The use of condensed tannins and nitrate to reduce enteric methane emission and enhance utilization of high-forage diets in sheep

Abstract

Methane emission in ruminant production is not only a global greenhouse gas, but also a loss of feed energy. Therefore, there is huge interest in identifying mitigation strategies that reduce ruminant derived methane, which guarantees sustainable ruminant production. Tannins have been reported to inhibit methanogenic activity, but may also limit feed intake and nutrient digestibility. While dietary nitrate has been noted as an efficient hydrogen sink in the rumen, its effectiveness may be dependent on the application method among others. The objectives of this study are: (i) to evaluate the effects of nitrate supplementation and nitrate treatment on the in vitro digestibility and methane production in Eragrostis hay; (ii) evaluate the effect of supplementing acacia tannin extract and nitrate on feed intake, nutrient digestibility, methane emission and health status of Merino lambs; (iii) prepare and evaluate an encapsulated acacia tannin extract formulation suitable for ruminants; (iv) evaluate the effect of encapsulated acacia tannin extract on feed intake, nutrient digestibility and enteric methane emission in Merino rams. A series of experiments (laboratory trials, and stall-feeding experiments) were conducted at the NUTRILAB and Hatfield Experimental Farm of the University of Pretoria, South Africa to evaluate the potential improvements in the use of nitrate and tannin extract using Sheep as model animal. Treating Eragrostis hay with urea improved its in vitro organic matter digestibility better than treatment with nitrate. However, nitrate treatment significantly reduced enteric methane and improved digestibility compared with the control, this suggests that nitrate could be incorporated as a hydrolytic agent in treating poor-quality roughage feeds, with the aim of improving their utilization. The combination of urea or calcium nitrate with or without Acacia tannin extract as dietary supplements in a growth trial revealed that lambs receiving nitrate-based TMR diets experienced superior growth performance compared to those on the urea-based TMR diets. Meanwhile, tannin inclusion did not improve growth or reduced methane emission from the lambs irrespective of the non-protein nitrogen source. A slight increase in haemoglobin, haematocrit and RBC count was associated with the use of calcium nitrate compared to urea. In this study, no clinical or subclinical signs of morbidity or tannin intoxication symptom was detected from the haematology and biochemical parameters evaluated in the Merino lambs. The trial on the preparation and evaluation of an encapsulated Acacia tannin extract (ATE) showed that Gum Arabic-maltodextrin and native starch could only encapsulate the tannin extract at low inclusion levels while the in vitro release was not sustained. However, palm oil was found to be an effective wall material in encapsulating ATE using the double phase solid-in-oil-in-water encapsulation method where up to 80% w/w inclusion of tannin extract in the lipid wall material was achieved. This extract exhibited good morphological characteristics and high encapsulation efficiency even under high loading percentage. The lipid-encapsulated extract significantly reduced enteric methane production in vitro. Under in vivo evaluation with cannulated Merino rams, encapsulated Acacia tannin extract resulted in considerable reduction in methane per neutral detergent fibre intake, compared to the crude extract.