Climate change is one of the most important environmental realities that human faces in this century. Not only because of the effects it has on the different human spheres, but because it represents a challenge for the development model that human has assumed since the industrialization period (Estenssoro 2010 and Lemaire et al. 2019).
High temperatures and variations in the relative humidity of the environment are common in tropical summers and are considered as the most influential factors in animal welfare. Often, animals exceed the capacity of their normal mechanisms for heat dissipation they generate. In this aspect, Correa-Calderón et al. (2004) argue that cattle are homeothermal animals and maintain their temperature at approximately 38 °C, by controlling internal heat production and external heat gain and loss.
The performance of the meteorological variables (temperature and humidity) causes stress conditions, which affect the physiology and homeostasis of the animal (Neri and Briones 2011), and are reflected in the decrease in voluntary intake, milk production and reproductive efficiency of production cows (Hernández et al. 2007 and Ghiano, et al. 2014). Several authors consider that dairy cattle are particularly sensitive to heat stress, due to the high metabolism of the dairy cow during lactation (Swan and Broster 1982, Bernabucci et al. 2014, Carabaño et al. 2016 and Nguyen et al. 2016).
In cattle livestock, animal welfare has become a determining factor to achieve their best productive expression (Sirven 2015). At present, the temperature-humidity index (THI) is the indicator of animal welfare most used to assess the comfort level of animals (Leva and Valtorta 1996, Olivares et al.2013, Carabaño et al. 2016 and Nguyen et al. 2016). The objective of this study was to determine the temperature-humidity levels (THI) and heat stress in dairy units of the Instituto de Ciencia Animal (ICA), Mayabeque province, Cuba.
Materials and Methods
The average monthly records of ambient temperature (maximum and minimum) and relative humidity, corresponding to the database of the agrometeorological station of ICA (22o02 north latitude and 82002 west longitude), for 2005-2016 period were analyzed. These records were applied to both dairy units, which were at a distance less than 5 km, in a straight line with the station.
Both dairy units had Siboney cattle, Guinea grass (Megathyrsus maximus). They also had a biomass bank with Cenchus purpureus cv. Cuba CT 115 (dairy 4) and with Leucaena leucocephala in silvopastoral system (dairy 3). The cows were supplemented with commercial concentrate or Norgold, depending on availability and production.
The proposed methodology for the analysis of both meteorological variables is inserted between the statistical procedures typical of the analysis of temporal series (Bruzual and Hernández-Szczurek 2005), which contemplates the seasonality and the tendency of each of variables.
The comfort temperature for this type of animal is equal to 25 ºC, with a high ambient humidity, from which the cow experiences heat stress (Espinoza et al. 2011).
The THI (Thom 1959) was calculated from the Valtorta and Gallardo (1996) equation:
THI = 0. 8*At + ((RH/100) *(At – 14.3)) + 46.4, where:
THI: temperature–humidity index
At: average temperature
RH: relative humidity
The stress in cattle, according to the THI, was classified as shown in table 1.
Results and Discussion
The maximum, average and minimum monthly temperatures in the ICA have a similar performance to that described for Cuba (Fonseca and García 2012), with two statistically well-established heat periods, which correspond to the seasonal precipitation (figure 1).
The average annual value of the average temperature during 2005 - 2016 was 23.0 ºC with a heat oscillation of 3.4 ºC, between the coldest month (February) and the warmest month (July). In the rainy season, it reached 23.9ºC. The maximum temperature had an average of 29.7ºC and the minimum 19.2 ºC, with a tendency to increase in the analyzed period, higher in the latter. The heat oscillation was also higher 1.8 ºC for the minimum temperature, with a value of 7.9 ºC between the coldest month, January, and the warmest, July.
During 2005 - 2016, the number of days with maximum temperatures higher than 35 ºC, and minimum temperatures lower than 15 ºC were not significant. In all months of this period, there were days with maximum temperatures higher than 27 ºC. Therefore, during all months the animals were subjected to stress (moderate or high), due to high temperatures (Arias, et al. 2008). The temperature-humidity index (THI) in the ICA, during 2005 - 2016, had a monthly performance similar to that of temperature (figure 2). Throughout the year, it maintained values not recommended for animal comfort (Valtorta et al. 1997). That is, the herd was maintained, as average, with slight stress levels (dry season) and moderate (rainy season).
This performance of extreme temperatures not only influences the daily mean, but also has a marked effect on daytime and nighttime temperatures (figure 3).
In the ICA, during 2005 - 2016, an average annual daytime temperature of 27.1ºC and nighttime temperatures of 21.8 ºC were reported. This showed a daily heat differential of 5.3 °C, being higher in the winter season of the year. Their tendencies showed opposite performances. In the same way happened with extreme temperatures. For the daytime, a significant decrease was observed.
The average annual relative humidity during 2005 - 2016 was 77 %. Seven months maintained their average monthly value above this value, with very similar variability in all months (figure 4). In general, an area with high relative humidity can be considered, which has a direct effect on heat sensation, since it accentuates the adverse conditions of high temperatures and the THI Domínguez et al. 2007, López et al. 2016 and Guerra 2019).
THI and heat stress. The average of the THI was 74 during 2005-2016, so it was classified as moderate with marked contrast in its annual distribution (figure 5).
The marked effect of the rainy and dry season is corroborated, which determines that during the dry months conditions of slight stress for cows were established. In this period, the average temperature is significantly reduced and variations in relative humidity occur in a significant way (P <0.001) with respect to the marked heat stress (moderate and severe) of the rainy season.
Several authors have described and discussed the vulnerability of dairy cattle, specifically of cows, to heat stress, and objectify it through THI (Bernabucci et al. 2014, Carabaño et al. 2016 and Nguyen et al. 2016).
This situation is more complicated, if it is taking into account that there are also marked differences between the day and night performance of the THI (figure 6). According to López et al. (2016), during the study period, the animals were maintained with an annual average of 80, which places them under severe daytime stress for ten months of the year, except in January and February, which was moderate. Hence the importance of taking into account not only the average THI, but the daytime.
In this study, the animals actually suffered ten months of severe stress, and not four, as it would have seemed if only the average THI had been taken into account. During the nights, according to the increase in night temperatures, the stress was slight in the dry season, and moderate in the rainy season, with an annual average of 70.
According to Alvarez (2014), in the ICA units there are structural and non-structural vulnerabilities, which can condition the comfort status of the animals, and accentuate the effect of high THI values. Among them, it can be mentioned that, although the location of the buildings is correct (east - west), they have concrete and fiber concrete roofs, which causes an increase in the internal temperature of the building and, consequently, in the animal’s body temperature. Another aspect that should be considered inside the buildings is that they are cleaned with water. This causes an increase in relative humidity, which, combined with the mentioned conditions, can cause higher levels of THI than those recorded outside. Although the units do not cover their accommodation capacities, overcrowding is sometimes observed, due to the poor management of the animals, which remain grouped in small spaces.
In grazing, there are also problems that accentuate the lack of comfort of cows, such as the lack of shade trees and the lack of water in the paddocks (Murgueitio et al. 2016 and Lemaire et al. 2019). In all cases, the animals graze from 6:00 a.m. at 10:00 a.m. and from 4:00 p.m. at 5:00 a.m. Water intake is carried out only in the resting buildings.
Different measures, such as tree sowing and the establishment of silvopastoral systems with multipurpose plants, can be applied to mitigate the effect of heat stress and contribute to improving the comfort of cows. In the case of Genetico 3, not all the dairy unit has silvopastoral system. In addition, the THI was calculated from the ICA meteorology station, and not from the temperature and relative humidity that exists under the trees. If so, surely the THI values would have been more benign in the silvopastoral area of that dairy unit.
To build artificial shadows with different alternatives (constructed of metal, shade mesh, nylon, among other materials, fixed or mobile), as well as having good access to fresh and clean water in paddocks and buildings, are measures of great importance for production, and that they can contribute to improving the physiological parameters in dairy cows, which are highly sensitive to stress, thus increasing productive reproductive indicators and animal welfare (FAO 2012, Gerber et al.2013 and Palma and González-Rebeles 2018).
The ICA can be considered an area with high relative humidity, which has a direct effect on heat sensation and the THI. In the rainy period, the highest values of THI are produced and, consequently, the animals are exposed to conditions of moderate to severe heat stress. While the rest of the year remains in slight conditions. The daytime THI showed heat stress every month of the year. The use of the temperature-humidity index, as an element to determine the heat stress in the animal, is a factor that can contribute to modifying the living conditions of animals in the enterprises.
Similar studies in other regions are recommended, where livestock activity constitutes the fundamental line of production.
Thanks to Dr. Pedro Carlos Martín for his help in rectifying this article and to the ICA Farm for the contribution of the analyzed information.
This article was originally published in Cuban Journal of Agricultural Science, Volume 54, Number 1, 2020.