Preliminary study on heritability of milk yield during the day of test

in Macedonian Awassi sheep
B. Palasevski, M. Gievski, G. Dimov and N. Pacinovski

 

Summary

 

About 400 ewe/test day records of Awassi sheep flock for the individual milk yield in the morning, midday and evening milking, in the day of test and fat content were used for assessment of heritabilities and genetic correlation by five-trait Animal model with two random uncorrelated effects.

It was found that the heritability for milk yield vary from 0.14 to 0.40 with the highest values in the morning. Genetic correlations between the four traits of milk yield were all very high and positive. Heritability for fat percentage was 0.21 and the correlations with yield traits were all high and negative.

The importance of the morning milking was emphasized and the necessity of more extensive studies with more data.

 

Keywords: awassi, test day, milk yield, fat percentage, heritability, genetic correlations

 

 

Introduction

 

The successful selection of dairy sheep depends to the knowledge for genetic variation of milk yield traits. During the last years more emphasis were made on the milk yield of sheep during the day of test (Ligda et all., 2002). The studies emphasize the importance of the trait in comparison to total milk yield obtained in the milking only period.

Milking frequency of sheep in the day of test depends on the flock stage of lactation and level of production. Higher production is a preposition for three times a day, while it is switched to two-fold in the third part of lactation.

The milk yield during separate milkings differ in amount, but of interest is also the extent of inheritance of each of these partial day yields in the morning, at noon and evening.

The aim of the study was to make a preliminary study on the additive, phenotypic variances and heritability estimates for each of daily milkings of Awassi sheep in AMF. 

 

 

Materials and methods

 

Milk yield of 35 the pure bred Awassi ewes was measured in the morning, in the middle of the day and in the evening. Sheep were reared in the Awassi Mediterranean Farm (AMF) in Kumanovo, Macedonia, which was established as a repro-centre of the breed. Ewes were on the age from first to eighth lactation, most of them were with one lam at lambing. Milk yield was measured on the monthly bases in two consecutive years: 2001 and 2002.

The traits in the study were the individual milk yield of ewes at morning, midday and evening measurement, daily yield and fat percentage. Each of these traits was measured on about 400 test day/ewe samples of 55 ewe/lactations. Distribution of records per trait and factor in the study were given in Table 1.

The influence of the non-genetic effects was tested previously (Dimov et al., 2005). The important factors were age, stage of lactation, month-year of test and suckling period. The number of lambs born did not influence the daily production, type of lactation curve affected only some of the measurements. All these were included in the fixed part of the five-traits Animal model used in the study:

 

Y = a + ipe + lpe = e,

 

where Y – the measurement of one of the five traits – morning, midday, evening, daily milk yield and fat % for the day of test,

a – individual additive effect,

ipe- individual permanent environmental effect of the lactation,

lpe- permanent effect of test day within the lactation.

 

The relationship matrix included all relatives of ewes up to 3 generations back. The across traits noncorrelated covariances were neglected, e.g. were set to zero. The applied programmes were these of the MTDFREML set (Boldman et al., 1993).

 

 

Results and discussion

 

1. Average productivity

Average values for the 5 measured traits were in the table 1. Milk yield decreased from 0.62 l in the morning to 0.42 l at noon and 0.43 l at the evening. Total average daily yield was 1.42 l with fat content of 7.52 %. The milk yields were with similar variation of 45-50% which showed the considerable influence of the factors on them. The averages for milk yield and fat % characterized the productivity of the ewes from the AMF as representatives of the specialized breed for milk production.

 

Table 1.

Average values for the milk yield (l) measured in different consecutive milkings during the day of test of Awassi sheep

 

Trait

Average

SD

CV

Min

Max

Morning

0.619

0.306

49.47

0.060

1.700

Noon

0.420

0.198

47.18

0.050

1.100

Evening

0.427

0.194

45.27

0.060

1.150

TDY

1.420

0.647

45.57

0.180

3.750

Fat %

7.517

1.438

19.14

4.000

9.940

 

2. Additive variances

Additive genetic variances slightly decreased during the day from 0.27 l in the morning to 0.21 l at noon and 0.17 l in the evening. The total yield was with higher value of 0.34 l. Similar values of variances suggested, that genetic variation could be assessed with similar accuracy. On the other hand lower evening variance in the evening milking and almost the same average milk yield at noon and at the evening is an indicator that the evening productivity is less important in selection assessment of the ewes. The results are tentative to some extent as the number of observations is very small.

Covariances between traits of milk yield (the off-diagonal elements of table 2) were also close in values. They were all positive and similar to the variances which showed that similar and positive correlations could be expected for these traits. The only negative covariances were with the fat% which was expected because of the nature of these two groups of traits – milk yield and fat content. 

 

Table 2.

Additive variances (on the diagonal) and co-variances for the milk yield in different parts of the day and for the total test day (TDY), l, and fat %

 

Traits

Morning

Noon

Evening

TDY

fat %

Morning

0.27365

0.23639

0.21515

0.29947

-0.61222

Noon

0.23639

0.20557

0.18585

0.25599

-0.53213

Evening

0.21515

0.18585

0.16993

0.23705

-0.48319

TDY

0.29947

0.25599

0.23705

0.33781

-0.66508

Fat %

-0.61222

-0.53213

-0.48319

-0.66508

1.38662

 

3. Variances of noncorrelated effects

Two variances of noncorrelated effects were included in the models. The first one was determined by different lactations of the same ewe, e.g. individual permanent effect,  σ2ipe. The second one presented the common lactation effect, σ2lpe.  The values for different effects and traits were very small. The proportion to total variance was also very small and amounted about 1 - 2% of it. The results suggested that for test day traits of dairy sheep these effects are not very important. The decision, however, for their inclusion or exclusion in genetic assessment have to be studied carefully and comparatively, as even with small effect they could influence the additive variance, heritability and the corresponding breeding values to considerable extent. From breeding point of view the proper modeling is important for accurate ranking and from here for the proper selection of the best parents for producing the replacement generation. A comparative study is a possible way for taking the proper decision for the best genetic evaluation model.

     

4. Heritabilities and genetic correlations

The highest heritability of 0.40 was found for the morning milking. The midday yield was with the lowest value of 0.14 and at the evening – with an intermediate one of 0.24. For the daily yield coefficient of inheritance was 0.24. Similar values of 0.35 were recently reported for the test day yield in Chios breed in Greece (Ligda et al., 2002).

The results suggested that most important for assessment of the milk yield are the results in the morning. It is difficult to speculate for the reason. One might be that during the night the milk secretion period is longer comparing to the midday and evening milkings. Another reason is in lower environmental effects at that time and in less stress factors, as the milk disturbs the normal production process in the flock.

A possible result of the study is that for less important sheep flocks, or for these that do not produce rams, the recording might be simplified with an individual test only in the morning and with utilization of coefficients for approximation of the daily yield.

Fat percentage was with similar heritability of 0.21 to these for milk yield. This value is small taking into account much higher values in majority of studies in dairy cattle, but it was very close to the studies with Chios where the same coefficient of 0.21 was found (Ligda et al., 2002).

Genetic correlations between the milk yield in different part of the day of test and the total yield during the test day were all positive, very high and close to 1. It also suggested that one of the milkings is a good indicator for the yield especially when the recording have to be carried out in larger flocks.

Genetic correlations of the yield traits and fat percentage were also very high and negative as was expected from the covariances. Intermediate negative correlations of -0.35 were reported for the discussed above study with the Chios breed (Ligda et al., 2002). The obtained results of the above correlations with the Awassi sheep are too high. All these have the meaning of a preliminary study because of the small number of records. The collection of more data and some future more extensive studies are in progress.

 

Table 3.

Heritabilities (on the diagonal) and genetic correlations for the milk yield in different parts of the day and for the total test day (TDY) and fat %

 

Traits

Morning

Noon

Evening

TDY

fat %

Morning

0.40

 

 

 

 

Noon

1.00

0.14

 

 

 

Evening

1.00

0.99

0.24

 

 

TDY

0.98

0.97

0.99

0.24

 

Fat %

-0.99

-1.00

-1.00

-0.97

0.21

 

 

Conclusions

 

- The test day milk yield of sheep in the Awassi MF was with intermediate heritability from 0.14 to 0.40.

- Fat percentage was with relatively low heritability of 0.21.

- Genetic correlations between measurements in the day of test were all high and positive.

- Genetic correlation between test day yield and fat percentage in the day of tes was high and negative.

- The morning milking is with the highest importance in selection of ewes.

 

 

References

 

1. Boldman, K., L.Kriese, L.D. Van Vleck, S.Kachman (1993). A Manual for use of MTDFREML. USDA. ARS.

2. Dimov,G., Pacinovski, N., Gievski, M. (2005). Preliminary study on the basic factors which influence daily milk production of sheep in the Awassi Mediterranean Farm. Scientific conference. Trojan. 26-27 May, 2005.

3. Ligda, Ch., Mavrogenis, A., Papadopoulos, Th., Georgudis, A. (2002). Genetic parameters for test day milk traits and somatic cell counts in Chios dairy sheep. In: “Breeding programmes for improving the quality and safety of products. New traits, tools, rules and organization” IZSC, Sassari, Italy, 9-11 May 2002.

4. Gootwine, E., Bor, A., Braw-Tal, R. Zenou, A. (1995). Reproductive performance and milk production of the improved Awassi breed as compared to its crosses with the Borola Merino. J. Anim. Sci. 60:109-115.

5. Gootwine, E., Alef B., and Gadeesh, S. (1980). Uder conformation and its hertibility in the Assaf cross of dairy sheep in Israel.

6. Mavrogenis, A.P. (1996). Environmental and genetic factor influencing milk and growt traits of the Awassi sheep in Cypruc. Small Rumin.Res.

7. Petrovic, M. (2000). Genetika i oplemenivanje ovaca. Naucna knjiga. Beograd.

8. Tjankov, S. (2000). Ovcevodstvo i Kozevodstvo, Stara Zagora.

9. Epstain H. (1985). The Awassi sheep with special reference to the improwed dairy typ. FAO.

 

Egyptian Journal of Sheep, Goat and Desert Animals Sciences (2006) 1(1): 41-46

 

Authors:

B. Palasevski, N. Pacinovski, Institute of Animal Science, Skopje, Macedonia

M. Gievski, Awassi Mediterranean Farm - Selection Center, Kumanovo, Macedonia

G. Dimov, AgroBioInstitute, Sofia, Bulgaria