Dietary strategies to reduce methane emissions from ruminants

Ruminant products form an important part of the human diet. The demand for ruminant products is expected to increase due to the increase in the size of the human population and its increasing wealth. The production of ruminant meat and milk is associated with a relatively large environmental impact when compared to other animal products. This is, for a large part, caused by the fact that ruminants produce enteric methane, a greenhouse gas, during the digestion of their feed. Many dietary strategies have been proposed to lower methane production in ruminants, although most of these have only been tested in vitro. In this thesis, a number of dietary strategies, that had been proven effective in vitro, were evaluated for their in vivo efficacy in methane reduction. A mixture of lauric acid, myristic acid, linseed oil and calcium fumarate lowered methane production by 10% in lactating dairy cows. However, fat and protein corrected milk production was negatively affected by feeding this mixture. Despite the methane reduction, energy balance was unaltered in this study. Diallyldisulfide, yucca powder, calcium fumarate, an extruded linseed product and a mixture of capric and caprylic acid did not affect methane production in lactating dairy cows, although their efficacy had been demonstrated in vitro. The addition of nitrate and sulfate to sheep diets lowered in vivo methane emissions (-32% and -16%, respectively), presumably by acting as a hydrogen sink in the rumen. No negative side-effects of feeding nitrate or sulfate were observed in this study.  The use of nitrate in methane mitigation was further evaluated in a long-term study with dairy cows. Dietary nitrate persistently lowered  methane  production  by  16%  in  dairy  cows  over  the  89-d  experimental  period. Despite this reduction in methane production, milk production or energy retention were not improved. Methemoglobin levels in blood were slightly elevated, when nitrate was fed to dairy cows. Further analysis of the efficacy of nitrate in methane mitigation demonstrated that the efficacy of nitrate in methane mitigation decreased with increasing dose of nitrate (expressed in g nitrate/kg ^0.75 per day).  The conversion of metabolizable energy gained from a lowering of methane production may be less efficient than is commonly assumed. This  could  originate  from  a  shift  from  methane  to  hydrogen  emissions,  when  methane is  specifically  inhibited,  or  from  erroneous  assumptions  made  in  the  calculation  of  heat production during indirect respiration calorimetry. Dietary fat addition may be an effective strategy to lower methane production from ruminants, although the fatty acid profile of the  added  fat  does  not  appear  to  have  additional  effects  on  methane  production  from ruminants. When assessing the environmental impact of ruminant products, it is generally overlooked that ruminants are capable of transforming feed not accessible to humans into human food.