Heat Stress as a Barrier to Successful Reproduction and Potential Alleviation Strategies in Cattle

Heat stress effect on fertility of two imported dairy cattle breeds from different Algerian agro-ecological areas

The present study investigates the susceptibility of two imported dairy cattle breeds to Algerian local climatic conditions, with a primary focus on heat stress (HS) and its repercussions on fertility traits. The dataset comprises 20,926 artificial insemination records involving 6,191 Prim'Holstein and 5,279 Montbéliarde cows. The animals originated from three distinct agro-ecological regions: littoral (L), semi-arid (SA), and arid (Ar), characterized by average annual Temperature-Humidity Index (THI) values of 75.2, 69.53, and 74.75, respectively. Logistic and linear regression models were performed to analyze the relationship between the THI on the AI day, season, and agro-ecological origin of the animals with the Conception Rate at 1st Artificial Insemination (CR 1stAI), Conception Risk (CR), Services per Conception (SPC), and reproductive period (RP). The results demonstrated a significant negative impact (P < 0.001) of THI > 72 compared to THI ≤ 72 on CR1st AI and CR for both cattle breeds (Prim'Holstein: -49.7% and - 17%, respectively; Montbéliarde: -20.7% and - 15%, respectively). Seasonal effects revealed a notably higher CR1stAI in winter and spring (≈ 25%) for Prim'Holstein and Montbéliarde cows compared to summer (19.41%) and autumn (19.12%), respectively. Furthermore, a reduced likelihood of conception at 1stAI and subsequent AI was observed during summer (0.839) and autumn (0.818) compared to winter for the Montbéliarde cows. Taking into account the littoral region as a reference, the likelihood of 1stAI success increased for both breeds in the SA region and decreased for the Ar region (P < 0.001). SPC increased for both breeds in THI > 72 categories (Prim'Holstein: 6.3%, Montbéliarde: 7.1%, P < 0.01), in the Ar region (Prim'Holstein: 30.9%, Montbéliarde: 26%, P < 0.001), and in the SA region (4%, P < 0.05) compared to the L region No significant seasonal effect on SPC was observed for either breed (P > 0.05). The RP increased in the THI > 72 category (Prim'Holstein: 4.1%, Montbéliarde: 7.4%, P < 0.001) and in the Ar region (Prim'Holstein: 122%, Montbéliarde: 73.4%) for both breeds. RP decreased in autumn compared to winter (Prim'Holstein: 15.3%, Montbéliarde: 8.4%). This study underscores the adverse impact of mild to severe heat stress (HS) and related factors (season, region) on fertility of Prim'Holstein and Montbéliarde cows under Algerian conditions, emphasizing the necessity for heat stress mitigation strategies, especially in adverse littoral humid and Saharan-arid environmental conditions.

Determinants of thermal homeostasis in the preimplantation embryo: a role for the embryo's central heating system?

A number of factors may impinge on thermal homeostasis in the early embryo. The most obvious is the ambient temperature in which development occurs. Physiologically, the temperature in the lumen of the female tract is typically lower than the core body temperature, yet rises at ovulation in the human, while in an IVF setting, embryos are usually maintained at core body temperature. However, internal cellular developmental processes may modulate thermal control within the embryo itself, especially those occurring in the mitochondria which generate intracellular heat through proton leak and provide the embryo with its own 'central heating system'. Moreover, mitochondrial movements may serve to buffer high local intracellular temperatures. It is also notable that the preimplantation stages of development would generate proportionally little heat within their mitochondria until the blastocyst stage as mitochondrial metabolism is comparatively low during the cleavage stages. Despite these data, the specific notion of thermal control of preimplantation development has received remarkably scant consideration. This opinion paper illustrates the lack of reliable quantitative data on these markers and identifies a major research agenda which needs to be addressed with urgency in view of laboratory conditions in which embryos are maintained as well as climate change-derived heat stress which has a negative effect on numerous clinical markers of early human embryo development.

Effect of heat stress during the dry period on milk yield and reproductive performance of Holstein cows

This study aimed to determine the influence of heat stress during the dry period on milk yield and reproductive performance of Holstein cows in a hot environment. Breeding and milk production records of cows, as well as meteorological data between 2017 and 2020 from a commercial dairy herd (n = 12,102 lactations), were used to determine the relationship between climatic conditions during the dry period (average of the temperature-humidity index (THI) at the beginning, middle, and end of the dry period) and reproductive efficiency and milk yield traits. THI was divided into < 70 (no heat stress), 70-80 (moderate heat stress), and > 80 (severe heat stress). First-service pregnancy rate of cows decreased (P < 0.01) with increasing hyperthermia during the dry period (9.5, 7.3, and 3.4% for THI < 70, 70-80, and > 80, respectively). All-service pregnancy rate was highest (P < 0.01) for cows not undergoing heat stress during the dry period (60.2%) and lowest (42.6%) for cows with severe heat stress during the dry period. Cows not experiencing heat stress during the dry period required a mean ± SD of 5.6 ± 3.8 services per pregnancy compared with 6.5 ± 3.6 (P < 0.01) for cows subjected to THI > 80 during the dry period. Cows not suffering heat stress during the dry period produced more (P < 0.01) 305-day milk (10,926 ± 1206 kg) than cows subjected to moderate (10,799 ± 1254 kg) or severe (10,691 ± 1297 kg) heat stress during the dry period. Total milk yield did not differ (P > 0.10) between cows not undergoing heat stress (13,337 ± 3346 kg) and cows subjected to severe heat stress during the dry period (13,911 ± 4018 kg). It was concluded that environmental management of dry cows during hot months is warranted to maximize reproductive performance and milk yield in the following lactation.

Evaluation of the expression of growth hormone and its receptor during the resumption of postpartum ovarian follicle development in dairy cows

Growth hormone is a key endocrine factor for metabolic adaptations to lactation and optimal reproductive function of the dairy cow. This study aimed to analyze the expression of GH and its receptor (GHR) in ovarian follicles, along with metabolic biomarkers, during the resumption of the postpartum follicular development, and to analyze the immunolocalization and protein expression of GH and GHR in preovulatory follicles. Thirty-six dairy cows were grouped according to the postpartum days (PPD) until the establishment of the first dominant follicle in: cows that established their first dominant follicle at fewer postpartum days (FPPD group; n = 15) and cows that established their first dominant follicle at more postpartum days (MPPD group; n = 22). For a second analysis, the same cows were regrouped according to the calving season (S), into cows calving in autumn (n = 20) and cows calving in winter (n = 17). During the PP, blood and follicular aspirates were obtained at two timepoints (T): when the first dominant follicle was established (T1, day 9 ± 2), and when the preovulatory follicle was established (T2, day 45 ± 2). Also, six dairy cows were ovariectomized in proestrus and ovarian histological sections were obtained. Growth hormone mRNA was detected in granulose cells from ovarian follicle sampled during PP. A PPD × T interaction was observed for GHR mRNA, where it was greater in the FPPD cows than in the MPPD cows at T1. Metabolic biomarkers and reproductive hormones showed differences or interaction between PPD, T, S, depending on the case. Also, GH and GHR were immunolocalized in granulosa and theca interna cells of preovulatory follicles. These results confirm the expression of GH and GHR in the mature ovarian follicles of dairy cows and show a possible association between greater GHR expression and an earlier resumption of postpartum follicular development.