1 Lactation records of milk and fat production provide important
information for managing a dairy goat herd and for breeding better goats. Environmental factors such as length of
lactation, age of doe, and season of kidding should be standardized through
appropriate adjustment factors to make genetic evaluations more accurate.
2 Length of Lactation
A lactation length of 305 days has been defined as the standard for dairy cattle and also is used for goats. This standard
assumes a 365-day interval between parturitions, which includes a 60-day dry period. However, many goats do not milk 305 days. A recent study shows that only one-third of all does with official records
ending with a dry date milked 305 days. One reason for shorter records is that
production of many does declines sharply with the onset of seasonal estrus and the does then are dried off. A standard lactation length of less than 305 days might be more useful for comparisons among does; however, the 305-day standard allows for reduced computing costs because
doe and cow records can be computed the same way.
3 If a doe's lactation ends on or before 305 days because her production declined to the point at which continued milking was not worthwhile, then her record is considered complete. Such records are
not projected to 305 days but are treated as complete 305-day records. If a lactation ends before 305 days for any reason other than going dry, such as the doe's being sold or the herd's discontinuing testing,
the record is considered incomplete and is adjusted. If a doe is still milking and has fewer than 305 days in milk, the record is considered incomplete, adjusted, and referred to as a record in progress. If a doe
milks for more than 305 days, the production for only the first 305 days is included in the 305-day record.
4 A method to adjust incomplete records and records in progress uses the USDA projection factors in Table 1. Different
categories of factors are required to adjust records because of variations in the lactation curve, particularly in the rate of decline of production after the peak
and the number of days milked. To select the appropriate factor, the following information is necessary: breed, herd
average production, month of kidding, days in milk, and age of doe at kidding.
5 An adjusted or projected record is the incomplete record plus an estimate of production for the rest of the lactation.
That estimate is the projection factor times the last sample-day production times the
number of days from the end of the incomplete record through 305 days.
6 Suppose a Nubian doe freshens in March at 25 months of age. She has an incomplete record at 130 days of 800 lb milk and 27 lb fat. Her last sample day production is 5 lb milk with 3.4 The herd average is 1,725
lb. Then, her projected record would be 800+0.69(5) (305-130) =1,404 lb milk and 27+0.76(5) (0.034) (305-130) =50 lb fat.
7 Age-Season Lactation production increases with age until maturity
and thendeclines. Month or season of kidding also influences lactation production. For example, does kidding in the early spring produce more milk than those kidding later in the year. Lactation
records can be adjusted to a common age and season of kidding to standardize the
effects of age and season. The factors in Table 2 standardize production to that expected from a doe kidding from January through March at 36 months of age.
8 The adjustment factors vary by breed, age, and season of kidding. Records are adjusted for age and season by multiplying
production by the appropriate factor from Table 2. To illustrate, consider the Nubian
doe from the previous example: Adjusted milk=1.07(1,404)=1,502 lb Adjusted fat=1.08(50)=54 lb
9 The factors in Table 2 were computed by assuming a smooth change by age and by ignoring lactation number. Recent results,
however, suggest that lactation number should be considered, particularly for does
kidding about 24 months of age. Equations recently developed at the University of California in Davis provide factors that
vary by parity. These factors may be more accurate; however, the factors reported here should contribute to improved comparisons among does.
