Analysis was made of the relationship between conformation traits and lifetime production efficiency of the cows that first calved in 2000 and represented the active population in the Pomorze and Kujawy regions of Poland. The CORR Pearson procedures of SAS package were used in the statistical calculations. It was found that there is a statistically significant relationship, weak or low on the Guilford scale, between conformation traits and lifetime production efficiency of the cows, which is slightly higher for milk yield than for longevity. The type and conformation traits appear to be more suitable than the detailed traits for predicting the lifetime production efficiency of cows. Lifetime performance was most strongly related to the overall score and udder score (r = 0.22), followed by the scores for type and conformation and legs and feet (r = 0.13), and detailed traits such as udder width and dairy character (r = 0.14). The highest positive effect on longevity was exerted by udder score and legs and feet (r = 0.11) and among detailed traits by udder placement (r = 0.14) and fore udder attachment (r = 0.10).

The effect of some factors on relationships between milk urea levels and fertility of 19 028 cows was analysed. The generalized linear model (GLM) of the SAS package (SAS 2004) was used in the statistical calculations. The increase in milk urea concentration was found to increase calving interval (r=0.05, P≤0.01), rest period (r=0.07, P≤0.01), service period (r=0.01, P≤0.01) and the number of services per conception (r=0.02, P≤0.01). Therefore, dairy breeders may find it advantageous to monitor urea concentrations, which could help to improve cow fertility. Herd milk production level, age of cow and lactation period were the factors that had the greatest effect on the relationship between milk urea levels and cow fertility parameters. The relationship between milk urea level and fertility was stronger in cows from herds with >6 000 kg milk yield, in first-calf heifers and in cows at 3 months of lactation.

Abstract. Data on 393 002 parturitions and performance in the following production cycle were collected from the SYMLEK database for 145 457 Polish Holstein-Friesian cows representing the active population in the Pomorze and Kujawy regions. The animals first calved during 2000-2012 and were used or culled by the end of 2013. GLM and FREQ procedures of the SAS package were used for the statistical calculations. The frequency of twin and triplet births depended on the cow's age (0.64 % in primiparous and 2.88 % in multiparous cows), calving season (1.79 % in winter, 2.41 % in summer), previous lactation milk yield (increase of 2.15 to 3.73 % with increasing milk production level), and year of calving of primiparous cows (increase from 0.43 % during 2000-2003 to 0.78 % during 2010-2012). Dams of twins were superior to dams of singles in terms of milk yield, but a significant difference was manifest only in multiparous cows. Despite the higher perinatal mortality of the twins and triplets (especially those born to primiparous cows), multiple pregnancy resulted in a greater number of calves born compared to single pregnancy. A negative effect of multiple pregnancies was a decrease in fertility, especially in multiparous cows (e.g. calving interval was longer by 18 days, reproductive rest period by 9 days, and service period by 10 days, while services per conception increased by 0.15). In addition, twin pregnancy (especially in heifers) and triplet pregnancy (especially in cows) considerably decreased chance of survival until the next calving.

The aim of this study was to analyse data on 74,081 calvings, subsequent lactation performance and culling of 23,588 Black-and-White cows improved with Holstein-Friesians. The animals represented the active population in Pomerania and Kujavia, first calved in 2000 and 2001, and were culled before the end of 2008. Frequency of calvings, which averaged 1.5% for twin births and just 0.02% for triplet births, increased with age of cow and also with increasing milk yield in the preceding lactation. Performance results showed that mothers of twins were superior to mothers of single calves in terms of milk yield (1.3 kg milk/day milking). Despite the greater perinatal mortality of twins and triplets, multiple pregnancies gave rise to a greater number of calves compared to single pregnancies. However, multiple pregnancies were accompanied by adverse effects such as increased proportion of complications requiring human assistance, mechanical assistance and veterinary intervention (1.2 × more). Fertility of the cows deteriorated after multiple pregnancies, with particularly unfavourable indicators of fertility found for triplet births, decreased chance of survival to the next calving, and increased culling rates in cows, especially due to udder diseases, infertility, reproductive diseases, old age, metabolic and gastrointestinal diseases, and locomotor system diseases. It was found that the increasing milk yield was paralleled by the increasing proportion of multiple pregnancies. This has highlighted the need for early and reliable diagnosis and management of twin pregnancies, which is supposed to facilitate parturition and ensure survival of calves.

The effect of dry period length on performance in the next production cycle was investigated in the active population of cows from the Pomerania and Kuyavia regions of Poland. Dry length period of the cows was calculated and classified (0, 1-20, 21-40, 41-60, 61-80, 81-100, >100 days). The GLM, FREQ and CORR PEARSON procedures of the SAS package 9.1 were used in the statistical calculations. The problem of dry period length was found to be debatable. A dry period of 40-60 days would be the most favourable in terms of milk, fat and protein yield in the next lactation, the course of parturition and the proportion of calves born alive, and a dry period of 21-60 days would be the most beneficial in terms of the proportion of cows surviving the next production cycle. Extending or shortening the standard dry period of 40-60 days by 20 days causes a slight decrease in performance determined by the above parameters, whereas extending and especially shortening the dry period by another 20 days, or the absence of a dry period, have a severely negative effect. Compared to cows that had been dry for 40-60 days, the cows whose calving was not preceded by a dry period had lower lactation yield (by 25 % for milk, by 24 % for fat and by 20 % for protein), lower daily milk yield (by 22 %), were significantly more prone to miscarriage, and more frequently gave birth to dead calves, calves with physical defects or grossly malformed calves. Too long a dry period (beyond 100 days) may carry a risk of higher culling levels in the herd, shorter lactations, health abnormalities, and poorer reproductive performance.