To assess the effect of adding solid manure fractions on the biomethane potential (BMP) of liquid dairy cow manure and on the biokinetic parameters of the process. The methanogenic potential of liquid dairy cow manure was strongly effected by adding a solid manure fraction. The 90/10 % (w/w) liquid/solid manure fraction mixture was the best substrate for CH4 production. This substrate mixture improved by 50 % the final CH4 production per g substrate and decreased the lag time by 220 % relative to the reference BMP test without the addition. Moreover, the addition of 20 % solid manure fraction adversely affected both the final CH4 production and the maximum methane production rate, while increased the lag time by 400 % compared to the reference BMP test without addition. Liquid dairy cow manure should be supplemented with no more than 10 % of solid manure fraction in order to improve the biomethane potential of this important agro-industrial residue.