age of heifers at first breeding?

My consultants have recently recommended that I back oof my age at first breeding from 12 months to 13 months. We\'ve had very good success getting the younger heifers pregnant (and the slightly older ones as well), and the helfers appear to be large enough at be bred at 12 months of age (according to me and my vet). They are fed aggressively from calves with three time a day feeding of pastuerized waste milk, and an aggressive diet after weaning. They believe that the younger heifers will give less milk in their first lactation and continue to give less milk in later lactations. I\'ve read a study about this before, but I believe the study that I read dealt only with the age of the animals, and didn\'t account for the size at breeding or the system used to raise the calf. Has anyone else seen heifers calving at 21 and 22 months give significantly less milk than those calving at 23 and 24 months? If so, what do you attribute the difference to?

Attached is a link that may help you with your question,
http://pubs.ext.vt.edu/404/404-285/404-285.html

Regarding the in depth question pertaining to age a first breeding and or calving age for first calf heifers there is no defining answer. Heifers can successfully calf at 21 months but multiple University studies have observed a loss in first lactation milk production as compared to heifers calving at 23 months. Losses in first lactation milk yield are accelerated when calving ages are further reduced 19, 20 months etc. In the past, milk losses associated with early calving were thought to be related to fat deposition in the developing udder. More recent research has redefined this issue as a function of time. In short, some extra time maybe needed during key mammary or growth development periods (prepuberty, prefresh etc).
In the larger scope of dairy management focusing a large amount of management attention on a minimum calving age for a herd is somewhat overrated. The bigger issue with calving age is to control the variance of it. That said, pick a breeding target age suitable for your herd (i.e. 13 months). Develop a prebreeding management strategy to get every heifer at or near breeding weight target at 13 months with a keen understanding of genetic size variance. Then get every heifer bred starting at 13 months with the highest degree of breeding efficiency possible. This strategy results in all heifers calving in the narrowest age window possible. As an industry we tend to focus our attention on the age of the first heifer bred but the age of the last heifer confirmed pregant is just as important. The brain teaser always is heifers that
calve at 22 months of age come from herds with an average age of first calving of 24 months of age (because there is a breeding decay curve). Shortening the breeding decay curve can decrease the herd age of first calving by a month without changing average daily gain.

I am currently breeding all heifers first service at 11.5 months. We started doing this over a year ago and have wound up with an average age at calving of around 22.5 months. I suppose we may lose some milk in the first lactation, but our first lactation animals are currently producing about 23500 305ME. I don't think that's too shabby. I haven't seen any notable trend involving difficulty conceiving at our 70 day VWP, and I don't forsee ending the 11.5 month protocol. Interesting note: Our first lactation animals don't have a lactation curve, they have a line which gradually increases until dry off.