At some point in the poem, things turn sour (with all that rotting and festering, how could it not?). A little more insight and a little more experience, and all of a sudden all that was pleasantly putrid is now revolting and scary. Maybe too much of a good (gross) thing really isn't a good thing, after all. Or maybe understanding that harmless, slimy frogspawn blast into beastly bullfrogs that look like grenades is a little intimidating. Either way, this poem turns from celebrating the icky in nature, to exposing its frightening and ugly side. Heaney's weapons of choice to get this idea of across are imagery (an old stand-by), and some wild wordplay using consonance.
Lines 22–24: This is the turning point in the poem where things start to turn sour. No longer is nature all rad, it's sickening and threatening. Heaney's imagery here engages three of our five senses to make this menacing introduction. The field is hot (sense 1: touch) and the fields absolutely stink of cow poop (sense 2: smell). The speaker sees the masses of frogs flooding the dam (sense 3: sight). Gone are the delicate descriptors Heaney uses in the first part of the poem to make the gross scene seem somehow charming. It's turning to a darker side.
Lines 25–27: Now we get an idea of what it sounds like. We can of course imagine what croaking frogs sounds like, but Heaney does one better: he uses consonance, in this case the repetition of the hard Csound to make the lines sound rough and threatening. Imagine, for contrast, the soft repetition of vowels and how they might seem soothing. For example: "Over the ocean and along the coast the herons roamed." You hear the pleasant repetition of the O sound (this is called assonance). Heaney's going for the exact opposite effect with those hard C sounds. He chooses "coarse croaking," "thick," "chorus," and "cocked" to create almost violent-sounding lines.
Lines 28–30: Here's a subtler example of consonance, but it's there nonetheless. This time Heaney uses the repetition of P sounds in "pulsed," "hopped," "slap," "plop" and "poised" to make the lines sound threatening. The imagery here is pretty wild too. Here are these frogs, cocked on sods—like a gun?—and they're huge necks are pulsing like sails. They're all over the place, launching from here to there, all the while making nasty fart noises with their croaking. Clearly the speaker is no longer charmed. (We really can't blame him, either. Can you?)
Lines 31–33: Not only is the speaker no longer charmed, he's horrified. Heaney creates this image of the frogs as "great slime kings" gathered for vengeance—it seems like the speaker thinks these frogs could do him actual harm. He's afraid if he goes in for his regular dose of frogspawn that they big frogs will snatch it. That's something straight out of a horror movie: Revenge of the Slime Kings.