The question should be where is this grain going. I have always been under the impression that the *America can starve Britain into submission* is mildly ridiculous.
Why?
The British have money. They can afford pay over the odds. Someone else might go hungry but it is unlikely to be the British. Europeans will probably realise the potential of buying American produce and then selling it off to the British if there is some rather implausable 'patriotic' ban on selling to the British.
There's a Royal Navy outside of America's harbors, remember? How will Prussian, Dutch, etc. ships get there?
If the US grain isn't on the market because it simply cannot be exported anywhere then American agriculture is about to suffer. On the otherhand its unlikely the British would blockade resources they need, especially when it is such a good tool for dividing the Union.
I do not think 67th Tiger will play armchair general, but suspect he will pursue the policy that Britain would have pursued.