1. Effect of the availability of artificial shade during grazing on progesterone concentration in West African lambs before puberty in the tropic
Pinto-Santini Livia; Pigliacampo Alberto; Drescher Karin; Fonseca Jeferson; Landaeta Antonio
Introduction
The predominantly elevated temperature-humidity index (THI) of tropical zones is a paramount reason for the heat stress that routinely affect farm animals from such zones. Concomitantly, heat stress has been largely recognized as an important cause for low productive and reproductive performance in farm animals from tropical zones. Among strategies for monitoring some reproductive parameters, measuring blood progesterone concentrations is a useful tool. However, results of blood progesterone seem to be distorted by stress sources due to the production of a progestin by-product during the cortisol synthesis (i.e., adrenal progesterone). Since heat stress is an almost permanent condition in tropical zones, it appears possible to obtain distorted results that cause a sort of confounded effects. Nevertheless, studies quantifying adrenal progesterone in tropical zones are scarce. Provision of shade is the most important cooling strategy in tropical zones, but its effect on the production of adrenal progesterone has not been investigated.
Our aim was to evaluate the effect of artificial shade availability during grazing on blood progesterone concentration in West African sheep before puberty
Material and methods
Location: The experiment was performed in the Sheep Unit of the Agronomy School – Universidad Central de Venezuela (10°14'49'' N, 67°35'45'' W) during five months of the rainy season (August to November).
Treatments and experimental design: Twenty pre-pubertal ewes between kg live weight and four months old were randomly assigned to two treatments (T): SHADE: paddock with Cynodon nlemfluensis grass and free access to artificial shadowed shelter made of green hill (High: 1.8 m, area: 36 m2) and, SUN: equal and contiguous paddock without any natural or artificial shade availability.
Variable evaluated
Climatic characterizations: Were determinate each hour and during every day of the experimental period. Climate variables were: ambient temperature (TA; °C), relative humidity (RH; %) and global solar radiation (SR; W/m2) both SHADE and SUN.
Additionally, temperature and humidity index (THI; units) was calculated using this equation (Valtorta and Gallardo, 1996):
THI = (1.8 TA + 32) – (0.55 – 0.55 RH/100) × (1.8 TA – 26)
Progesterone concentration (P4): Blood samples for P4 determinations were collected every three days during experiment, when animals were between 118 to 192 days old. The serum P4 was measured by commercial ELISA kit (Progesterone ELISA; DRG Instruments GmbH Germany®). P4 by day of the evaluation and the accumulated progesterone (ACCUP4; sum of all the values obtained at each sampling) were showed. The intra-assay and inter-assay coefficient of variation were 10.7 and 12,2%, respectability.
 Statistical analysis
Statistical analysis of data was performed using ANOVA for longitudinal measures (PROC MIXED SAS) and a model that included the effect of T (SHADE vs. SUN), day (Day) of the measurement and their interaction (T*Day).
Results
Climatic characterizations: The THI (LSM ±EE) for SHADE and SUN were 74.6 ± 1.2 and 73.8 ± 1.3; respectively. During most of days of the experiments, THI were higher than 72 units in both treatments. Likewise, during some days, THI was above 72 for more than 14 hours. The SR differed (P≤0.01) among treatments (Figure 1). SR at noon reached ~600 W/m2 in SUN even though the experiment was conducted in rainy season.
It seems likely that sheep tolerate THI above 80 units (Reyes, 2016). However, to our knowledge, there is not scientific information on the THI level at which heat stress starts in tropical sheep.
Figure 1. Global solar radiation
(means for each hour during the experiment)
Progesterone concentration (P4): Treatments did not affect progesterone concentration. Pooled data from both groups are presented (Figure 2). Progesterone concentrations increased in 121, 150, 155 and 161 day (P≤0.01) but were not ≥ 1 ng/mL. From the day 168 and to 185 progesterone concentration increased again and from day 175 was ≥ 1 ng/mL for more than two consecutive samplings. Thus, it was considered like day of the first ovulation or puberty age.
Accumulated progesterone (ACCUP4): ACCUP4 were different between treatments (P≤0.05). The maximum values were presented in SUN with means that 9,97±1.51 ng/mL in SUN (P=0.04) vs. 5.25±1.51 ng/mL in SHADE. In ewes without shade availability, the production of progesterone from adrenal origin was likely to occur which explains the higher level of ACCUP4 observed in the SUN group.
Figure 2. Mean progesterone concentration in West African female tropical sheep. When two or more consecutive progesterone samples were ≥ 1 ng/mL was considered an ovulation. Different letters indicate differences at P<0.01.
Conclusion
The availability of shade during daily grazing time can modify the secretion of progesterone without effect on age of puberty in female West African lambs under tropical conditions.
Figure 3. Means accumulated progesterone concentrations in West African female tropical sheep. Different letters indicate differences at P<0.05.