No abstract

Twenty male crossbred Texel lambs were used in a 2 × 2 factorial design experiment to assess the effect of dietary addition of nitrate (2.6% of dry matter) and sulfate (2.6% of dry matter) on enteric methane emissions, rumen volatile fatty acid concentrations, rumen microbial composition, and the occurrence of methemoglobinemia. Lambs were gradually introduced to nitrate and sulfate in a corn silage-based diet over a period of 4 wk, and methane production was subsequently determined in respiration chambers. Diets were given at 95% of the lowest ad libitum intake observed within one block in the week before methane yield was measured to ensure equal feed intake of animals between treatments. All diets were formulated to be isonitrogenous. Methane production decreased with both supplements (nitrate: -32%, sulfate: -16%, and nitrate+sulfate: -47% relative to control). The decrease in methane production due to nitrate feeding was most pronounced in the period immediately after feeding, whereas the decrease in methane yield due to sulfate feeding was observed during the entire day. Methane-suppressing effects of nitrate and sulfate were independent and additive. The highest methemoglobin value observed in the blood of the nitrate-fed animals was 7% of hemoglobin. When nitrate was fed in combination with sulfate, methemoglobin remained below the detection limit of 2% of hemoglobin. Dietary nitrate decreased heat production (-7%), whereas supplementation with sulfate increased heat production (+3%). Feeding nitrate or sulfate had no effects on volatile fatty acid concentrations in rumen fluid samples taken 24h after feeding, except for the molar proportion of branched-chain volatile fatty acids, which was higher when sulfate was fed and lower when nitrate was fed, but not different when both products were included in the diet. The total number of rumen bacteria increased as a result of sulfate inclusion in the diet. The number of methanogens was reduced when nitrate was fed. Enhanced levels of sulfate in the diet increased the number of sulfate-reducing bacteria. The number of protozoa was not affected by nitrate or sulfate addition. Supplementation of a diet with nitrate and sulfate is an effective means for mitigating enteric methane emissions from sheep.

1. An isotope tracer method for estimating methane production in sheep is described.2. The technique was used to estimate methane produced in both the upper and lower digestive tract and to determine the routes by which it was excreted.3. Four Merino ewes given lucerne chaff (33 g every hour) were used.4. Total methane production rate was 21±1.1 (se) ml/min; production in the rumen accounted for 87±1.2% of the total production; 95±1.4% of the methane produced in the rumen was excreted by eructation.5. Of the methane produced in the lower digestive tract, 89±2.3% was excreted through the lungs and 11% through the anus.

I. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with ['SN]ammonium sulphate and [Wlurea and [14C]urea.2. Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates.3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 gjd.4. It was estimated that 59 yo of the dietary N was digested in the reticulo-rumen; 29 yo of the digested N was utilized as amino acids by the micro-organisms, and 71 % was degraded to ammonia.5. Of the 14'2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4'3 g N/d represented recycling, largely within the rumen itself (through the pathways : ruminal ammonia + microbial protein + amino acids --z ammonia).6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through the rumen wall and 16.4 g N/d apparently arising from deaniination of amino acids and ammonia absorbed from the lower digestive tract. 7.In the 24 h after intraruminal injection of [15N]ammonium salt, 40-50 % of the N entering the plasma urea pool arose from ruminal ammonia; 26% of the lSN injected was excreted in urinary N.8. Although 5-1 g N/d as urea was degraded apparently in the digestive tract, only 1.2 g N/d appeared in ruminal ammonia; it is suggested that the remainder may have been degraded in the lower digestive tract. 9. A large proportion of the urea N entering the digestive tract is apparently degraded and absorbed and the ammonia incorporated in the pools of nitrogenous compounds that turn over only slowly. This may be a mechanism for the continuous supply to the liver of ammonia for these syntheses.10. There was incorporation of 16N into bacterial fractions isolated from rumen contents after intraruminal and intravenous administration of [15N]ammonium salts and [lSN]urea respectively. 11. A model for N pathways in sheep is proposed and, for this diet, many of the pool sizes and turn-over rates have been either deduced or estimated directly.

Nitrogen metabolism is reviewed with emphasis on methods for quantitating various nitrogen-transactions in the rumen of animals on a variety of diets. Ammonia kinetics, microbial cell synthesis, the inputs of endogenous nitrogen, degradation of dietary protein, and availability to the animal of dietary bypass protein are discussed. The efficiency of microbial protein from the rumen is discussed in relation to the ratio of protein to energy in the nutrients available to meet the requirements of the animal. The ratio is determined largely by the maintenance requirements of microbes and the breakdown of microbial materials, which result in the recycling of microbial nitrogen in the rumen. Emphasis is placed on the role of rumen protozoa in decreasing the ratio of protein to energy in absorbed nutrients in ruminants on diets that are marginally deficient in protein. Recent studies of the dynamics of protozoa in the rumen and their contribution to microbial protein outflow are summarized.