Diagnosing the crop disorder
A hen is behaving lethargically, not eating much, if at all, but she has a full crop. This is the first sign something's not right. The full crop points to this hen having crop issues and not being sick for some other reason such as a bacterial infection, which would result in loss of appetite and empty crop.
Before you treat for a crop issue, though, you need to verify that your hen or roo actually has a crop issue and whether it's sour crop or impacted crop or both. So you need to monitor the crop overnight, checking the condition before bedtime, and first thing in the morning before your patient eats anything.
A crop that is still full in the morning has issues. So, is the issue sour crop? If the crop feels squishy and full of liquid, it's likely sour crop. If there's an odor coming from the head of the chicken that smell similar to sauerkraut, then you are probably dealing with sour crop. The cause is a yeast called Candida albicans.
If the crop is extremely full and hard, or perhaps lumpy, and maybe feels like it's full of fibrous material, you are probably looking at impacted crop. If the chicken has been drinking lots of water, yet not eating anything, this is further indication of impacted crop.
Impacted crop/sour crop
If the crop is full and hard and lumpy and the chicken has been drinking lots of water and it smells like sauerkraut, you likely have an impacted crop that has developed a yeast infection. You will be treating the impacted crop first, followed by treatment for the yeast infection.
Treatment for impacted crop
I use coconut oil for the ease in administering it. When chilled, it is solid and easy to break into small chunks and slip into the beak of the patient without creating a huge mess or getting oil into the airway. You want to measure two teaspoons for an adult chicken and one teaspoon for a baby chick.
After getting the oil into the patient, you want to massage the crop gently in a circular and slightly upward motion. This will direct the contents toward the crop "drain". Massage for five to ten minutes. If the crop refuses to empty, repeat the oil and massage again in 30 minutes. If the crop still refuses to empty, then give a stool softener such as Dulcolax (docusate sodium). Wait 30 minutes and massage the crop. The crop should empty. Add more oil if it doesn't and massage again. This should do it.
Treatment for sour crop
I advise against trying to make your chicken vomit because it may cause them to aspirate the sour liquid. Besides, it's very unpleasant for your hen, and she may hate you if you do it. (Curiously, most sour crop victims are hens.)
Nystatin is the best treatment for yeast infections, but it requires a prescription. Or you can try to locate medistatin which is for birds and doesn't require a prescription.
The easiest (and cheapest) to obtain yeast treatment, though, is miconazole, found on the women's hygiene shelf in the pharmacy. You can use either the suppositories or the vaginal cream. Measure a quarter inch of suppository or about half an inch of cream and give orally twice a day for seven days. Do not stop treatment before the full seven days are completed or the yeast may return.
Following treatment for sour crop, offer plenty of plain fresh water and boiled egg to get the crop operating again. I like to also give a probiotic or Greek yogurt to restore good microbes in crop and intestines.
If you have treated for these crop issues and the crop still refuses to empty by morning, the hen may have a condition called pendulous crop. This is caused by poor muscle tone that causes the crop to sag and the contents are below the crop "drain" so the crop doesn't fully empty. The solution is a crop bra.
You can buy these or easily create one yourself by gluing or sewing a four inch square piece of sturdy cloth to two twelve-inch long strips of elastic bandage such as Vet wrap.