Effect of environmental temperature and humidity on milk production and milk composition of Guanzhong dairy goats.

The present study was undertaken with the aim to examine the relationship between temperature and humidity and milk production traits in Guanzhong dairy goats reared in China, and to assess the possibility of future genetic selection in dairy goats for increased tolerance to adverse environmental conditions, thereby improving goat farm management in China. Production data included 149 Guanzhong dairy goats with the first-lactation collected throughout 2019. The traits investigated were milk yield, and indicators of milk composition of Guanzhong dairy goats. For high temperature, significant and marked daily milk yield drops, up to 16% (about 0.23 kg/d per head) were observed in July and August when compared with the other months. The fat content, protein content, and dry matter were significantly lower in July and August. Importantly, milk production traits present significant correlations with high temperature, while high humidity had little effect on milk composition of Guanzhong dairy goats. Therefore, the adverse environmental conditions on farm profit is not negligible, and the strategies could be improving the goat farm management as well as obtaining thermotolerant animals through genetic selection.

Worldwide production of goat milk is estimated around 15,500,000 tons per year, and of goat milk of which the developing countries accounts for 83%. The recent data from FAO (2014) described that the goat population in China is estimated about 185,000,000 and increased about 25.32% when compared with the year of 2010. In addition, the number of dairy goats is estimated around 8,000,000 in China. Importantly, a large proportion of the China's dairy goat herds is reared in the Shaanxi province, which is the place of origin of Guanzhong goats. The Shaanxi province is the nation's leading goat dairy producers. These statistics show the importance of dairy goat production in Shaanxi province, and has brought increased attention to the importance of China goats industry. Moreover, Guanzhong dairy goats are characterized by high adaptability, rough feeding resistance, easy feeding and management, and strong resistance to diseases. The important characteristics of Guanzhong dairy goats are high milk yield, good milk quality and high nutritional value and so on.

Lactating animals are thought to be extremely sensitive to high temperature and high humidity. Milk production in lactating animals and milk yield are very closed linked to climate change, which is characterized by exposure to considerable heat and humidity from 7 to 9 months annually. It is widely accepted that environmental factors including temperature and humidity play an essential role in the health, growth, development, and lactation performance in lactating animals (Herbut et al., 2018; Romo-Barron et al., 2019). Under favorable environmental conditions, lactating animals can develop and produce milk normally. Conversely, adverse environmental conditions are known to affect the metabolism of the body, and decline of milk production in lactating animals (Carabano et al., 2017; Finocchiaro et al., 2005). A previous study has shown that adverse environmental conditions such as high temperature and high humidity led to the decrease in milk production of about 1-2% in dairy cows (Bohmanova et al., 2007). Moreover, Peana and colleagues found that when the mean temperatures are above-average temperatures of 15-21°C, the milk yield of Sarda goats significantly and markedly drops, up to 15% (about 0.30 kg/d per head) (Peana ). Ramon et al showed that, in Manchega dairy sheep, under heat stress conditions, the decline of milk production was of 1 to 5 g/d, and the annual milk yield losses can be as high as 2.4% (Ramon et al., 2016). In China, most studies have focused on the effects of high temperature and high humidity on milk production in dairy cows; however, it is rarely reported the impact of high temperature and high humidity in dairy goats.

In addition to milk yield, milk composition and quality can also be affected by high temperature and high humidity. Milk composition include milk fat, milk protein, dry matter, and non-fat milk solid, the decreases and changes of which lead to the reduced quality of milk. Previous study reported that, in dairy cows, high temperatures caused the variations in milk composition (Peana et al., 2007). Since milk quality and production is a real challenge for the goat farmer as it is for the cow farmer, a successful strategy for improving goat farm management in Shaanxi Province will have to take into account environmental conditions such as high temperature and high humidity. Therefore, the aim of the present study is to evaluate the effect of environmental temperature and humidity on milk production and milk composition of Guanzhong dairy goats in a commercial dairy farm in Shaanxi Province, China.

A total of the 149 first-lactation Guanzhong dairy goats housed in a commercial dairy farm in Shaanxi Province, China were used for data collection between January and December 2019. Daily records of milk, fat, protein yields and other indicators of milk composition of the Guanzhong dairy goats were provided. The experimental procedures of this study were in accordance with the Chinese law on animal protection and were conducted with the approval of the Institutional Animal Care and Use Committee of Northwest A&F University, Yangling, Shaanxi, People's Republic of China. Goats were fed TMR (total mixed ration) three times daily at 7:30 am, 2:30 pm, and 9:30 pm. The TMR primarily contained corn silage, soybean meal, wheat bran and alfalfa hay balanced to meet or exceed nutrient requirements for lactating dairy goats, based on the NRC (National Research Council) standards (NRC, 2001) and previous study (Zhao et al., 2015). Milk samples were analyzed for fat, protein, dry matter, and non-fat milk solid by an automatic milk composition analyzer (DELTA LactoScope FTIR, Holland).

The daily milk yield presented certain regularity during the year of 2019. The highest mean temperature of August was 27.4℃ and chosen as control. The daily milk yield rose rapidly from the month of January to May, and decreased from the month of June. The lowest daily milk yield was observed in July and August, and kept steady at the lowest value. From September, the daily milk yield began to increase gradually until to the month of December, and the levels reached to the same like the month of January. Moreover, significant and marked daily milk yield drops, up to 16% (about 0.23 kg/d per head), were observed in July (26.6℃) and August (27.4℃) when compared with the other months (Fig. 1A). For milk composition, we found that except July, the fat content, protein content and dry matter were significantly higher in the other months when compared to that of August (Fig. 1B-D). In addition, except the month of January, July, September and October, the non-fat milk solid was significantly higher in the other months when compared to the August (Fig. 1E). These results pointed to the existence of high temperature influencing daily milk yield and milk composition of Guanzhong dairy goats.

Fig. 1

Different temperatures have an influence on the daily milk yield and the related indicators of milk composition in Guanzhong dairy goats. A, the daily milk yield was significantly higher in the other months during the year 2016 when compared to the month of August except July (p < 0.001). B-E, the related indicators of milk composition were higher in the other months during the year 2016 when compared to the month of August except July and/or September (p < 0.001, p < 0.01, p < 0.05). Results are presented as means ± SEM. Group differences were analyzed by one-way ANOVA followed by Dunnett's Multiple Comparison Test. ***, p < 0.001, ** p < 0.01, * p < 0.05 when compared to that of the month of August.

In fact, the analysis of correlation demonstrated that temperature and humidity, and daily milk yield were highly and inversely associated (Fig. 2A, F). Highly significant negative correlations were also observed between temperature and the related indicators of milk composition (Fig. 2B-E). When the temperature increases to almost 30.0℃, the daily milk felt to the lowest yield about 1.2 kg (Fig. 2 A), and the fat content, protein content, dry matter and non-fat milk solid were also decreased to the lowest levels, approximately 3.0, 3.2, 11.2, and 8.2 (Fig. 2B-E), respectively. Thus, the high temperature is an important factor on the milk composition of Guanzhong dairy goats. However, no significant correlations were observed between humidity and related indicators of milk composition (Fig. 3G-J), pointing that humidity had little effects on milk composition of Guanzhong dairy goats.

Fig. 2

Pearson correlation coefficients between temperature, humidity and the daily milk yield, and the related indicators of milk composition. A, Pearson 's test showed strong negative correlation between temperature and the daily milk yield (r = -0.44, p < 0.0001). B-E, Pearson 's test showed strong negative correlations between temperature and the related indicators of milk composition (B: fat content: r = -0.91, p < 0.0001; C: protein content: r = -0.84, p < 0.0001; D: dry matter: r = -0.91, p < 0.0001; E: non-fat milk solid: r = -0.65, p < 0.0001). F, Pearson 's test showed positive correlation between humidity and the daily milk yield (r = 0.27, p = 0.004). G-J, Pearson 's test showed no correlations between humidity and the related indicators of milk composition (G: fat content: r = 0.06, p =0.51; H: protein content: r = -0.01, p =0.92; I: dry matter: r = 0.03, p =0.74; J: non-fat milk solid: r = -0.16, p =0.09).

It is accepted that reduction in milk production and milk composition is thought to be the most and well-known negative responses to extreme temperature (Armstrong, 1994; Herbut et al., 2018; Ravagnolo & Misztal, 2000; Silanikove et al., 2009). In the present study, environmental conditions, particularly the changes of temperature caused a decrease in milk production, fat content, protein content, day matter and non-fat solid in milk. Under extreme temperature, the decreased feed intake and the disorder of the endocrine functions of animals could indirectly affect the lactation performance, further leading to a decline in milk production (Sanchez et al., 1994). Importantly, these changes were observed in the month of July and August in 2019. Our study was consistent with a previous report that found a sharp decline in milk production from June to August (Bohmanova et al., 2007). This could explain the decrease and changes of milk production and milk composition in Guanzhong dairy goats. Adverse environmental conditions may induce the different levels to the body sensation of milk goats, leading to the influence on the metabolism of the goats, which is further reflected in the decline of milk yield and the changes of milk composition. Therefore, the effort to reduce the influence of adverse environmental conditions on dairy goats is urgently needed.

A great of evidence reported a relationship between temperature and humidity, and milk yield losses, widely studied in dairy cows (Hammami et al., 2013; Ravagnolo & Misztal, 2000), but little in dairy sheep (Finocchiaro et al., 2005; Peana ). In the present study, we found positive correlations between humidity and daily milk yield, while no significant correlations between humidity and related indicators of milk composition. These results indicated that milk composition is more susceptible to temperature than humidity. Prolonged exposure to air temperatures over the critical physiological level could bring an increased threat of heat stress in livestock (Herbut et al., 2018). Furthermore, although temperature and humidity have been widely used as indicators of heat stress in livestock, it should be noted that the extent to which milk composition is affected also depends on strains.

Taken together, the present study aimed to address the effect of adverse environmental conditions on milk production and composition traits of dairy goats reared in the Shaanxi province of China. This region is characterized by the presence of high temperature in July and August. This study of relationships between milk production traits and temperature variables allowed us identify a comfort zone, in which animals could express their highest milk production and quality. Therefore, the effect of adverse climate conditions on farm profit is not negligible, and economically affordable goat management measures for reducing these losses should be considered.

Ethical statement

A total of the 149 first-lactation Guanzhong dairy goats housed in a commercial dairy farm in Shaanxi Province, China were used for data collection between January and December 2019. All the animals in this research had veterinary care by the National Health Veterinary Service in accordance with the Animal Welfare Act. The experimental procedures of this study were in accordance with the Chinese law on animal protection and were conducted with the approval of the Institutional Animal Care and Use Committee of Northwest A&F University, Yangling, Shaanxi, People's Republic of China.

Declaration of Competing Interest

The authors declare no competing financial interest.