestible nutrient (TDN) concentrations were varied by changing ration ingredients and by altering their relative proportions. Following minor ration changes (e.g., proportion of soybean meal in concentrate), animals were adapted to a ration for a minimum of 1 wk before sampling. Longer adaptation periods were used following major ration changes (e.g., t y p e of roughage). Proportions of ration ingredients as well as dietary CP and TDN concentrations are in Table 1. Crude protein (N x 6.25) was determined by the Kjeldahl procedure. Ration TDN was calculated from National Research Council (NRC) tabular values (16). All CP and TDN values are on a dry matter basis. Most of the 1033 ruminal ingesta samples were collected via stomach tube; a few samples were obtained via rumen fistula. Direct comparison of method of sampling showed close agreement in ruminal ammonia values. Animals were sampled at least four times during the day, with mean ruminal ammonia concentrations obtained by averaging results from the individual samples. When ration ingredients were fed separately, sampling times were spaced to allow all ingredients to influence the average. Ammonia was determined in the supernatant fraction of centrifuged rumen fluid by steam distillation over magnesium oxide (1). Simple and multiple regression equations relating mean ruminal ammonia concentration to ration composition were computed. Because the number of observations varied among trials, regression analyses (24) were weighted. RESULTS A N D DISCUSSION
Mean ruminal ammonia concentrations ranged from .8 to 56.1 mg NH3-N/100 ml rumen fluid (Table 1) and increased as the percent CP in the ration dry matter increased
1881
60
"E 50
8_
~=10.57-2.50 X+ .15gX "~ Y=RUMINAL NH3-N (mcj/100 m,D X=°~ CP IN DM J
I
10
•• o • •
•
" 8
10
•
/
#-=.ss
~3o z_
•
12
14
•
16 18 a.~ CP IN DM
b
"
•
01
20
22
I 24
26
FIG. 1. Relationship between dietary crude protein (CP) and ruminal ammonia. (Fig. 1). The positive relationship between mean ruminat ammonia concentration and percent dietary CP is described by the equation: Ruminal NH3-N (rag/100 ml) -- 1 0 . 5 7 -2.5 %CP + .159 %CP2; r 2 = .88.. Both linear and quadratic effects were highly significant (P
