Reproductive Performance of Holstein Dairy Cows Grazing in Dry-summer Subtropical Climatic Conditions: Effect of Heat Stress and Heat Shock on Meiotic Competence and In vitro Fertilization

Abstract

The present study was designed to evaluate how environmental factors in a dry-summer subtropical climate in Terceira-Azores (situated in the North Atlantic Ocean: 38° 43' N 27° 12' W) can affect dairy cow (Holstein) fertility, as well as seasonal influence on in vitro oocytes maturation and embryos development. Impact of heat shock (HS) effects on in vitro oocyte's maturation and further embryo development after in vitro fertilization (IVF) was also evaluated. For such purpose the result of the first artificial insemination (AI) performed 60 to 90 days after calving of 6,300 cows were recorded for one year. In parallel, climatic data was obtained at different elevation points (n = 5) from 0 to 1,000 m and grazing points from 0 to 500 m, in Terceira island, and the temperature humidity index (THI) was calculated. For in vitro experiments, oocytes (n = 706) were collected weekly during all year, for meiotic maturation and IVF. Further, to evaluate HS effect, 891 oocytes were collected in the cold moths (December, January, February and March) and divided in three groups treated to HS for 24 h during in vitro maturation at: C (Control = 38.5°C), HS1 (39.5°C) and HS2 (40.5°C). Oocytes from each group were used for meiotic assessment and IVF. Cleavage, morula and blastocyst development were evaluated respectively on day 2, 6, and 9 after IVF. A negative correlation between cow's conception rate (CR) and THI in grazing points (-91.3%; p<0.001) was observed. Mean THI in warmer months (June, July, August and September) was 71.7±0.7 and the CR (40.2±1.5%) while in cold months THI was 62.8±0.2 and CR was 63.8±0.4%. A similar impact was obtained with in vitro results in which nuclear maturation rate (NMR) ranged from 78.4% (±8.0) to 44.3% (±8.1), while embryos development ranged from 53.8% (±5.8) to 36.3% (±3.3) in cold and warmer months respectively. In vitro HS results showed a significant decline (p<0.05) on NMR of oocytes for every 1°C rising temperature (78.4±8.0, 21.7±3.1 and 8.9±2.2, respectively for C, HS1, and HS2). Similar results were observed in cleavage rate and embryo development, showing a clear correlation (96.9 p<0.05) between NMR and embryo development with respect to temperatures. Results clearly demonstrated that, up to a THI of 70.6, a decrease in the CR occurs in first AI after calving; this impairment was confirmed with in vitro results.

Keywords: Artificial Insemination; Environmental Stress; Heat Shock; Maternal Heat Stress; Oocyte Meiotic Maturation.

Figure 1

Mean of temperature humidity index (THI) calculated through the sensors placed in different points located in the island, (Av. Elev. Points), grazing points (GP) and higher than 500 m (Av. H. Elev. Points). Results obtained in cold months (December, January, February, and March) are statistically lower (p<0.001) as compared with warm months (June, July, August, and September).

Figure 2

Relation between temperature humidity index (THI) calculated though the sensors placed in grazing points and conception rate (CR) as measured by the non-return in estrus at least 90 days after the first insemination after calving. Results obtained for CR in cold months are statistically higher (p<0.001) as compared with warm months.

Figure 3

Embryo cleavage and development after in vitro fertilization (IVF). Every point represents the mean of 40 oocytes fertilized on each iteration. C = control group, HS1 and HS2 represents heat shock 1 and heat shock 2. Insemination and embryo development was performed at 38.5°C. Cleavage was determined 48 h after insemination. Embryo development was evaluated on day 6 (morula) and day 9 (blastocysts). Results marked with a,b,c; a1,b1,c1; a2,b2,c2, differ statistically (p<0.05).