The aim of the study was to determine the effect of photoperiod and exogenous melatonin on milk yield and chemical composition of sheep’s milk. Sheep (n = 60) were randomly divided into three groups: lambing in February (Group 1—n = 20), lambing in June (Group 2—n = 20), and lambing in June and treated with subcutaneous melatonin implants (Group 3—n = 20). Milk yield was higher for Group 1 and Group 2 than for Group 3 (p < 0.01). The milk of ewes of Groups 2 and 3 had a significantly (p < 0.01) higher content of dry matter, protein, and fat. Group 3 sheep’s milk contained significantly more (p < 0.01) of SFA (Saturated Fatty Acids). The highest content of MUFA (Monounsaturated Fatty Acids) and PUFA (Polyunsaturated Fatty Acids) was found in the samples collected from Group 1, the lowest was in the milk of Group 3 animals. The highest (p < 0.01) CLA, content was identified in the milk of Group 1, while the lowest was recorded for the milk obtained from sheep treated with exogenous melatonin (Group 3). The experiment carried out has shown that day length and treatment with exogenous melatonin modulate the chemical composition of milk.

Abstract. Previous studies demonstrated that milk yields in sheep displaying strong seasonal sexual activity depend on the day length. The objective of the studies was to determine whether the introduction of melatonin in high pregnancy affects milk secretion in seasonally sheep. The studies were carried out on 60 Polish Longwool sheep. Sheep were allocated to three groups: Group I (n = 20 – the control group, lambed in February), Group II (n = 20 – a group of sheep lambed in June and kept under natural day-length conditions), Group III (n = 20 – a group of sheep with melatonin implants injected six weeks before lambing, sheep lambed in June). Lambs were reared with mothers up to 56th day of their life. When lambs were weaned, ewes were milked mechanically twice a day up to the dry period. Once a month collective milk samples were drawn from six sheep from each group in order to determine the concentration of melatonin. Milk yields were subjected to individual checks every 10 days. The studies demonstrated that sheep lambed in February (Group I) displayed the highest milk yields in the milking period (37.8 ± 8.1 l). The milk performance of the two other groups was lower and amounted to 30.2 ± 9.4 litres in case of sheep lambed in June and to 29.2 ± 7.6 litres in sheep with melatonin implants. The introduction of melatonin signal to produce a short-day condition in state of high pregnancy in ewes caused a drop of milk yields both in the period of lambs raising and during milking.

Molik E., Błasiak M., Misztal T., Romanowicz K., Zięba D.A. (2017): Profile of gonadotropic hormone secretion in sheep with disturbed rhythm of seasonality. Czech J. Anim. Sci., 62, 242-248.The effect of artificial conditions of a short daylight period (16 h darkness (D): 8 h light (L)) and exogenous melatonin on milk yield parameters of sheep during spring and summer was examined to determine the impact of using sheep for milk on the secretion level of gonadotrophic hormones. The research was conducted on 60 sheep lambed in February. After raising the lambs, the sheep were divided into 3 groups and assigned for dairy use (May-September). The mothers in the control Group 1 (G1) were maintained under natural daylight conditions. The sheep in Group 2 (G2) were maintained under conditions of an artificial photoperiod (16 h D : 8 h L). Meanwhile, the mothers in Group 3 (G3) were given melatonin implants. A 6-hour collection of blood from 6 sheep of each group was performed every 4 weeks. The concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in plasma were measured using radioimmunoassay. The average LH concentration in G1 gradually increased since May (5.32 ± 0.2 ng/ml), reaching the highest value in August (6.70 ± 0.2 ng/ml). In G2, the increase in LH occurred 4 weeks after the introduction of the 16 h D : 8 h L condition (6.26 ± 0.2 ng/ml). The maximum LH concentration in G3 was noted in August (7.31 ± 0.2 ng/ml). The average FSH concentration in G1 gradually increased since May (6.59 ± 0.2 ng/ml), reaching the highest value in August (10.50 ± 2.6 ng/ml). In G2, there was a significant increase in the FSH concentration in June (9.00 ± 0.3 ng/ml). In the final period during lactation, the FSH concentrations in G2 (13.51 ± 1.3 ng/ml) and G3 (13.60 ± 1.9 ng/ml) were higher than in G1. The results indicate that using sheep for milk does not inhibit the secretion of gonadotropic hormones induced by the simulation of short daylight conditions and exogenous melatonin.

Many animals show a cyclic pattern of physiological processes depending on seasonal changes. These particularly concern the mating period, migration, aestivation and hibernation, which occur during seasons of the year characteristic of a given species. These phenomena require the animals to predict environmental changes and to metabolically prepare for them in advance. Such endogenous rhythms and cycles *Work financed from statutory activity (KBZ-2015).

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