It's a tricky one, trish. It's easiest to get the second calf on if you do it straight away; the longer she's had just her own calf the more she's reinforced the bond and the 'count to one'
. With Hillie's first calf I added the second the very next day; with Plenty the timing was less lucky and it was a week or so before I got another calf for her. It took her a lot longer to settle to having the two.
To get the calf on, you will probably have to tie her up with some cake at first. (Seems counterintuitive to be caking when you are worried about excess milk! but it's just for a while, and you can use a 16% mix, bulked up with mollichaff or alfalfa or something to create a longer eating experience
) Bring her own calf round to the suckling position so she can see and smell it - if it will suckle, so much the better of course - then latch the foster on behind. It's best to teach the foster to come from behind to the rear teats, then it will soon learn to slip in and sneak a drink when she's suckling her own calf, and she'll notice less.
I've always had my girls inside for calving, in a large well-strawed pen, and I just bring the foster calf into the pen with mum and her own calf and keep them all together. Then twice a day, mum to where she can be tied and caked, get the foster on for a suck. Take a litre of milk in the morning to be able to offer the foster a midday drink without tying mum up again - you may find you don't need this and it's pinching enough, or is managing well enough on two feeds.
It does need to be a
large pen to do it this way, because mum will probably chase the foster calf away quite a bit at first. It needs to be able to skip out of her way and not get rammed against a wall or pen side
. Telling mum off when she does this does have an effect with mine, they pretty quickly stop being physically aggressive and settle for shaking their heads at it
.
Usually within a week I have spied the foster suckling away while she's feeding her own calf while they're all loose in the pen. Once that's happened twice I will let them all go out for the day as a family. As long as foster calf stays with her, and is clearly getting fed, you can then keep them outside and dispense with the tied-up feed.
I dehorn my Jerseys - not sure I'd leave a new foster calf alone in a pen with a new mother if she still had her horns
So all of which said, I'm not sure how to answer your question about how to tell if she needs another calf to use up all the milk. The first three or four days the bag will be humungous and very taut anyway, and her own calf drinking very little yet, so if you haven't a foster on you will probably need to handmilk a bit off to ease the tension. But don't take litres and litres the first few days; firstly the more you take the more you stimulate production, which is not what you want in your situation, and also she may be prone to milk fever in the early days so you don't want to overload her.
Just an aside - as this is a heifer, I would definitely be training her to be handmilked this first time. She may need to be handmilked in future lactations, so it's best to get her used to that right away.
So I guess if you want to see if she can manage with only her own calf before getting a foster...
Well, I think I would teach her a routine where she gets tied up and gets a bit of cake twice a day anyway, and handmilk a little off each time. If after say five or six days you are finding she still has a big taut udder when you come to handmilk your little bit off, then it would indicate to me that she is overproducing. Now you can get a foster calf in, she is already used to fiddling about going on down there while she is tied up and eating cake, so the addition of the foster behind will be less of a shock. In introducing the foster after a week you may have more difficulty getting her to allow it to suckle while they're all loose, so you may need to keep the calf in for the first week, feeding it twice a day on the cow and a midday bottle or bucket feed if it needs it. By that time it should smell of her and she should be more accepting of it being around so you could try them all loose together.
