254 M
. Serrano et al. Livestock Production Science 67 2001 253 –264
1. Introduction systematic, environmental and genetic effects direct-
ly, where they are expressed, on the day of record- In Latxa and Manchega Spanish dairy sheep, test
ing. day record TD collecting is carried out under the
Effects that influence TD production are not International Rules for Milk Sheep Recording
different from those that affect complete lactation ICAR. Most records in both breeds are collected
yield in general Danell, 1990. Nevertheless, for TD monthly under an alternate morning evening system
measures there are differences from lactation records following ICAR rules. To standardize records to 120
and, therefore, different approaches have been used days, at least four TD per ewe are required, other-
to consider environmental effects. Modeling the herd wise, in Manchega ewes, lactation is projected to 120
effect as herd-test day HTD instead of herd-year- days by extension factors developed previously
season of parturition HYS leads to reduced residual Serrano et al., 1996a. In Manchega, 16 of the
variances Ptak and Schaeffer, 1992, 1993. Includ- available lactation records lack the third and or four
ing new environmental effects, such as the stage of TD and need to be projected. When the first and or
lactation measured in number of days in milk, allows second TD records are missing, lactations are not
a better estimation of environmental variability. projected and the records are discarded 3.
However, modeling TD records causes some The goodness of the standardization and projec-
problems with respect to the contemporary group tion procedures depends on the quality of milk
CG definition, the treatment of within and among recording with regard to temporal aspects. TD re-
parity information and the increase of computational cords are frequently collected at highly variable time
demands due to the larger number of records to be periods due to animal management. This implies that
managed. estimation of standardized yields depends not only
Various authors Meyer et al., 1989; Ptak and on TD yield but also on the lactation stage of the
Schaeffer, 1992, 1993; Stanton et al., 1992; Strabel animal from which test samples were collected. This
and Szwaczkowski, 1995; Swalve, 1995; Rekaya, traditional approach of using lactation records has
1997 have suggested that models with HTD classes been criticized as inconsistent, since the aggregation
are superior to HYS based on residual variances. of records taken at defined locations and time is
However, the HTD model is rather problematic rather trivial.
regarding the size of the CG. In several studies Numerous studies in dairy cattle Danell, 1982;
Reents et al., 1995; Swalve, 1995; Van Bebber et al., Meyer et al., 1989; Pander et al., 1992; Ptak and
1997 it has been pointed out that this may be an Schaeffer, 1993; Reents et al., 1995; Swalve, 1995;
important obstacle when trying to fit a HTD model. Rekaya, 1997; Wiggans and Goddard, 1997 and
In dairy cattle, different models have been pro- fewer in goats Schaeffer and Sullivan, 1994 and
posed to estimate the covariance structure among sheep Baro et al., 1994; Barrillet and Boichard,
TD measures. Repeatability models that consider TD 1994; Georgoudis et al., 1997; El-Saied et al., 1998;
as repeated measures of the same trait within lacta- Serrano et al., 1998 have dealt with the use of TD
tion and as multiple traits across lactations Reents et records as an alternative to standardized lactation
al., 1995. Models where TD are considered as yields. Use of test days has the advantage of directly
different traits within and along lactations Wiggans considering records at their origin. Under TD
and Goddard, 1997, for which the main constraint is models, records on the sample day are considered
the high computational demand. Random regression directly in analysis and no assumption about the
models RRM are also being applied to fit the shape length of a lactation has to be made Visscher and
of the lactation curve Schaeffer and Dekkers, 1994; Goddard, 1995. Models that consider single TD
Jamrozik and Schaeffer, 1997; Rekaya et al., 1999. records allow the removal of abnormal measures
This kind of model seems to be adequate for traits avoiding the elimination of entire lactations. On the
which repeated measurements are taken on a trajec- other hand, records do not have to be projected using
tory of time and allows us to obtain information on a ‘extension’ factors and all information can be used.
trait that changes gradually over time. Finally, the With TD models an attempt is made to account for
use of covariance functions to fit different covar-
M . Serrano et al. Livestock Production Science 67 2001 253 –264
255
iances among repeated records has been proposed yield and protein percentage TD records. Given the
Kirpatrick et al., 1994; Meyer and Hill, 1997 to large size of the data set available 94,191 lactation
reduce the number of parameters to fit when there records and 70,207 pedigree records, several edits
are many measures for each individual taken over were performed so that the data set was more
time which are considered as different traits. manageable and a better correspondence between test
In this study, a genetic evaluation model is defined day number and date of test was achieved. First TD
and genetic parameters are obtained for TD milk between days 30 and 72 from parturition and time
yield and protein percentage records in two different interval between successive tests of more than 25
Spanish dairy sheep breeds. Some considerations are days and less than 35 days were forced. In addition,
made to develop the models. Firstly, since in dairy a minimum milk yield in each test of 200 ml and no
ewes there are generally four TD records in a missing values for any TD were required. Identifica-
lactation in contrast to the 10 in dairy cattle, compu- tion of dam and number of lambs born plus existence
tational demands are much smaller and allow a of both milk and protein percentage records were
multivariate approach within lactation. Secondly, also required. Finally, data were sampled by HYS
dairy sheep herds have a larger average size than CG, discarding CG with less than six observations.
dairy cattle herds, which partially avoids problems After these edits, 22,804 milk yield and protein
with HTD CG size. percentage TD records from 5701 first lactation ewes
of 32 herds were kept and used to estimate genetic parameters. The pedigree file consisted of 10,883
2. Material and methods animals.
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