It appears that nobody is interested in this topic so I'll close it down. I actually have a much more complicated scenario than I've described and I worked it all out manually so I now don't need any help.
But this looks to be a really useful and interesting site. I'm sure I'll be back.
I want to estimate the number of male and female cattle produced over a period of time (say, five years and ten years) from an initial stock of 30 heifers, all 18 months of age at which age they are first joined (mated). Assume that the first calf is born when the mother is 27 months old...