What passing-bells for these who die as cattle?

The speaker opens by asking what "passing-bells" (that is, church bells that are typically rung when someone dies) can be heard on the battlefront as soldiers die. He also uses a simile to show the inhumane ways the soldiers die (like cattle) and convey a slaughter in large numbers. He uses these instead of those in an attempt to convey a sense of proximity to the battlefield and bring readers closer to its horrors.

Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.

While the soldiers are compared to cattle, the rifles are personified as "loud" and "monstrous anger" that is seen "stuttering" here to announce the soldiers' deaths. They provide the background rhythm for the soldiers' prayers. These lines also show the quick way soldiers are killed, as words like "rapid," "patter," and "hasty" are repeated in quick unison.

Nor any voice of mourning save the choirs,—
The shrill, demented choirs of wailing shells

These lines show the contrast between death at home and death on the battlefront. While a choir might be heard as a "voice of mourning" to memorialize a death, the background "music" of the battlefield is a "demented choir of wailing shells." In death during war, there is no sense of peace. The dead are not remembered with religious music. Instead, they are surrounded by further death and fighting. These lines also convey an absence of traditionally comforting religious traditions on the battlefield.

And each slow dusk a drawing-down of blinds.

This line reinforces the idea of death and dying that is woven throughout the poem. The dusk can be seen as the lives of the young soldiers who lie in their final moments on the battlefields during this war. It can also be read to show the ability of those back home to safely put down their blinds each evening, their sense of security resting on the deaths of these soldiers each night. By closing their blinds each evening, they are able to shut out the realities of what is happening in the world far from their homes.