TECHNICAL

NOTES

Relationship Between Ruminal Ammonia and Nonprotein Nitrogen Utilization by Ruminants. II. Application of Published Evidence to the Development of a Theoretical Model for Predicting Nonprotein Nitrogen Utilization 1 R. E. ROFFLER and L. D. SATTER Department of Dairy Science Universityof Wisconsin Madison 53706

ABSTRACT

INTRODUCTION

Results from published experiments dealing with several aspects of nitrogen utilization by ruminants were used to test the concept of zero utilization of nonprotein nitrogen under conditions where more ammonia (>5 mg ammonia nitrogen/100 ml) is in the rumen than can be converted to microbial protein. Results from experiments where the flow of nonammonia nitrogen to the abomasum of sheep was measured indicate that when urea was the source of supplemental nitrogen, a constant quantity of amino acids reached the abomasum for all rations ranging from 10 to 23% crude protein. From growth studies, addition of nonprotein nitrogen to low protein, high energy rations caused an improved rate of gain. Additions of nonprotein nitrogen to rations resulting in predicted ruminal ammonia concentrations greater than 5 mg ammonia nitrogen/100 ml rumen fluid were without benefit. From lactation studies, nonprotein nitrogen supplementation did not improve milk production if the ration contained more than 12.5% crude protein prior to supplementation or if the predicted ruminal ammonia concentration was greater than 4 mg ammonia nitrogen/100 ml rumen fluid. The importance of the amino acid requirement of the animal as well as the composition of the ration in designing and evaluating nitrogen supplementation studies is discussed.

When more ammonia is in the rumen than can be converted to microbial protein, there is an "overflow" of ammonia from the rumen. As long as the concentration of dietary protein remains low enough to prevent ammonia accumulation, nonprotein nitrogen (NPN) and protein are nearly equal in terms of furnishing amino acids to the intestines. However, once the point of ammonia accumulation or "overflow" is reached, the requirements of the ammonia-utilizing rumen microbes have been satisfied, and microbial protein synthesis is not stimulated by further addition of NPN. When NPN is used as the sole source of supplementary nitrogen, the point of ammonia accumalation coincides with a plateauing of the quantity of amino acids leaving the rumen. When protein is used, the quantity of amino acids available postruminally continues to increase beyond the point of ammonia accumulation because of the contribution of dietary protein which escapes ruminal degradation. A graphic representation of the influence of the point of ammonia overflow and the t y p e of nitrogen supplement on the quantity of amino acids absorbed postruminally (metabolizable protein) is in (34).

Received September 26, 1974. 1 Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison. 1889

Satter and Slyter (35) demonstrated that an ammonia nitrogen concentration of 5 mg/100 ml rumen fluid was adequate for supporting maximal microbial protein production. By studying the influence of ration composition on ruminal ammonia, Roffler and Satter (32) were able to describe dietary conditions which resulted in excessive concentrations of ruminal ammonia. This information was used to develop a model for estimating NPN utilization by cattle (32). The proposed model for predicting efficacy of NPN supplementation is based upon

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the assumption that NPN is not utilized when ruminal ammonia concentrations exceed the requirement of the ammonia-utilizing bacteria. This paper reviews data from published studies to test the validity of the model developed for estimating NPN utilization by cattle (32). MATERIALS AND METHODS

Published data were used to study the influence of NPN supplementation on: (a) the quantity of nonammonia nitrogen (NAN) reaching the abomasum, (b) growth response, and (c) lactation response. To obtain an overall picture of responses to NPN supplementation, data were combined from many trials representing a variety of feeding situations. Effect of Supplemental Protein or NPN on the Flow of N A N Through the Abomasum

The influence of protein or NPN supplementation on the quantity of NAN passing daily through the abomasum was studied in results from published trials with sheep fitted with omasal, abomasal, or duodenal cannulas. Measurements of NAN flow with duodenally-fistulated sheep were adjusted by subtracting 1 g N/day to allow for nitrogenous abomasal secretions (27). To compare results from trials involving diverse quantities of feeding, abomasal NAN values were adjusted to a dry matter (DM) intake of 1000 g/day, with proportionality. An increase in feed intake resulted in a proportional increase in the amount of NAN presented to the intestine (4, 9, 22). The quantity of NAN flowing through the abomasum was examined in relation to the crude protein (CP) content of 24 rations containing urea. For comparison, results from trials involving 36 rations where protein was the source of supplemental nitrogen were studied also. Rations are identified and described in Table 1. Growth of Feedlot Steers in Response to Supplementation with Either Protein or NPN

Recent results (3) from feedlot trials involving adequate numbers of steers (18 per treatment) were used to test the accuracy of the proposed model (32) for predicting whether NPN supplements would be beneficial to growing cattle. Treatments selected for our paper are identified in Table 2. Three comparisons were Journal of Dairy Science Vol. 58, No. 12

chosen, and each involved four treatments: a negative control, two amounts of urea supplementation, and a positive control. Percent crude protein and total digestible nutrients (TDN) for each treatment are in Table 2, and proportions of preformed protein and NPN are in Fig. 3, 4, and 5. For each negative control ration, the point of ammonia overflow was calculated by methods described (32). These values (represented by dashed horizontal lines drawn through the left half of Fig. 3, 4, and 5) were 10.0% CP for Comparison 1 and 11.0% CP for Comparisons 2 and 3. Body weights at the start of the feeding period averaged about 170 kg for steers in Comparisons 1 and 2 and about 370 kg for steers in Comparison 3. Milk Production Response to NPN Supplementation

Data from published lactation trials of 38 comparisons of NPN-supplemented rations with unsupplemented, negative control rations were used to test the accuracy of the proposed model (32) for predicting whether NPN supplements would be useful to lactating cattle. The total was 406 cows. Supplemented rations were formulated by addition of NPN to either the concentrate or the forage. The latter group included corn silage treated with either urea or ammonia solutions. Identification and ration composition are in Table 3. Total digestible nutrient contents of the negative control rations were calculated by National Research Council (NRC) tabular values (23). Ruminal ammonia concentrations were predicted from the percent CP and TDN in the unsupplemented rations by multiple regression equation (32). Simple regression equations related percent increase in milk production following NPN supplementation to composition of the ration prior to supplementation. RESULTS A N D DISCUSSION Flow of N A N Through the Abomasum

The quantity (g/kg DM intake) of NAN reaching the abomasum ranged from 13.2 to 23.6 for rations containing urea and from 11.4 to 37.0 for rations containing natural protein (Table 1). The relationships between the flow of NAN through the abomasum and the CP content of rations containing urea or natural protein are in Fig. I and 2, respectively.

T A B L E 1. I n f l u e n c e o f d i e t a r y c r u d e p r o t e i n (CP) c o n c e n t r a t i o n o n q u a n t i t y o f n o n a m m o n i a n i t r o g e n ( N A N ) r e a c h i n g t h e a b o m a s u m o f sheep. Diets c o n t a i n i n g u r e a

Diets c o n t a i n i n g n a t u r a l p r o t e i n

CP in DM

DM i n t a k e

N intake

NAN a reaching abomasum

(%)

(g/day)

( g / k g DM)

(g/day)

3.3 5.9 6.3 8.8 9.2 10.1 11.4 12.1 12.1 12.1 12.4 14.4 14.6 15.1 15.4 15.9 16.1 16.4 16.6 16.9 18.9 20.6 20.8 23.4

480 467 945 624 945 624 506 624 1025 945 745 814 751 945 1025 1025 814 744 772 816 1025 485 1025 1025

5.3 9.5 10.1 14.0 14.7 16.2 18.2 19.3 19.4 19.4 20.0 23.1 23.4 24.2 24.5 25.5 25,8 26.2 26.6 27.0 30,2 32.9 33.3 37.5

13.2 15.4 19.1 19.4 23.4 20.2 17.4 22,6 20.6 20.8 23.0 18.9 22.5 23.6 20.1 22.2 21.6 22.3 22.0 22.4 21.7 19.1 22.0 20.6

Relationship between ruminal ammonia and nonprotein nitrogen utilization by ruminants. II. Application of published evidence to the development of theoretical model for predicting nonprotein nitrogen utilization.

Results from published experiments dealing with several aspects of nitrogen utilization by ruminants were used to test the concept of zero utilization...
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