Wilfred Owen constructed this poem in iambic pentameter. This means most of the lines are broken up into five feet (this is the penta- part), and each foot consists of one unstressed syllable followed by one stressed or accented syllable (this is the iambic part). An iamb is a metrical foot (or unit) that has one unstressed syllable followed by one stressed syllable.
Although the majority of the poem is fairly regular iambic pentameter, the first three lines are irregular. In line one, the narrator asks, "What passing-bells for these who die as cattle?" There's an extrametrical syllable at the end of the line: the "tle" sound in "cattle." Also, lines two and three vary from the regular meter as well. They read "— Only the monstrous anger of the guns. / Only the stuttering rifles' rapid rattle." The word "Only" is a trochee (a foot that consists of one stressed syllable followed by one unstressed), so both of these lines begin with trochees instead of iambs. This is called a metrical substitution, and such extrametrical syllables and substitutions are often meant to call readers' attention to something in the line's content. Further, the repetition of the initial word, "Only," makes it seem as though Owen wants to emphasize how very little ceremony is paid to these young men who die "like cattle," how they get none of the trappings associated with death and funerals and the woeful inadequacy of the recognition of their sacrifice. Therefore, it seems as though Owen might begin the poem with three irregular lines in order to grab our attention and emphasize the main idea of the poem.