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Effect of Heat Stress on Early Embryonic Development in the Beef Cow
Abstract
Hereford and Hereford X Angus cows (n = 31) were utilized to determine the effects of heat stress on early embryonic development and survival. After acclimation to handling, cows were cannulated via the jugular vein on d 7 and assigned to either a control (C) chamber environment of 22 C, 35% relative humidity (RH) or one of two heat stress treatments. Ambient temperature was maintained at 37 C for 12 h followed by a decrease to 33 C for the remainder of the day in both treatment groups. Relative humidity was maintained at 27% in treatment 1 (TRT 1) and 38% in treatment 2 (TRT 2). On d 8 to 16, daily measurements of respiration rate (RES), rectal temperature (REC) and water intake were taken along with samples of blood, which were analyzed for hematocrit (HEM) and plasma concentration of protein (PP), progesterone (P4), estradiol-17 beta (E2), thyroxine (T4) and glucose (GLU). The uterus was recovered and flushed with saline on d 17 to recover the conceptus and uterine contents. Conceptus (if present) and corpus luteum (CL) wet weight were determined. Cows subjected to TRT 2 had increased RES and REC (P less than .01), while HEM was decreased (P less than .05) compared with C cows. Plasma T4 concentration was decreased (P less than .10) in TRT 2 compared with TRT 1 and cows, while P4 concentration were not significantly different. Corpora lutea wet weights were reduced (P less than .10) in heat-stressed cows vs C cows.(ABSTRACT TRUNCATED AT 250 WORDS)
... La eficiencia reproductiva en los animales domésticos disminuye considerablemente al someterse a altas temperaturas de verano (Gwazdauskas et al., 1975;Wolfenson et al., 1988;Ozawa et al., 2005, Mellado y Meza, 2002. Los efectos del estrés calórico en la reproducción de la hembra se manifiestan principalmente en trastornos fisiológicos que 46 incluyen disminución en la duración (Monty y Wolf, 1974;Abilay et al., 1975) y expresión del estro (Gangwar et al., 1965), así como en el desarrollo embrionario (Putney et al., 1988), flujo sanguíneo uterino (Roman-Ponce et al., 1978), relaciones hormonales (Ozawa et al., 2005) y crecimiento fetal (Biggers et al., 1987). Cabe señalar que algunos trabajos reportan que en climas templados la cabras lecheras tienen menos riesgo de abortar en verano, sugiriendo que factores ambientales como hipoglucemia, plantas tóxicas, deficiencias minerales (Cu y Se), pérdida de peso durante la sequía, son factores que causan mayor perdidas fetales (Mellado y Pastor, 2006), inclusive que el mejor desempeño reproductivo se ha observado mejor en épocas calurosas al Norte de México (Mellado y Meza, 2002). ...
... Otra hormona que es modificada durante el verano es el Estradiol-17β, cuyas concentraciones en plasma dependen del cambio de las estructuras ováricas (Ozawa et al., 2005;Biggers et al., 1987;Bandinga et al., 1993). Esto se ha atribuido a que el estrés por calor puede alterar la dinámica folicular al reducir el diámetro y volumen de los folículos dominantes (Bandinga et al., 1993), observándose un incremento en el número de folículos >10mm de diámetro, la temprana formación del folículo dominante en la segunda onda folicular y una tendencia en la reducción de las concentraciones de inhibina (Wolfenson et al., 1988), Durante la fase de reclutamiento folicular se suprime el crecimiento para la siguiente ovulación, acompañado por un decremento en los receptores de la Hormona Luteinizante (LH) y la síntesis de estradiol, aun cuando los niveles de la Hormona Folículo Estimulante (FSH) no se alteren (Ozawa et al., 2005). ...
La crianza de la cabra para la producción de carne, leche y ocasionalmente piel, representa un importante sustento económico principalmente en el sector rural en México, que dado las características de rusticidad y adaptabilidad de la especie, ésta se ha desarrollado mayormente en zonas áridas y semiáridas del país. La eficiencia productiva de la cabra es dependiente no solo de la capacidad genética, sino también del nivel de adaptación al medio con el que interactúa; esto se refiere básicamente a enfermedades, disponibilidad y calidad de alimento, nivel tecnológico, manejo zootécnico y el medio climático que exista en su zona. En mayor o menor intensidad, el clima es un importante modelador en la producción animal, ya que influye en su comportamiento en cualquiera de los sistemas de producción. Como cualquier ser vivo, la cabra tiene requerimientos climáticos; básicamente hablamos de temperatura, radiación solar, humedad relativa, velocidad de viento y aunque la presión atmosférica, nubosidad y precipitación pluvial no son variables que intervienen directamente, también son consideradas. Es por ello que las primeras razas caprinas se distribuyeron geográficamente dependiendo de la disponibilidad de recursos, donde a través de la modificación en sus hábitos y disponibilidad de alimento fue que se adaptaron (color y tipo de pelaje, ciclo reproductivo, etc.) y se establecieron en diferentes zonas climáticas. La temperatura neutral en la cabra se encuentra entre los 20 y 26°C, sin embargo, debido a la necesidad de domesticación de la cabra por el ser humano, fue que la explotación de algunas razas se extendiera a regiones donde sus necesidades climáticas (zona termoneutral) y capacidad termoregulatoria son rebasadas con facilidad en algunas estaciones del año (>34-36°C), ocasionando la pérdida de su homeostasis. Ante un clima extremo o combinación de variables climáticas, la cabra requiere modificar su comportamiento, estado fisiológico, endócrino, lo que conocemos como Estrés térmico, lo que repercute negativamente en la fisiología celular. Bajo esta situación, la suma de todos estos factores se reflejan en deficiencia o ausencia reproductiva, reducción en producción de leche, estrés, abortos, enfermedades, bajo peso al nacer, deficiencia en crecimiento, entre otras no menos importantes. El clima a su vez favorece la presentación de enfermedades dependiendo si las condiciones ambientales son favorables para el desarrollo de parásitos, bacterias, virus, etc. por el simple hecho de ser un organismo vivo
... Berthelot and Paccard (1990) demonstrated early embryonic mortality is primarily caused by an increase in uterine temperature. Additionally, heat stress also affects the metabolism of the developing embryo and the maternal recognition processes (Biggers et al., 1987). Whatever the method, maternal heat stress has detrimental effects on the embryos it produces that last a lifetime and cannot be reversed by postnatal care . ...
... When an embryo is still in the zygote stage of development, exposure to high temperatures can impair its ability to advance to the blastocyst stage (Sakatani et al., 2012). Biggers et al. (1987) found as a result in their study that the heat stress during the first days of pregnancy from days 8 through 16 in beef cows, conduct to decreasing the size of the embryo on day 17. Several experiments in vivo and in vitro were carried out on the negative effects of heat stress on the embryo's development at early stages. ...
Heat stress becomes a serious problem in the livestock sector as it affects cows' performance negatively. The objective of this paper review is to investigate the effects of heat stress during the different phases of the life cycle of cows; embryos, calves, heifers, and cows. Heat stress during early maternal gestation affects the ability of embryos to develop increasing the risk of abortion and early embryonic death. Heat stress during late maternal gestation affects the performance of calves and heifers later in their life, as it reduces growth performance, conducts physiological changes, impaired immunity, changes the behavior, and reduces the length and intensity of the estrus in heifers with decreasing in milk production in the first lactation. On the level of cows, milk quality and production, meat quality, and the final body weight decrease under hot temperatures. Heat stress decreases the conception rate, alters follicle growth, and estrous symptoms. Hormones secretion and physiological changes because of the heat stress conduct to impair the immunity system, and in oxidative stress and death in some cases. Same as for calves and heifers a change in the behavior of cows was detected in order to decrease their temperature.
... Se ha publicado desde la década de 1990 que la tasa de preñez por inseminación artificial (IA) en el ganado lechero disminuye drásticamente en los veranos (García-Ispierto et al., 2006;Huang et al., 2009;De Rensis et al., 2017). Se considera que una de las principales razones de este efecto adverso es la elevación de la temperatura corporal, que altera las funciones ováricas y uterinas; las cuales están acompañadas de una elevada mortalidad embrionaria (Biggers et al., 1987;Dobson et al., 2001;Drost et al., 1999;De Armas y Pérez, 2002). ...
La influencia del ITH, temperatura vestibular vaginal y el estrés calórico diurno fueron evaluados sobre los resultados de la transferencia de embriones producidos in vitro frescos o congelados en 160 hembras cruzadas tipo leche. Las receptoras fueron sincronizadas con el esquema convencional para IATF y la transferencia se realizó a tiempo fijo [46 receptoras recibieron embriones frescos producidos in vitro (EPIVF) y 114 receptoras recibieron embriones producidos in vitro congelados (EPIVC)]. El ambiental al momento de la trasferencia de los embriones y en lo sucesivo fue tipificado por psicrometría (temperatura, humedad relativa e índice temperatura humedad para bovinos). El estrés calórico al momento de la trasferencia de los embriones fue severo (ITH de 82.92 ± 2.84°C%). La transferencia de embriones EPIVC resultó en 25.2% de preñeces y la transferencia con embriones EPIVF, la preñez fue 39.1%. Las temperaturas tegumentarias (TT) y vaginal vestibular (TVV) no fueron diferentes según los tipos de embrión y las categorías de receptoras (p>.05). La TV influyó sobre el índice de preñez con embriones EPIVF y EPIVC (P<.01). La falla gestacional aumentó a partir de la TV de 39.5°C con los embriones frescos y congelados (p<.01). Los embriones EPIVF presentaron un menor índice de gestación negativa a partir de la TV de 39.5°C; superando a los EPIVC en -15.6%. Según la relación la TVV con las preñeces; debe utilizarse EPIVF si la TVV ? a 39.5°C y utilizar EPIVC si la TVV es ? a 39.0 °C para mejorar el índice de preñez en el trópico.
... In cattle, seasonal influence on the size of the corpus luteum and its functionality has been reported by Rhodes et al. (1982), Badinga et al. (1994), and Wilson et al. (1998b). Such influence on the CL size are mainly represented by a deleterious effect of stress on reproductive physiology (Biggers et al. 1987, Dobson and Smith 1995, Wilson et al. 1998b, Wolfenson et al. 2002 including reduction of CL size, weight, and functionality (Fernandez-Novo et al. 2020). In this study, no significant difference (p>0.05) was recorded for the maximum diameter of CL, day of maximum diameter, and day of onset of regression of CL (Table 2), which was not in agreement with the findings of Stahringer et al. (1990) who reported a reduction in size and functionality of the corpus luteum during the winter season. ...
The objective of study was to investigate the influence of season, viz. summer, winter, and isothermic (spring and autumn season), on follicular and luteal dynamics in dairy cows. Thirty dairy cows (n=10 in each season) were subjected to the ultrasonographic examination of ovarian structures, i.e. follicle and corpus luteum, during two- and three-follicular waves in an estrous cycle using B-mode and colour doppler mode of ultrasonography. The daily temperature-humidity index (THI) was recorded to envisage its variation among different seasons. In results, the size of the dominant follicle was recorded to be significantly different in all seasons during the second follicular wave. However, all other parameters for follicle, i.e. day of wave onset, length of growth phase, duration of dominance, the maximum diameter of the largest sub-ordinate follicle, as well as luteal morphometric analysis had no significant difference among different seasons. On the other part, the THI varied significantly in different seasons with maximum THI in the summer (72.54), followed by the isothermic (64.7) and lowest in the winter season (54.12). As a part of summation, the seasonal impact was barely noteworthy on different aspects of follicular and luteal dynamics except for the size of dominant and sub-ordinate follicle in dairy cows.
... In cattle, seasonal influence on the size of the corpus luteum and its functionality has been reported by Rhodes et al. (1982), Badinga et al. (1994), and Wilson et al. (1998b). Such influence on the CL size are mainly represented by a deleterious effect of stress on reproductive physiology (Biggers et al. 1987, Dobson and Smith 1995, Wilson et al. 1998b, Wolfenson et al. 2002 including reduction of CL size, weight, and functionality (Fernandez-Novo et al. 2020). In this study, no significant difference (p>0.05) was recorded for the maximum diameter of CL, day of maximum diameter, and day of onset of regression of CL (Table 2), which was not in agreement with the findings of Stahringer et al. (1990) who reported a reduction in size and functionality of the corpus luteum during the winter season. ...
... Heat stress results when the animal's means to dissipate body heat are surpassed by its heat gain, resulting in a body temperature increase (West, 2003;Kumar et al., 2011). Such thermal effects limit dry matter intake O'Brien et al., 2010), decrease rate of weight gain (Ray, 1989;Mitlohner et al., 2001), and impair reproductive function (Biggers et al., 1987;De Rensis and Scaramuzzi, 2003), thus adversely affecting performance in cattle. Selection for heat tolerant breeds in tropical regions is, therefore, imperative to achieve productive efficiency. ...
Este estudio evaluó la influencia del tipo y color del pelaje sobre la regulación de la temperatura vaginal (TV) en novillas de carne. Se registró la TV, cada cinco minutos durante cuatro días, de cinco novillas Senepol de pelo corto y rojizo (SENEPOL); seis novillas cruzadas de pelo corto y claro (PELONAS); y cuatro novillas cruzadas de pelo largo y claro (REGULARES). Las novillas cruzadas se obtuvieron del cruce Charbray x Senepol x Charolais. También se registró la temperatura y humedad relativa del aire en sincronía con la TV para calcular el índice de temperatura y humedad (ITH). Las novillas ocuparon un cercado grande con libre acceso a sombra. Los datos se analizaron mediante los procedimientos GLIMMIX y CORR (SAS). El tipo-color de pelo interactuó con la hora del día para afectar la TV; siendo las TV mayores en novillas REGULARES que en las PELONAS y SENEPOL entre las 2200 a 2400 h (39.33±0.03, 38.84±0.01, y 38.74±0.02 °C, respectivamente). Se obtuvieron las mayores correlaciones positivas entre el ITH y la TV al usar el valor de ITH observado siete horas previas en las novillas SENEPOL (r=0.25) y PELONAS (r=0.30) y de ocho horas previas en las novillas REGULARES (r=0.32). Independientemente del color, el pelo corto facilitó una superior capacidad termorreguladora. Se sugiere que la introducción de genes para el fenotipo pelo corto provenientes del Senepol en otras razas de ganado de carne puertorriqueño podría ayudar a reducir el efecto negativo del estrés por calor.
Livestock production and human civilization are interlinked. Since the start of human civilization, livestock has been playing vital roles. Livestock serves as a source of milk and protein, an agricultural business, and a pet. Every individual requires animal proteins on a daily basis, such as meat and dairy products, the majority of which are derived from livestock animals. Industrialization of livestock production has become an important component of global GDP and a source of income for both industrialists and low-income marginal communities around the world. Production of livestock depends on intrinsic factors as well as environmental factors. Environmental factors such as temperature, humidity, rainfall, and so on have a significant impact on farm animal production, survivability, and disease load. The level of environmental stress differs in different climatic conditions. Therefore, a concrete understanding of environmental stress factors is required to ensure the optimum production of livestock. In this chapter, we discuss thermal stress and its detection methods, adaptive thermal stress management, the effects of temperature on bovine health, milk production, beef production, and reproductive performance, thermal effects on water availability, livestock diseases and intramammary infection, uses of small ruminants for livelihood, livestock sources of methane, and strategies to mitigate thermal stress and methane emissions.
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Elevated temperatures disturbed sperm physiology. Bovine sperm cells exposed to heat shock led to diminished mitochondrial activity, fertilizing ability, increased oxidative stress and caspase activity concomitant with a delay in embryonic developmental kinetics and modulation of sperm-borne microRNAsmiRNAs.
Abstract:
Sperm function is susceptible to adverse environmental conditions. It has been demonstrated that in vivo and in vitro exposure of bovine sperm to elevated temperature reduces sperm motility and fertilizing potential. However, the cascade of functional, cellular, and molecular events triggered by elevated temperature in the mature sperm cell remains not fully understood. Therefore, the aim of this study was to determine the effect of heat shock on mature sperm cells. Frozen-thawed Holstein sperm were evaluated immediately after Percoll purification (0 h non-incubation control) or after incubation at 35, 38.5, and 41°C for 4 h. Heat shock reduced sperm motility after 3-4 h at 41°C while mitochondrial activity was reduced by 38.5 and 41°C when compared to the control. Heat shock also increased sperm reactive oxygen species production and caspase activity. Heat-shocked sperm had lower fertilizing ability, which led to diminished cleavage and blastocyst rates. Preimplantation embryo developmental kinetics was also slowed and reduced by sperm heat shock. The microRNA (miR) profiling identified >300 miRs in bovine sperm. Among these, three and seven miRs were exclusively identified in sperm cells exposed to 35 and 41°C, respectively. Moreover, miR-181d was enriched in sperm cells exposed to higher temperatures. Hence, elevated temperature altered the physiology of mature sperm cells by perturbing cellular processes and the miR profile, which collectively led to lower fertilizing ability and preimplantation development.
This experiment was designed to evaluate the effects of increasing doses of estradiol cypionate (ECP) on reproductive function of lactating dairy cows during the summer. In Exp. 1, 643 lactating Holstein cows were blocked by parity and assigned to receive 1) an intravaginal P4 device (1.9 g of P4) and 2.0 mg of estradiol benzoate on day −11, 25 mg (i.m.) of dinoprost tromethamine on day −4, 1.0 mg (i.m.) of estradiol cypionate and CIDR withdrawal on day −2, followed by TAI on day 0 (n = 326; ECP-1) or 2) the same synchronization protocol with 2.0 mg of ECP on day −2 (n = 317; ECP-2). In both treatments, cows were TAI on day 0 of the protocol, and cow rectal temperature was measured on days −2, 0, and 7. In Exp. 2, 608 lactating crossbred Holstein × Gir dairy cows were blocked by parity and enrolled to the same treatments as in Exp. 1, but on day 7, cows received one viable embryo into the uterine horn. In Exp. 1, a greater percentage of ECP-2 cows were detected on estrus (81.3 vs. 91.1%, respectively). A treatment × body condition score (BCS) interaction was observed on day 60 pregnancy per AI (P/AI), as ECP-2 cows with a BCS <2.75 had a greater P/AI vs. ECP-1, but an opposite result was observed in cows with a BCS ≥2.75. Regardless of treatment, there were effects of mean rectal temperature and heat stress events on P/AI. Treatment affected the diameter of the ovulatory follicle at TAI (ECP-1 = 15.3 mm vs. ECP-2 = 14.8 mm) and a greater percentage of ECP-1 cows had larger follicles (≥16.5 mm), but ECP-2 resulted in a greater incidence of early ovulatory cows (ovulating before day 0). Therefore, follicle diameter at TAI affected P/AI on day 60 in cows receiving ECP-2 and tended to affect ECP-1 cows. A treatment effect was observed on time to estrus following ECP treatments and a reduced proportion of ECP-2 cows showed estrus at TAI. Regardless of treatment, cows detected on estrus 48 h after ECP administration had a greater P/AI on day 60 vs. cows detected on estrus 24 h. In Exp. 2, a greater percentage of ECP-2 cows were detected on estrus (68.3 vs. 81.4%, respectively). In summary, a greater dose of ECP increased the percentage of animals expressing estrus, but it did not benefit the reproductive function of lactating dairy cows during the summer, regardless if animals were assigned to a TAI or timed-embryo transfer (TET) protocol.
Sixty-three Angus x Hereford calves from first-calf heifers were assigned at birth to remain with their dams until weaning (NW) at 7 months or to be early weaned (EW). Calves were horn between February 18 and April 17 and those assigned to be EW were weaned every 2 weeks at 6 to 8 weeks of age. EW calves were raised in drylot and self-fed high concentrate diets until July, when one-half of the calves were moved to pasture and creep-fed. Heifers with EW calves and heifers with NW calves were bled at weekly intervals between 6 and 13 weeks postpartum for plasma progesterone analysis. All heifers were exposed to Angus bulls from May 29 to July 31. NW calves weighed 169.3 kg, at 7 months of age, compared to 169.8 kg for EW calves. Moving calves to pasture with creep feed reduced (P<.05) gains of EW calves by 20 kilograms. Conception rate was greater (P<.05) among EW (96.8%) than among NW heifers (59.4%) during the 64-day breeding season. The average interval from parturition to conception was reduced (P<.05) from 90.5 to 73.0 days by early weaning. Ovarian activiw, as indicated by progesterone levels greater than 1 ng/ml, was initiated by 85 days postpartum in 90.3% of the EW heifers, compared to 34.3% of the NW heifers (P<.005). EW heifers gained more rapidly (P<.05) during the summer.
Twenty-four Angus X Hereford steers (155 +/- 4 kg) were used to examine thyroid function during exposure to ambient temperatures of 4, 18 and 32 C. Jugular cannulae were inserted after steers were acclimated to individual stalls in environmentally controlled chambers at 18 C for 3 d. The day following cannulation, ambient temperatures were changed 2 C/h for 7 h and serum samples were collected hourly. After steers were exposed to either 4, 18 or 32 C for 1 and 72 h, thyrotropin releasing hormone (TRH; 50 micrograms, iv) was rapidly infused. Serum samples were collected hourly for 8 h after each treatment with TRH and every 8 h for 3 d between treatments. Rectal temperatures and respiratory were greater (P less than .05) in steers exposed to 32 C compared with steers at 4 C. During the change in environmental temperature, the concentrations of thyroxine (T4) and triiodothyronine (T3) over time tended (P less than .10) to decrease in steers exposed to 32 C compared with those at 4 C. Concentrations of T4 and T3 after the second treatment with TRH were significantly less in steers exposed to 32 C compared with those at 4 C. The response of T4 to TRH was reduced (P less than .01) after the second treatment with TRH compared with the first for steers exposed to all three temperatures, whereas, the response of T3 was reduced (P less than .05) after the second treatment with TRH only in steers exposed to 32 C.(ABSTRACT TRUNCATED AT 250 WORDS)
Serum-luteinizing hormone and progesterone values were measured in lactating and nonlactating Holstein-Friesian cows to evaluate their relationship with reduced fertility observed during hot summer months in Arizona. The stress of hot weather had no effect on frequency of preovulatory increase of luteinizing hormone nor on the interval between the preovulatory increase and ovulation in either lactating or nonlactating cows. Progesterone values were significantly (P less than 0.05) increased in lactating cows which became hyperthermic during hot weather, yet progesterone values did not change in nonlactating cows which had remained homeothermic. A similar relationship was observed between fertility and serum progesterone values, and between fertility and degree of thermal stress caused by hot weather. Decreased fertility and increased serum progesterone values were associated with increased environmental temperature and hyperthermia.
Exposure of male and female pigs to elevated ambient temperatures can result in reduced reproductive efficiency. When boars and gilts are exposed to heat stress, respiratory rates increase to enhance evaporative cooling because minimal sweating occurs. During early pregnancy, gilts are especially susceptible to heat stress. Decreased conception rates and reduced litter size occur when gilts are exposed to elevated ambient temperature during Days 0 to 16 after mating. Concentrations of progesterone in peripheral plasma were reduced during Days 13-19 after mating and luteal function was extended to Day 25 in heat-stressed gilts that did not conceive. Increased concentrations of oestradiol during Day 10 to 12 of heat stress may interfere with normal maternal recognition of pregnancy. Heat stress reduced the amount of embryonic tissue present at Day 16 of pregnancy but the protein synthetic activity of the tissue was not altered. Thus some embryos may be lost and a reduction in litter size may occur. The production of oestrogen by the conceptus and uterus is not altered on Day 16, after exposure to heat stress for the previous 8 days. These studies suggest that heat stress during early pregnancy can alter the reproductive endocrine system, especially the control of luteal function. In addition, heat stress may have a direct effect on embryo or conceptus development. Exposure of boars to elevated ambient temperatures causes reductions in semen quality, sperm output and fertility. About 5 weeks are required for boars to recover from the detrimental effects of heat stress and to produce semen with potential for maximal fertility. Increased temperature has an inhibitory effect on spermatid maturation and on testicular androgen biosynthesis. Improvements in reproductive performance can be achieved by increasing evaporative cooling of boars.
Conception rates of a Holstein dairy herd near Culiacan on the west coast of Mexico were evaluated with respect to the average temperature-humidity index of the 2 days prior to breeding, the day of breeding, and the day following breed- ing. Average daily index for 3 yr ranged from 62 to 86. Average daily ambient temperature and relative humiditv for indexes 76 and 82 were 26.5 C," 68% and 29.7 C, 75%. Conception rates for 191 cows serviced on days with an av- erage index under 66 was 67% as com- pared to 21% for 818 cows serviced on days averaging above 76. The average temperature-humidity index of the 2nd day prior to breeding was the most re- lated to conception rate. Conception rate declined from 55 to 10% as the average index of this day increased from 70 to 84 (r = --.995). Between 68 and 82 the average conception was 42.3% ff the average index of the 2 days prior to breeding was less than on the day of breeding. Conception rates averaged 10.4% lower (31.9%) ff the index of the 2 days prior to breeding was higher than on the day of breeding indicating that temperature~humidity of individual days prior to breeding influenced breed- ing efficiency.