The effects of melatonin and Light treatments on the reproductive performance of yearling Suffolk rams were investigated. The two groups (A, n = 8; B, n = 8) were given a priming period of long days (18 h Light (L): 6 h dark (D) during 1st February to 14th March. During 14th March to 9th September, group A was exposed to local light + melatonin Implants (Regulin) at 5 weekly intervals, group B was exposed to local light + melatonin Fed (M- 5250) alongwith the feed. The group (C, n = 8) in the local light during 1st February to 14th March and during 14th March to 9th September remained on local light + melatonin implants (Regulin) at 5-weekly intervals. However, the group (D, n=8) was treated as control and kept on local light environment (Lat. 51 degree 43'N) throughout the experiment. Testis diameter, sexual behaviours and semen quality were used to assess reproductive performances. The treatment to group A & B resulted in a significant advancement in high reproductive performance. The treatment to group C indicated that the abrupt application of melatonin in mid March resulted in a weak response in terms of all the criteria of assessment. The investigation indicated that seasonal fluctuations in reproductive behaviour and semen quality may be rephrased by the application of light and melatonin treatments and that these treatments may be considered for the preparation of rams for out of season breeding.

In the conditions of the Republic of Belarus there was studied the possibility of application of biotechnological methods of correction of sexual function of infertile cows and heifers (oestrus stimulation and synchronization) a dairy cattle breeding farms with high breeding productivity indexes and simultaneous metabolic function diseases. In course of the study there were applied clinical, cytodetection technique, haemotological, biochemical and analytical methods. Results of three-year studies of pathological metabolic disorders of cows (chronic acidosis; chronic ketoacidosis; dystrophic liver affection; renal irritation syndrome; disturbance of calcium and phosphorous ratio; trace element deficiencies; hypovitaminoses) were analyzed. Structure of obstetric-gynecologic pathology in experimental dairy cows herd was presented. Research results showed that high reproduction indexes of cows were the contributory cause for development of metabolic disorders. In such conditions for the efficient stock reproduction it was proposed to apply the mandatory hormonal stimulation of sexual function by means of application of the Ovsinkh scheme.

In the conditions of the Republic of Belarus there was realized a determination of an optimal time for cow insemination with the help of oestrus detector by Draminsky's method. In course of the experiment there were used black-and-white nonpregnant cows at the stage of oestrus (sexual heat). In course of the study there were used clinical, biochemical and hematological research methods. Dynamics of changing of estrous cycle slime depending on a day of sexual cycle was analyzed. Indexes of the reproductive function of cows in experimental and control groups were compared. Research results showed that revelation of cows with oestrus with the help of oestrus detector by Draminsky's proved to be efficient and informative. Insemination of animals at the stage when the resisting of oestrus mucilage after the minimal indexes (180-200) rose sharply up to the indexes 300-320 proved to be efiicient. This made it possible to realize single insemination of animals.