Praca oryginalna Original paper
Recent years have witnessed a significant, nearly two-fold increase in milk production levels in high--yielding dairy herds, as a consequence of crossing domestic cows with HF bulls (1, 10, 23). Such an improvement in dairy cattle performance has resulted from rapid changes in the environmental conditions of milk production in Poland, access to the global gene pool and advanced breeding programs in developed countries (6). HF bulls, used for improving the local dairy cattle, contributed not only to increased producti-vity but also to significant changes in the body confor-mation of cows (particularly in the active population), including improvements in growth performance, udder quality and leg conformation. Dairy cattle should be characterized not only by high milk production in the first lactation, but also by high lifetime productivity that contributes to stable and high milk production profit-ability (10). High lifetime productivity allows for the
reduction of herd replacement costs. However, the pro-ductive life of dairy cattle tends to become shorter (15). It is very difficult to improve productive longevity of cows, since this is a low heritability trait (17).
In recent years, dairy cattle improvement schemes have involved primarily breeding bull selection and evaluation. Liberalization of international trade has facilitated access to the worlds animal genetic resources. As a rule, bull mothers are inseminated with semen of elite progeny-tested HF sires, mostly from the US, Canada the Netherlands and France. Young breeding bulls are also imported for the purpose of insemination and embryo transfer, which promotes genetic progress in the local dairy cattle population (15).
The aim of this study was to determine the effect of local (PHF) and imported (HF) sires on the performance, fertility, conformation and productive longevity of their daughters.
Relationships between sire effect, milk production
in young cows and their productive longevity
KAZIMIERZ KONSOWICZ, JANINA POGORZELSKA*, JAN MICIÑSKI*, WIES£AW SOBOTKA**, GRZEGORZ ZWIERZCHOWSKI*
Animal Breeding and Insemination Center, Zamczysko 9a, 85-868 Bydgoszcz, Poland *Department of Cattle Breeding and Milk Evaluation, Faculty of Animal Bioengineering, University of Warmia and Mazury in Olsztyn, Oczapowskiego 5, 10-957 Olsztyn-Kortowo, Poland
**Department of Animal Nutrition and Feed Management, Faculty of Animal Bioengineering, University of Warmia and Mazury in Olsztyn, Oczapowskiego 5, 10-957 Olsztyn-Kortowo, Poland
Konsowicz K., Pogorzelska J., Miciñski J., Sobotka W., Zwierzchowski G.
Relationships between sire effect, milk production in young cows and their productive longevity
Summary
The objective of this study was to determine relationships between sire effect, milk production in young cows, their performance in subsequent lactations, fertility, conformation and productive longevity. A total of 1724 cows that first calved between 2003 and 2005 were analyzed during 4475 lactations. The cows were kept in 10 barns located in the Provinces of Pomerania and West Pomerania, Poland. The analysis covered 1085 daughters of 145 local Polish Friesian (PHF) sires and 639 daughters of 128 imported Holstein--Friesian (HF) sires, including 437 cows culled after the first lactation, 445 cows culled after the second lactation and 140 long-lived cows. Research has shown that among first-calf heifers and the second lactation cows more than 55% of daughters of imported bulls and about 48.75% of daughters of domestic bulls were culled. Essentially higher results of sort ratings concerning the overall appearance, body conformation, udder quality and frame size, efficiency of milk and its components were acquired after the imported bulls. High milk production levels, exceeding 8000 kg of milk in the first and second lactation, had an adverse effect on the productive life of cows. A correlation between milk production of milk cows and their productive longevity was proved as evidenced by the fact that cows which were used for at least five lactations produced less milk and its components as the first-calf heifers than cows which were scrapped later.
Material and methods
A total of 1724 cows that first calved between 2003 and 2005 were analyzed during 4475 lactations. The cows were kept in 10 barns located in the Provinces of Pomerania and West Pomerania, Poland. The analysis covered 1085 daughters of 145 local Polish Holstein-Friesian (PHF) sires and 639 daughters of 128 imported Holstein-Friesian (HF) sires.
The yield (kg) of milk, milk fat and protein, and the content (%) of milk fat and protein were determined during 305-day lactations: first, second and subsequent lactations. The following parameters were also analyzed: age at first calving, the length of lactation and the calving inter-val, cow productive life and longevity, body conformation scores, reasons for culling, and the breeding values of bulls sires of the analyzed cows.
The performance results of the following were compared: a) long-lived cows in their first lactation and cows culled after the first and second lactation, b) long-lived cows in their second lactation and culled cows. Long-lived cows (i.e. cows that continued to be productive for five or more lacta-tions) were analyzed in view of the country of origin of their fathers. The analysis covered 437 cows culled after the first lactation, 445 cows culled after the second lactation and 140 long-lived cows.
PHF and HF bulls were compared based on their breeding values for production and functional traits (PF index) and the values of subindices proposed by the National Research Institute of Animal Production in Balice. The yields of milk and major milk components were determined based on data supplied by the Polish Federation of Cattle Breeders and Dairy Farmers.
The results were processed statistically, arithmetic means and standard deviations were calculated, and non-orthogonal ANOVA was performed. The significance of differences between means was estimated by Fishers LSD test. All cal-culations were made using SPSS ver.18.0.
Results and discussion
Table 1 data show that approximately half of cows in both groups daughters of PHF sires (48.75%) and HF sires (55.24%) were culled after the first or second lactation. Only 23.23% of daughters of PHF sires and 25.20% of daughters of HF sires were productive for more than three lactations. Long-lived cows, i.e. cows
that continued to be productive for at least five lacta-tions, accounted for only 6.55% and 10.80% among the daughters of PHF and HF sires, respectively. Due to small differences in culling rates after successive lacta-tions, the average productive life of cows was similar in both groups (2.6 lactations).
Table 2 presents the breeding values of PHF and HF sires whose daughters were evaluated in this study. Imported bulls had significantly higher values of the PF index, and production (SI-PR) and conformation (SI-CO) subindices. The values of the fertility sub-index (SI-FE) and somatic cell score (SCS) differed insignificantly between groups.
HF sires, which had significantly higher values of the conformation subindex, added better body conforma-tion to their offspring (Tab. 3). Daughters of HF sires, compared with daughters of PHF sires, received highly significant higher scores for overall appearance, body conformation (type), udder quality and frame size. Cows of both groups did not differ significantly with respect to leg conformation. The percentage of long-lived cows was considerably lower in the group of daughters of PHF sires than in the group of daughters of HF sires, which could result from less
Primiparous cows daughters of PHF and HF sires were characterized by very high productivity (Tab. 4). Age at first calving was 823.2 days and 843.3 days in daughters of PHF sires and HF sires, respectively (significant difference at p £ 0.01). In daughters of PHF sires, the first lactation lasted 363.2 days, and first lac-tation milk production reached 8173.7 kg. Daughters of HF sires were characterized by a significantly (p £ 0.01)
Tab. 1. Culling of cows-daughters of PHF and HF sires in successive lactations numbers of cows (%)
n o it a t c a L DaughtersofPHFsries DaughtersofHFsries 1st 265(24.42) 172(26.92) 2nd 264(24.33) 181(28.32) 3rd 304(28.02) 125(19.56) 4th 181(16.68) x92(14.40) 5th 44(4.06) 41(6.42) 6th 23(2.12) 18(2.82) 7thandsubsequent x4(0.37) 10(1.56) l a t o T 1085(100.00) x639(100.00)
Tab. 2. Breeding values of PHF and HF bulls (x ± s)
Explanations: means in columns followed by different letters are significantly different: A, B at p £ 0.01; PF production and functional trait index; SI-PR production subindex; SI-CO conformation subindex; SI-FE fertility subindex; SCS soma-tic cell score
d e e r B Breedingvalues F P S-IPR S-ICO S-IFE SCS s ll u b F H P 09.60.690A 909.0.669A 09.41.10A7 09.908.28 09.41.196 s ll u b F H 100.30.423B 100.50.675B 100.30.786B 01.0100.98 09.51.123
Tab. 3. Body conformation of cows-daughters of PHF and HF sires (x ± s) Explanation: as in Tab. 2 n o it a c if i c e p S DaughtersofPHFsries DaughtersofHFsries e zi s e m a r F 81.6A±0.070 83.4B±0.055 s g e L 78.5 ±0.066 78.3 ±0.066 r e d d U 77.8A±0.060 78.9B±0.060 e p y T 79.5A±0.064 81.5B±0.053 e c n a r a e p p a ll a r e v O 78.9A±0.049 80.2B±0.040
higher milk yield (by 727 kg) during the first lactation, which was 16 days longer than in daughters of PHF sires. The yields of milk fat and milk protein in the first lactation were also significantly (p £ 0.01) higher in daughters of HF sires, by 36.2 kg and 23.2 kg, respecti-vely. The fat content of milk was significantly (p £ 0.05) higher in daughters of HF sires than in daughters of PHF sires (4.35% vs. 4.28%). Differences in the protein content of milk between groups were statistically non--significant (3.37% vs. 3.38%).
The obtained results point to a correlation between the productive life of cows and first lactation milk production. Table 5 data show that long-lived cows, which were productive for five or more lactations, first calved at an older age (840.2 day on average) and were characterized by significantly (p £ 0.01) lower first lac-tation milk production (7302.6 kg), compared with cows culled after the first and second lactation (8789.2 kg and 8486.7 kg, respectively). The yields of milk fat and protein were significantly (p £ 0.01) higher in cows culled after the first lactation than in long-lived cows. Milk from long-lived cows had the highest fat content and the lowest protein content (4.37% and 3.30%, respectively). The first lacta-tion of long-lived cows lasted for only 334.2 days, and it was significantly shorter than the first lactations of cows culled after the first and second lacta-tion (391.6 days and 359.5 days, re-spectively).
Very high milk production levels in the first and second lactation of young growing cows, considerably exceeding 8000 kg, had an adverse effect on their productive longevity. This could be due to an overly long period of negative energy balance after calving, leading to health status deterioration and culling.
As demonstrated by Tab. 6 data, the inter--calving interval was significantly (p £ 0.01) longer (422.2 days) in cows culled after the second lactation. As a result, the lactation pe-riod lasted for 356.6 days and milk production exceeded 9650 kg. In long-lived cows in their second lactation, the average inter-calving interval was 403.4 days, the second lactation period lasted for 324.6 days and milk produc-tion reached 8251.1 kg, at a lower milk fat and protein content. The above results are indicative of a correlation between milk production levels and productive longevity of cows.
Long-lived cows-daughters of PHF sires first calved at around 835 days of age, i.e. 12-13 days later than cows culled after the first and second lactation (Tab. 7). In all groups, daughters of HF sires first calved at an older age than daughters of PHF sires. Cows that were oldest at first calving had the shortest productive life, and were
Tab. 4. Age at first calving, length of 1st lactation, milk yield
and composition in primiparous cows (x ± s)
Explanations: means in lines followed by different letters are significantly different: A, B at p £ 0.01; a, b at p £ 0.05 s r e t e m a r a P oDfaPuHgFhsteriress oDfaHuFghsterierss ] d a e h [ s w o c f o r e b m u N 1085 639 ] s y a d [ g n i v l a c t s ri f t a e g A 823.2A±0.012 843.3B±0.013 1 f o h t g n e L stlactaiton[days] 363.2A±0.062 379.4B±0.066 1 n i d l e i Y stlactaiton[kg] k li M 8173.7A±0.004 8900.7B±0.003 t a F x347.7A±0.095 x383.9B±0.151 n i e t o r P x277.2A±0.118 x300.4B±0.109 ] % [ t n e t n o C t a F 4.28a±1.208 4.35b±1.000 n i e t o r P 3.38 ±1.846 3.37 ±1.000
Tab. 5. Age at first calving, length of 1st lactation, milk yield and composition in
primiparous cows depending on productive life (x ± s)
Explanations: as in Tab. 4 s r e t e m a r a P Onelactaiton Twolactaitons Filvaectoaritomnosre ] d a e h [ s w o c f o r e b m u N 437 445 140 ] s y a d [ g n i v l a c t s ri f t a e g A 835.7 ±0.014 825.0 ±0.013 840.2 ±0.011 1 f o h t g n e L stlactaiton[days] 391.6A±0.060 359.5Ba±0.070 334.2Bb±0.055 1 n i d l e i Y stlactaiton[kg] k li M 8789.2A±0.036 8486.7A±0.040 7302.6B±0.004 t a F x371.7A±0.091 x366.0A±0.156 x317.1B±0.093 n i e t o r P x298.8A±0.111 x287.3A±0.111 x241.9B±0.127 ] % [ t n e t n o C t a F 4.25A±1.287 4.35B±1.349 4.37B±1.352 n i e t o r P 3.39 ±1.778 3.39 ±1.793 3.30 ±1.900
Tab. 6. Average inter-calving interval, length of 2nd lactation, milk yield
and composition in cows culled after 2nd lactation and in long-lived cows
(productive for five or more lactations) (x ± s)
Explanations: as in Tab. 4 s r e t e m a r a P Cullleadctaatfiteorn2nd Porromdourceitvleacftoaritoifvnes ] d a e h [ s w o c f o r e b m u N 445 140 ] s y a d [ l a v r e t n i g n i v l a c -r e t n i e g a r e v A 422.2A±0.039 403.4B±0.023 2 f o h t g n e L ndlactaiton[days] 356.6A±0.066 324.6B±0.062 2 n i d l e i Y ndlactaiton[kg] k li M 9651.4A±0.003 8251.1B±0.004 t a F x397.6A±0.081 x358.9B±0.086 n i e t o r P x328.1A±0.098 x275.4B±0.115 ] % [ t n e t n o C t a F 4.22A±3.751 4.17B±3.880 n i e t o r P 3.41a±1.900 3.36b±1.774
culled after the first lactation. The fact that daughters of
HF sires first calved at an older age than daughters of PHF sires is difficult to explain, since usually daughtersof imported sires calve at a younger age than daughters of local sires. In the first lactation, the yields of milk, milk fat and protein were significantly higher in daughters of HF sires. In young cows, culled after the first and second lactation, milk production exceeded 9000 kg. In long-lived cows-daughters of HF sires first lactation milk production was lower by over 1000 kg. Greater differences in milk yield, which was significantly lower, were noted in primiparous cows-daughters of PHF sires. Cows culled after the first lacta-tion produced the largest amounts of milk 8517.7 kg. First lactation milk production levels in long-lived cows were considerably lower, at 6701 kg. The above data indicate a correlation between the productivity and longe-vity of cows (Tab. 8).
In long-lived cows-daughters of HF sires, second lactation milk production reached 9138.5 kg (Tab. 8), and it was significantly (p £ 0.01) higher (by over 1600 kg) than in daughters of PHF sires. In the group of daughters of PHF sires, the length of the second lactation varied widely, from 309.5 days in long-lived cows to 358.2 days in cows culled after the second lacta-tion. Inter-calving intervals were longer in cows culled after the second lactation (417.7 days in daughters of PHF sires, 429.7 days in daughters of HF sires). In long-lived cows, inter--calving intervals lasted 395 days and 412.9 days, respectively. There were no differences in milk yield between cows culled after the second lactation
Tab. 7. Age at first calving, length of 1st lactation, milk yield and composition in
long-lived cows in their 1st lactation and in cows culled after 1st and 2nd lactation
depending on the country of origin of sire (x ± s)
Explanations: means within traits in columns are significantly different: xx at p £ 0.01; x at p £ 0.05; means in lines followed by different letters are significantly different: A, B at p £ 0.01; a, b at p £ 0.05 s r e t e m a r a P oPrromdourceitvlaecftoaritoifnves Cullleadctaaittfoenr1st Cullleadctaatfiteorn2nd ] s y a d [ g n i v l a c t s ri f t a e g A s e ri s F H P f o s r e t h g u a d 834.8±0.012 822.6A±0.013 821.3±0.013 s e ri s F H f o s r e t h g u a d 846.3±0.011 853.7B±0.014 832.0±0.012 1 f o h t g n e L stlactaiton[days] s e ri s F H P f o s r e t h g u a d 325.6±0.050 383.8±0.059 353.8a±0.070 s e ri s F H f o s r e t h g u a d 344.4±0.058 395.0±0.062 374.0b±0.071 1 n i d l e i Y stlactaiton[kg] k li M s e ri s F H P f o s r e t h g u a d 6700.9Axx±0.005 8517.7Bx±0.004 8192.0Bxx±0.003 s e ri s F H f o s r e t h g u a d 7977.3A±0.003 9050.3B±0.003 9022.9B±0.004 t a F s e ri s F H P f o s r e t h g u a d 288.5Axx±0.119 358.5Bx±0.098 351.1Bxx±0.078 s e ri s F H f o s r e t h g u a d 349.7A±0.063 387.5B±0.078 392.8B±0.053 n i e t o r P s e ri s F H P f o s r e t h g u a d 223.0Axx±0.161 291.3Bx±0.115 277.0Bxx±0.106 s e ri s F H f o s r e t h g u a d 263.8A±0.088 307.9B±0.068 306.5B±0.114 ] % [ t n e t n o C t a F s e ri s F H P f o s r e t h g u a d 4.28±2.768 4.22±3.609 4.33xx±3.610 s e ri s F H f o s r e t h g u a d 4.44a±3.444 4.30a±3.647 4.19b±3.294 n i e t o r P s e ri s F H P f o s r e t h g u a d 3.30a±1.830 3.41bx±2.035 3.38±1.861 s e ri s F H f o s r e t h g u a d 3.31a±1.955 3.37b±1.766 3.40b±1.676
Tab. 8. Length of 2nd lactation, average inter-calving interval, milk yield and composition in cows in their 2nd lactation culled
after 2nd lactation depending on the country of origin of sire (x ± s)
Explanations: as in Tab. 4 2 r e tf a d e ll u C ndlactaiton Producitvefor ifveormorelactaitons s e ri s F H P f o s r e t h g u a d daughtersofHFsries daughtersofPHFsries daughtersofHFsries 2 f o h t g n e L ndlactaiton[days] 358.2±0.067 342.0±0.085 309.5a±0.049 337.1b±0.072 ] s y a d [ l a v r e t n i g n i v l a c -r e t n i e g a r e v A 417.7±0.038 429.7±0.043 395.0A±0.020 412.9B±0.024 2 n i d l e i Y ndlactaiton[kg] k li M 9501.2±0.004 9561.4±0.004 7459.6A±0.004 9138.5B±0.003 t a F x392.3±0.082 x393.4±0.095 x325.6A±0.100 x396.2B±0.066 n i e t o r P x324.7±0.099 x326.9±0.110 x255.9A±0.131 x297.5B±0.096 ] % [ t n e t n o C t a F 4.19±3.866 4.17±4.192 4.40±2.739 4.38±3.639 n i e t o r P 3.40±1.874 3.41±2.046 3.38±1.707 3.33±1.829
daughters of local and imported sires. In the second lactation, the yields of milk, milk fat and protein were high, at 9500 kg, over 390 kg and ca. 325 kg, respectively (Tab. 8).
High lifetime productivity of cows culled after the first lactation resulted primarily from the length of their pro-ductive life (over 527 days). Cows that remained productive for at least two lactations were also characterized by high lifetime productivity. The life-span of long-lived cows was 3007.9 days, and their lifetime productivity exceeded 8000 kg milk (Tab. 9).
Table 10 presents lifetime
produc-tivity of cows depending on the country of origin of sires. The average lifetime productivity of daughters of PHF sires was ca 21 254 kg milk, and the length of their productive life was 887 days. Daughters of HF sires stayed productive for 124 days longer, and their lifetime productivity reached 23 459 kg milk. Our findings sug-gest that cows lifetime productivity is affected not only by the length of productive life but also by the sires country of origin.
The length of the productive life of daughters of PHF and HF sires was only 886.6 days and 1010.7 days, re-spectively (Tab. 10). Their lifetime productivity reached 45 372 kg and ca. 51 013 kg of milk, respectively (Tab. 11); the noted difference was statistically signifi-cant (p £ 0.01). The yields of milk fat (p £ 0.01) and protein (p £ 0.05) were significantly higher in daugh-ters of imported sires. In the group of long-lived cows, lifetime milk, fat and protein production levels were significantly higher in daughters of HF sires than in daughters of PHF sires (Tab. 11).
The results of these studies confirm that the productive life of dairy cows in Poland has decreased recently, and the percentage share of primiparous cows has increased in herds included in the milk recording program (http:// www.pfhb.pl/?strona=owutib_archiwum.htm). A shorter productive life of cows decreases milk production pro-fitability and restricts cow selection for herd replace-ment. Daughters of imported sires are characterized by higher milk performance (22).
High milk production is an important economic consideration. Other key factors are the total amount of milk sold, reproductive performance and culling rates (15). Today HF cattle are characterized by decreased fertility. According to Krychowski (8), an average of
1.66 semen doses are required per successful concep-tion in Montbeliarde and Normance cows, compared with 1.95 in HF cows. An increase in milk yield is ac-companied by a decrease in reproductive performance (11, 13, 25) and reproductive problems are the main reason for culling. Sobek et al. (21) reported that 58% of cows born in 1966-1999 in the Wielkopolska region were culled for reproductive disorders, and 73% for infertility.
Cows productivity is largely determined by the sire effect. Van Tassel and Van Vleck (24) estimated genetic trends in a dairy cattle population and found that the sires of bulls and sires of cows paths were responsible for 40% and 30% of genetic gain, respectively, whereas the dams of bulls and dams of cows paths for 20% and 10%, respectively. The role of bulls in herds has increased as they can sire many offspring (www.wycena. izoo.krakow.pl).
Today, dairy cattle breeders and pro-ducers focus on improving functional traits such as health status, udder quality and leg conformation, and dif-ferent selection strategies are applied to achieve this goal. In Scandinavia, much attention is paid to herd health and recording clinical cases, whereas in the US and the Netherlands to indicators indirectly related to
func-Tab. 9. Lifetime productivity and productive life of cows (x ± s)
Explanations: means within groups in lines followed by different letters are significantly different: A, B, C at p £ 0.01 s r e t e m a r a P Producitvelfie n o it a t c a l e n o twolactaitons ifveormorelactaitons ] d a e h [ s w o c f o r e b m u N 437 445 140 ] s y a d [ e fi l e v it c u d o r P x527.3A±0.049 x853.3B±0.013 2167.9C±0.007 ] s y a d [ n a p s e fi L 1363.0A±0.019 1678.3B±0.010 3007.9C±0.006 ] g k [ y ti v it c u d o r p e m it e fi L k li M 10888.6A±0.002 19769.0B±0.001 48032.1C±0.001 t a F 460.2A±0.072 825.4B±0.029 2047.6C±0.001 n i e t o r P 366.0A±0.087 667.0B±0.036 1590.2C±0.016
Tab. 10. Lifetime productivity of cows-daughters of PHF and HF sires (x ± s) Explanations: as in Tab. 2 s r e t e m a r a P oDfaPuHgFhtseriress oDfaHuFghsterierss ] d a e h [ s w o c f o r e b m u N 1085 639 ] s y a d [ e fi l e v it c u d o r P x886.6A±0.006 1010.7B±0.059 ] s y a d [ n a p s e fi L 1709.8A±0.032 1854.0B±0.031 ] g k [ y ti v it c u d o r p e m it e fi L k li M 21254.1±0.003 23459.2±0.003 t a F 913.9±0.069 1024.4±0.068 n i e t o r P 701.4±0.089 789.0±0.085
Explanations: means within traits in columns followed by different letters are signifi-cantly different: xx at p £ 0.01; x at p £ 0.05
Tab. 11. Lifetime productivity of long-lived cows-daughters of PHF and HF sires (x ± s) s r e t e m a r a P Mlik(kg) Fat(kg) Protein(kg) y ti v it c u d o r p e m it e fi L s e ri s F H P f o s r e t h g u a d -s w o c 45372±0.001 1928±0.014 1513±0.018 s e ri s F H f o s r e t h g u a d -s w o c 51013xx±0.001 2181xx±0.010 1676x±0.013
tional traits, such as body conformation and somatic cell score. According to Rogers (16), selection indices de-veloped to improve functional traits have to be accurate and based on information from 50-100 daughters. In view of the above, a new selection index has been pro-posed by the Polish Federation of Cattle Breeders and Dairy Farmers (14). Research results show that impor-ted semen and breeding materials can significantly con-tribute to genetic progress (4, 20, 26). The cited authors demonstrated that the import of sires or their semen from the countries where the above process has already been initiated is the most effective strategy to increase the protein content of milk in the local cattle population. The breeders concerns regarding the use of semen from young progeny-tested bulls in cattle breeding herds seem unjustified. Due to fast breeding progress and the young age of such sires, their daughters are not expected to produce less milk than daughters of evaluated sires. In a study by Sitkowska and Mroczkowski (20), per-formed on 1132 cows used in 1990-2002, daughters of local sires were characterized by considerably lower milk production over their first 305-day lactation, compared with daughters of sires imported from the US, Canada, France, the Netherlands and Germany. Higher milk, fat and protein yields in the first lactation of imported HF cows were also reported by Czerniawska-Pi¹tkowska et al. (2), Dymnicki and Reklewski (3), Gnyp (4).
In a study by Dymnicki and Reklewski (3), the pro-ductivity of daughters of PHF sires was lower only when compared with daughters of Italian and German sires. According to the cited authors, differences between cattle populations can be reliably determined based on the milk performance of imported cows. It should be noted, however, that the amount of milk produced by a cow is affected not only by individual traits, but also by envi-ronmental factors. Gnyp (4) demonstrated that cows kept under good management conditions had higher lifetime milk, fat and protein production than daughters of the same sires in kept in herds with low milk production levels. A significant effect of various environmental factors on milk production was also observed by other authors. Some authors (7, 9, 10, 12, 18) noted a negative influence of high milk production levels in the first and second lactation on cow fertility, health and culling rates. Summarizing the research results it should be noted that among first-calf heifers and the second lactation cows more than 55% of daughters of imported bulls and about 48.75% of daughters of domestic bulls were cul-led. Essentially higher results of sort ratings concerning the overall appearance, body conformation, udder quality and frame size, efficiency of milk and its com-ponents were acquired after the imported bulls. High milk production levels, exceeding 8000 kg of milk in the first and second lactation, had an adverse effect on the productive life of cows. A correlation between milk production of milk cows and their productive longevity was proved as evidenced by the fact that cows which were used for at least five lactations produced less milk and its components as the first-calf heifers than cows which were scrapped later.
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Corresponding author: dr hab. Jan Miciñski prof. nadzw. Department of Cattle Breeding and Milk Evaluation, Faculty of Animal Bioengineering, University of Warmia and Mazury in Olsztyn, Oczapowskiego 5, 10-957 Olsztyn-Kortowo, Poland; e-mail: micinsk@uwm.edu.pl
