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Indices of Efficiency of Rumen Fermentation of Sheep Grazing Desert Range Forage as Influenced by Supplements of Nitrogen and Phosphorus¹

Utah Agricultural Experiment Station, Logan

Abstract

Twenty-seven rumen-fistulated young wethers, in three replications of a 3x3 factorial experiment, were used to study the influence of nitrogen and phosphorus supplements on the composition of the ruminal ingesta of sheep grazing desert range forage. The three levels of supplementation were: 0, 3 and 6 gm. of N from wheat gluten and 0, 2.0 and 2.65 gm. of P from monosodium orthophosphate per sheep per day. Supplements were supplied in a capsule inserted through the rumen fistula each morning immediately before grazing. Rumen samples were taken at 0, 3, 6, 9 and 12 hr. after administration of supplements and commencement of grazing. Esophageal fistulated sheep were used to sample the range forage.

A marked diurnal pattern was evidenced in the pH values of the rumen, but the range was only from 5.8 to 6.5. The mean pH for 810 determinations was 6.18. With the exception of one sampling time, the pH of the rumen mirrored the steam VFA concentration. The mean rumen VFA concentration derived from 540 measurements was 100 mmole/1. The molar percent compositions were as follows: acetic, 75.1; propionic, 17.9; butyric, 5.9; and valeric, 1.1. The concentration, but not the molar percent of valeric acid, was increased by gluten supplementation. The pH values and the total VFA concentration were not affected by either nitrogen or phosphorus supplements.

The rumen ammonia concentration curves for sheep fed the three levels of nitrogen supplementation paralleled each other and exhibited maximum concentration 9 hr. after the beginning of grazing. Phosphorus supplementation did not affect rumen ammonia concentration.

The administration of 2.0 or 2.65 gm. of P per day significantly elevated the phosphorus concentration in strained rumen liquor from 30.0 to 48 and 48.7 mg./100 ml. and in centrifuged rumen liquor from 11.6 to 31.8 and 34.6 mg./100 mg., respectively. Twenty-four hours after administration of the phosphate supplement, the concentration of phosphorus in the rumen was greater in treated than control animals.

Neither phosphorus nor nitrogen supplements influenced the rate of cellulose digestion (as measured by the cotton thread technique), the apparent digestibility coefficient of cellulose, or gross energy.

The mean daily forage dry matter and metabolizable energy intakes were 985 gm. and 1223 kcal., respectively. Phosphorus supplementation significantly reduced the calculated metabolizable energy intake from 1303 kcal. for control sheep to 1086 kcal. for sheep receiving 2.65 gm. P/day.

The average distance traveled by the sheep under study was 4.6 km. per day.

¹ Report on Project 421, Regional project W-34, Journal paper No. 447.

² Formerly graduate assistant, now Senior Husbandry Officer, Animal Research Institute, Yeerongpilly, Queensland, Australia; Professor of Animal Science; Associate Professor of Animal Science; and Professor of Range Management, respectively.

³ The authors wish to acknowledge the assistance and advice of J. C. Street pertaining to the chemical procedures.