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When I read things on the internet, I can't help but be a little skeptical about what is written. Anyone can write anything they want, and there's often no way to tell what is someones opinion and what is fact.
Who can you trust about animal information?
When I'm looking at information about animals, I tend to rely heavily on articles or studies that are written by either universities or SARE. Universities because they are usually the least financially involved in the subject being studied, and SARE because the topics that they study are often closest to the small farm situation that I'm in. What I'm going to write about here is drawn from a number of sources, but mostly from this study, from the Virginia Cooperative extension, a publication of Virgina State University. I mean no disrespect to anyone, and in fact, the study confirms a lot of what I've read on blogs about dairy cows.
Dairy cow math - warning! cow geek information follows
That compares well with the auction-reported weight of 415 pounds, so as an estimate the tape appears to be pretty good.
The reason that I want to be able to track the weight is so that I can manage her growth rate. If she grows too fast, or too slow I'm going to want to change her feed situation appropriately. I'm also going to be breeding her by weight, not by age.
I'm making that decision because I don't know her exact age. The seller reported that she was born in October, about 6 months ago, and in fact, she's right at the average weight for a 6 month old holstein heifer. From here on I want her to have steady growth, not too fast, not too slow, until she reaches a good breeding weight. And during her pregnancy she needs to continue to gain weight at about the same rate while growing the calf.
I'd also like to breed her so that she calves at at time of year that works for me, and for my farm, and for my grass; so I've got a little bit of a deadline, too.
At a weight of 420 pounds, I have another 310 pounds to put on her before she's bred. But it's not as simple as that. From the study:
"There is a critical period when overfeeding can have a detrimental effect on udder development. This begins at about 3 months of age and ends at puberty or approximately 9 to 10 months of age. This is referred to as the allometric period of mammary growth. During this period, udder growth and development is 3.5 times that of other body systems. Studies indicate that when overconditioning during this period occurs, mammary secretory or milk producing tissue in the udder is greatly reduced and replaced with fat. Temporary periods of rapid gain after puberty are acceptable and may allow compensatory adjustments for weight gain to our target at 24 months and weighing 1350 pounds the day of calving. However, ADG's should be limited to not exceed 1.7 lbs. per day during this 3 to 9 months of age period. "
ADG is Average Daily Gain. I need to manage her so that she doesn't gain more than that per day. So I need to add 310 pounds to her at a max of 1.7 pounds a day, which gives me a rough goal of 182 days from now, or 6 months. Which means that the recommended time I can breed her will be in October.
Having learned this, this is a reason that you might consider buying a calf that is 3 months old if you want to control its milk-cow potential during this critical period. My little heifer looks like she's right on the average growth line for holsteins, so I think I'm good there.
So if I breed her in October, I'll be calving in July.
Is that a good time of year to calve? That's a question for you dairy cow folks. In a commercial dairy I suspect that they'd just calve as soon as they could given the limitations above.