The most common and most likely answer to this crossword clue is the 7 letter word RAGWORT
I've seen this crossword clue in The Daily Telegraph
R
A
G
W
O
R
T
More clues leading to the result RAGWORT
- Weeds grow wild, eaten by rodent
- See plant grow wild in desert
- See weed grow out of control – rodent will eat it
- See one grow wild in middle of garths
- I'm toxic to livestock, and grow wild in desert
- Weed‘s value mainly seen by tabloid
- Weed mainly grows wild in desert
- Value mainly applied to tabloid’s wild growth
Buddy explains!

The straight part
We know that "I'm toxic to livestock" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i'm toxic to livestock = ragwort.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RAGWORT mean?
RAGWORT noun- Any of a number of wild flowering plants with yellow flowers in the family Asteraceae, mostly belonging to Senecio and related genera.