Anim Sci J. 2020;00:e13381. wileyonlinelibrary.com/journal/asj | 1 of 9 https://doi.org/10.1111/asj.13381 © 2020 Japanese Society of Animal Science 1 | INTRODUCTION Livestock contributes about 14.5% of human induced greenhouse gas (GHG) emissions globally in terms of carbon dioxide equivalent (Gerber et al., 2013). The GHG emission from pig production is about 13% of global GHG from livestock and this is the second contributor to GHG in the livestock sector (FAO, 2011). Pork meat has been re- garded as the most consumed meat out of all other livestock species constituting about 37% of world's consumption (McGlone, 2013). The future growth in the world's population is expected to increase the demand for pork meat, hence increased intensive pig production. Subsequently, the emission of gases such as methane (CH 4 ), ammo- nia (NH 3 ), carbon dioxide (CO 2 ), nitrous oxide (N 2 O), and hydrogen sulfide (H 2 S) will increase with negative consequences on the envi- ronment as well as pose health risks (Bellarby et al., 2013). Carbon dioxide, CH 4 , and N 2 O are known to contribute to the depletion of the ozone layer resulting in global warming. These gases have vary- ing potentials causing the depletion of the ozone layer (IPCC, 2007) while hydrogen sulfide has been reported to pose negative impact on health of humans (EEA, 2009). Pig diets are reported to often contain more protein than other animal, thereby leading to the release of unutilized pro- tein in resultant manure (Montalvo, Morales, Pineiro, Godbout, & Bigeriego, 2013). Dietary constituents fed to livestock have been reported to influence the nutrient composition of the resultant manure produced (Lukehurst, Frost, & Al Seadi, 2010). Protein Received: 16 December 2018 | Revised: 23 September 2019 | Accepted: 10 March 2020 DOI: 10.1111/asj.13381 ORIGINAL ARTICLE Impact of reduced dietary crude protein levels and phytase enzyme supplementation on growth response, slurry characteristics, and gas emissions of growing pigs Adebukunola Olufunmilayo Lala 1,2 | Abimbola Oladele Oso 2 | Emmanuel Lartey Osafo 1 | Jos G. M. Houdijk 3 1 Department of Animal Science, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana 2 Department of Animal Nutrition, Federal University of Agriculture, Abeokuta, Nigeria 3 Monogastric Science Research Centre, SRUC, Edinburgh, UK Correspondence Adebukunola Olufunmilayo Lala, Department of Animal Nutrition, Federal University of Agriculture, P. M. B. 2240, Alabata, Abeokuta, Nigeria. Email: funmilala02@yahoo.com Funding information Department for International Development; African Academy of Sciences; Association of Commonwealth Universities Abstract This experiment was carried out to evaluate the effect of reduced dietary crude pro- tein (CP) levels supplemented with or without exogenous phytase on growing pigs. Six dietary treatments arranged in a 3 × 2 factorial arrangements of 3 CP levels (con- taining 14%, 16%, and 18% CP) supplemented each with or without 5,000 FTU/g phytase enzyme. Thirty growing pigs (average weight of 17.80 ± 0.10 kg) were allot- ted to the six dietary treatments in a complete randomized design. The final weight, daily weight gain, and feed conversion ratio (FCR) increased significantly with in- creasing CP levels. While, phytase supplementation improved ( p = .044) FCR in pigs. Total solid and volatile solid content of the slurry were higher ( p = .001) in pigs fed 14% and 16% CP diets supplemented with phytase when compared with other treat- ment groups. Concentration of methane gas emitted was lowest ( p = .001) in the slurry of pigs fed 14% CP diet with or without phytase and those fed 16% CP diet with phytase supplementation. In conclusion, reduction in dietary CP levels resulted in reduced weight gain and poor FCR. While, reduced CP with phytase supplementa- tion reduced concentration of methane gas emitted. KEYWORDS carbon dioxide, crude protein, methane, nitrogen, phytase