"Mametz Wood" is a poem with a specific historical context that is important to note before this discussion. Mametz is a village in Northern France, and the nearby wood was the scene of a brutal battle during the First World War. In this battle, a section of the British Army—known as the 38th (Welsh) Infantry Division—was tasked with attacking the German stronghold in the wood, or forest. Around 4,000 men on the British side were killed during the conflict.
As referred to in the poem's opening, farmers have been finding body parts of the fallen ever since. The alliterating and consonant /f/ sounds in the first line come across almost like discoveries themselves, as though they were poking out of the line above the other letters:
For years afterwards the farmers found them –
The speaker then emphasizes the youthfulness of those killed. Over half of all WWI soldiers were between 18 and 23 when they enlisted; an incredible amount of young life was "wasted." Additionally, the caesura just after "young" gives the line a short pause, allowing the reader to reflect on this fact.
There is something deeply unsettling about this stanza's mention of the bodies "turning up under [the farmers'] plough blades," but this is nothing compared to the graphic details that are to come. It's also worth noting that there is something vaguely threatening about the "plough blades"—even though they are an agricultural tool, they seem to hint at man's capacity for destructive technology, the kind that made WW1 so devastating.
In line 3, "tended" works as a kind of pun. It relates on a literal level to the farmers' actions of tending the land, but also sets up one of the poem's metaphors regarding the land as somehow injured—"tending" its wounds. This also gently develops the tension between nature (which humans are a part of) and the fearsome power of man-made technology, which ravaged the earth and has left scars on the Mametz landscape that can still be seen today.