If it's June, "this month" means June.
So if today is June 25th, and you ask "which cows are due this month", you'll only see 5 days worth of cows listed.
If you want to see a month's (or a weeks) worth of cows listed, use "due in the next 30 days", or "due in the next 7 days".
If it's currently June and you want to see cows due in July, you can use
Which cows are due next month ?
Which cows are due in July ?