Greenhouse gas emissions from livestock manure (cattle) in different

Introduction

Global warming seriously affects the agroecological, flora and fauna growth conditions and agricultural production (Rosegrant et al., 2007; Aydinalp & Cresser, 2008), and greenhouse gasses (GHG) are a significant contributor to climate change. Agriculture has been identified as a prominent source of anthropogenic greenhouse gas emissions (Anita et al., 2010), and livestock gas emissions are one of these. According to FAO (2015), the livestock sector contributes about 75 percent of agricultural nitrous oxide (N2O) emissions, with an equivalent of 2.2 billion tons of carbon dioxide (CO2). Besides, livestock is a significant driver of the global trends in land utilization and changes in land use, including deforestation and desertification. According to the United Nations Food and Agriculture Organization, the livestock sector generates more greenhouse gas emissions measured in CO2, with 18% compared to the transport sector. Moreover, according to Steinfeld et al. (2006), livestock is one of the most substantial contributors to today's most dangerous environmental problems.

Livestock contributes to climate change by emitting GHG directly through enteric fermentation and manure management or indirectly through feedproduction activities and forest conversion into the pasture. The life cycle assessment (LCA) approach estimated that the livestock sector emits about 7.1Gt of CO2-eq or about 18% of global anthropogenic GHG emissions. Moreover, LCA estimated that chemical fertilizer in livestock feed emitted about 0.4 Gt of CO2-eq. Furthermore, enteric fermentation in ruminants contributed 1.9 Gt CO2-eq of GHG, while manure management accounted for 2.2 Gt CO2-eq. These cover manure storage, application, and deposition (CH4, N2O) (Steinfeld et al., 2006). In 2015, the livestock sector contributed approximately 75 percent of the agricultural nitrous oxide (N2O) (2.2 billion tons of CO2) emissions and 2.7 billion tons of carbon dioxide (CO2) emissions.

Most livestock animals are ruminants, and cattle are one of those. Ruminant has a unique digestive system that allows them to use the energy from fibrous plant material efficiently. In the ruminant's digestive system, methane emission is a natural part of its digestive process (enteric fermentation). Also, Methane gas is a by-product of manure management operations in ruminants. Ruminants account for up to one-third of the anthropogenic methane (CH4) emissions worldwide (Huhtanen et al., 2014). In 2011, the United States emitted about 137 million metric tons of methane (in CO2 equivalents) from the enteric fermentation of livestock, comprising about 70% of the total agricultural methane emissions. Beef cattle are by far the most significant species contributor of enteric methane. Beef cattle contribute nearly 100 million metric tons of CO2 equivalents annually in the U.S., nearly three (3) times that of dairy cattle and 50 times greater than swine (Smith, 2014). On the other hand, a software called HOLOS, a farm model developed by Agriculture and Agri-Food in Canada, revealed that enteric methane (CH4) accounted for 63 percent of the total GHG emissions. Also, cattle manure was recorded with the highest nitrous oxide (N2O) at 23 percent (Beauchemin et al., 2010).

In early 1991, the Philippines started its efforts to address the issue of climate change. From 1990 to 1994, the Philippines launched its national greenhouse gas emissions inventory. The agricultural and forestry sectors have integrated into their development plans measures on greenhouse gas mitigation through the Philippine Clean Air Act of 1999 (Merilo, 2001).

The implementation of the Philippine Clean Air Act of 1999 becomes the venue for the implementation of other environmental policies such as the Asia Least- Cost Greenhouse Gas Abatement Project (ALGAS), National Action Plan on Climate Change and the "Enabling Activity on Climate Change." However, more than these endeavors are needed to mitigate GHG emissions. In 1990, the Philippines recorded 96Mt of carbon dioxide (CO2) emissions; in 2018, it increased to 159 Mt of carbon dioxide. It is garnering an average of 139 Mt of carbon dioxide (CO2) and an average of 0.31% of the total global shares, which leads the Philippines to rank 6th in Southeast Asia (Ilea, 2006).

The continuing increase of GHG emissions despite the existing law leads to the birth of the "Greenhouse Gas Emission Atmospheric Removal Act of 2015" and "Low Carbon Economy Act of 2016". These acts hoped to minimize GHG emissions by setting up an emission cap-and-trade system in the industry sector and facilitating the development, demonstration, and implementation of technology that shall remove GHGs from the atmosphere (Merilo, 2001). However, with the increasing economic growth and population, the demand for meat and dairy products is increasing yearly. Global meat production is projected to double from 229 million tons in 1999/2001 to 465 million tons in 2050, while milk output is set to climb from 580 to 1043 million tonnes. Hence, the need to balance production and mitigate GHG emissions from cattle is undeniably vital. That is why this study was conducted to determine the relationship between feeding methods and practices and the volume of GHG in decomposing manure.