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.

Insulin resistance (IR) is a state in which the tissues show a reduced sensitivity to insulin despite its normal or even increased concentration in blood. IR causes inability of peripheral tissues to respond to the hormone and to increase the glucose uptake from blood nor to raise the metabolism rate. The tissues resistance to insulin may refer to the prereceptor malfunctions, which reduce the concentration of insulin or decrease the number of insulin receptors; however, in most cases changes in signal transduction after insulin binding to the receptor are observed. The development of IR is determined by a number of genetic and environmental factors. One of the most relevant causes is obesity, which develops into a growing problem among the horse population all over the world. The accumulation of lipids in the tissues (lipotoxicity) and a mild inflammation caused by the release of proinflammatory adipokines and cytokines from the fat tissue are a common denominator that links obesity with IR. Both IR and obesity seem to cause metabolism malfunctions in horses and are the key components of equine metabolic syndrome; moreover, both may be the reason for endocrinological laminitis. The methods of prevention and treatment of IR include feeding the animals with low glycemic index food and increasing physical activity as well as pharmacological treatment, where levothyroxine sodium and metformin are of the highest importance. ARTICLE HISTORY