Protein-energy relationships in the digestion products of sheep fed on herbage diets differing in digestibility and nitrogen concentration

Abstract

The digestion of organic matter and nitrogen in 17 herbages was studied in sheep fitted with duodenal re-entrant cannulas. Each herbage was in chopped form, and was given at levels close to ad libitum but offered at an even rate hourly by automatic feeder. Diets ranged from 0.4 to 5.1% nitrogen and from 49 to 79% digestibility of organic matter (OM). At the levels of intake achieved, the apparent digestibility of OM in the stomachs ranged from 28 to 45%; between 54 and 72% of the apparently digested OM disappeared before the duodenum. Though the percentage of the digestible OM digested across the stomachs tended to increase with decreasing overall digestibility, the relationship was poor (r² = 0.25). Of the OM apparently digested between duodenum and faeces, 5ndash;40% could be accounted for as crude protein. Non-protein OM digested in the intestines contributed about 25% of total apparently digested OM. The nitrogen intakes extended over a range of 1.8–57.1 g/day, and the apparent digestibility of nitrogen rose from negative values to 84% with increasing nitrogen intake. Small gains (1–3 g) in nitrogen at the lower end and substantial losses of up to 40% of nitrogen at the upper end of the dietary range resulted in yields of non-ammonia nitrogen (NAN) of 2.4–32 g/day at the duodenum, of which 75–80% was accounted for as α--amino nitrogen. Crude protein yielded at the duodenum ranged from 10.3 to 42.5 g/100 g OM apparently digested across the stomachs. Metabolic faecal nitrogen losses were estimated at 0.65 g/100 g OM entering the duodenum of which 0.60 g was estimated to arise in the intestines. Over all diets the estimated mean true digestibility of NAN entering the intestines was 84%. Crude protein truly digested in the intestine contributed 5.6–21.2 g of each 100 g OM apparently digested, or an estimated 5.3–20 g of each 100 g OM truly digested. This would have provided from c. 7% to c. 24% of digestible energy as protein, or 14–45 g truly digestible true protein/Mcal metabolizable energy (ME) to the animal. While a 12-fold range of dietary crude protein concentration was condensed to a fourfold range of protein concentration in the OM digested by the animal, it is suggested that diets 1–13 still provide insufficient protein per unit energy to meet requirements for normal lamb growth even if adequate ME intake could be achieved, and that diets 1–8 would not provide adequate protein for maintenance. The protein yield as grams truly digested true protein per Mcal ME (P) was related to the dietary nitrogen concentration, expressed as grams per 100 g digestible OM (N), by the equation

P = 11.50 + 4.58N (&plusminus;3.19; r² = 0.88)

However, despite the high correlation, prediction on this basis would not separate diets in the mid-range of nitrogen concentration (i.e. 3.0 g N/100 g digestible OM) which provided, in terms of eventual protein yield per unit energy, 19–42 g truly digestible true protein per Mcal ME (or roughly 11–22% of calories).