FAO Agricultural Studies   No. 73




J.K. Loosli

Professor of Animal Nutrition and
Head, Department of Animal Science,
Cornell University, Ithaca, New York, U.S.A.


I.W. McDonald

Chief of Division, CSIRO,
Ian Clunies Ross Animal Research Laboratory,
Prospect, N.S.W., Australia


The Animal Production and Health Division of FAO has established a number of expert panels to give it advice and guidance. The members of these panels are appointed for their individual ability and not on a country basis, and this book, Nonprotein nitrogen in the nutrition of ruminants, is an illustration of the manner in which panel members may assist the Organization. The authors and contributors are all well-known scientists and are members of the Animal Nutrition Panel. They have given of the limited time they could spare from the execution of their normal duties to compile this book, and I wish to thank them most sincerely for their activities. It is hoped that this book will be the forerunner of a series compiled and published in this way.

In addition to the two co-authors, assistance was given by and is gratefully acknowledged to K. Breirem and T. Homb of Norway, J. Kielanowski and M. Chomyszyn of Poland, and F. Whiting of Canada.

K.V.L. Kesteven
Director, Animal Production
and Health Division

Rome © FAO 1968
Printed in Italy

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1. Importance and use of nonprotein nitrogen and urea

Brief historical summary
Importance of urea as a feed ingredient
Nonprotein nitrogen compounds used in ruminant feeds
Mechanism of urea utilization
Biological value of microbial protein

2. Factors affecting urea utilization

Effect of level of protein
Effect of carbohydrates
Effect of alcohol in liquid supplements
Effects on digestibility and feed intakes
Influence of sulfur
Influence of other factors

3. Applied studies on the use of urea

Drought feeding for survival
Supplementing low quality pasture
Supplementing low quality rations (hand-feeding) for maintenance
Supplementing green pasture forage
Maintenance of breeding cows and ewes
Feeding growing and fattening lambs
Feeding weaner cattle for growth
Preparing beef cattle for fattening
Fattening cattle in feedlots
Supplementing rations for dairy cattle

Growing dairy cattle
Lactating dairy cows

Spraying pastures
Urea in salt licks or salt blocks
Addition of urea to silage
Addition of urea to hay
High-urea supplements
Urea not useful for nonruminants
Toxicity of urea

4. Use of other nonprotein nitrogen compounds

Use of ammoniated feeds

Ammoniated beet pulp and citrus pulp
Ammoniated straw and silage
Ammoniated molasses
Ammoniated sugarcane bagasse
Ammonium salts
Ammoniated rice hulls
Other nonprotein nitrogen compounds

Sewage sludge
Poultry litter

5. Suggested methods of feeding urea and other NPN compounds

Urea in concentrate mixtures
Urea in high-protein supplements
Nonprotein nitrogen additions to silage and hay
Salt licks containing urea or biuret
Precautions when using urea

6. Conclusions

Summary of findings
Further research requirements



This report on the use of urea and other nonprotein nitrogen (NPN) containing compounds in feeding ruminants is presented with the hope that it will serve as a useful summary of research findings and as a guide for feeding cattle, sheep and other ruminants, particularly in countries where little or no research has yet been done.

Protein is often the major limiting nutrient for ruminants. Protein-rich leguminous forages are not widely grown in many areas grazed by ruminants, and vegetable protein supplements are usually expensive or not available. The manufacture of urea and ammonia for use as fertilizer has been greatly expanded in many countries, but these compounds could be used more widely in feeds for ruminants. The ability of the micro-organisms in the rumen of cattle and sheep to utilize these NPN sources to form true protein, that can be converted to meat and milk by the animals, represents an important contribution to man's food supply.

The purpose of this review is to bring together selected examples of research in which urea has been used successfully to replace the short supplies of vegetable protein feeds as well as to direct attention to limitations in the use of urea and other NPN sources. No attempt has been made to compile a complete list of all published evidence, but rather to illustrate the varied types of comparisons which have been made and to offer suggestions regarding the practical usage of urea in livestock feeding practices. Earlier reviews have been made by Krebs (1937), Reid (1953), Goss (1942–43), Annison and Lewis (1959), Barnett and Reid (1961), and Blackburn (1965), while Stangel, Johnson and Spellman (1963) have presented an extensive compilation of abstracts and titles of research papers dealing with the use of urea and other NPN compounds in ruminant rations. The reader is referred to these sources for additional information.