Applied Animal Nutrition-I

PROTEIN REQUIREMENTS FOR MAINTENANCE

• Loss of protein continuously occurs through wear and tear of body tissue, for renewal of hairs, nails, feathers, hooves  etc., which represents the amount of protein required for maintenance. If the losses are not completed promptly by proper amount of protein either in the form of tissue protein or NPN substances, the animal will rundown in condition and its reproducing ability or productivity will be adversely affected.
• The losses of body protein in the animal when kept on a protein free ration occurs through urine and faeces in negligible amount, through shedding of hairs, loss of nail, skin etc.
• The loss, which occurs through urine, is known as EUN or endogenous urinary nitrogen loss and loss, which occurs through faeces, is called MFN or Metabolic faecal nitrogen loss.

Endogenous -nitrogen metabolism : 

• Endogenous Urinary Nitrogen (EUN): Here the loss of nitrogen is due to the catabolism incidental to maintenance of the vital tissues of the body, which can be measured at the minimum urinary excretion on a nitrogen free otherwise adequate (particularly energy adequacy) diet. The greater part of the nitrogen in the urine of mammals not receiving food N is in the form of urea, the typical byproduct of amino acid catabolism, which arises from the turnover of body proteins.
• Like basal metabolism, endogenous -nitrogen metabolism is a function of body size. Terroine and Sorg-Matter in 1927 reported that a relationship exists, for the first time, 2.3 to 2.9 mg nitrogen per kilcalorie BMR. Later in 1934, Brody and coworkers confirmed that realtionship and is indicated by the following formula,

EUN mg/d =146 W^0.72kg

• This interspecies equation was arrived at by the analysis of a body of data on minimum EUN excreted by mature animals of different species,ranging in weight from 0.02 to 500 kg. It was suggested that mammals excrete 2 mg of EUN per kilocalorie of basal metabolism or 140 mg N/kg^0.75/day. EUN is highest in young animals and lowest during hibernation, since EUN tends to reflect energy metabolism. EUN of Indian cattle was 0.2g/kg BW, while that of Bos taurus was 0.289g/kg BW. 

Metabolic faecal nitrogen (MFN or FN_m):

• It consists principally of spent digestive enzymes, abraded mucosa and bacterial nitrogen. It is difficult to obtain MFN using a nitrogen-free diet in ruminants.Mitchell used a well balanced protein such as 4% of defatted egg protein in rats instead of nitrogen free diet. Egg protein was 100% absorbed. MFN is proportional to feed intake and it is about 2 mg per g DMI in rats. 
• In 1927 Titus introduced a technique with steers, which involved plotting of the total N intake as a function of the total faecal N excretion, using rations of varying protein content but of constant feed intake. He extrapolated the straightline thus obtained back to the point of zero protein intake and arrived at the estimated metabolic faecal N excretion for the test feed. It is about 5 mg /g of DM intake.This is over twice the value for rats and would appear logical since both microbial residues and tissue desquamation would appear to be higher in ruminants. The MFN values determined in Indian cattle were 0.35 g/100 g DMI and in buffaloes 0.34 g/100 g DM intake.These values are lower than the values determined in Bos taurus.
• Endogenous urinary nitrogen and metabolic faecal nitrogen put together  has come to 350 mg N/kg metabolic body size per day in ruminants.It is two to three times as great as in non-ruminants.