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Introduction
Goat milk is very important for the economy and survival in diverse regions and populations worldwide (Yangilar, 2013). The increasing demand of consumers for natural products is having a positive effect on the development of organic production systems. Among them, grazing is becoming important in goat production. Nonetheless, a better understanding of the relationship between grazing and production yield is required (Charpentier and Delagarde, 2018). Goat studies have reported that they are efficient in feed conversion and use for milk production (Bedoya, 2012). Goat milk has stood out during the last decades due to its macronutrient contribution (fat, protein and minerals) and its vitamin and fatty acids contents, particularly linoleic acid (Kondyli et al., 2012).
López et al. (2011) reported that marginality of small farming production systems, especially goats, is characterized by the absence of products with high added-value. The rationale of those families to increase their income is by increasing their flock size instead of reducing it and obtaining greater productivity.
In arid and semi-arid rangelands, the soil physical-chemical characteristics (mineral, pH, electrical conductivity, organic matter, among others; Debra et al., 2009) and climate variables (temperature, relative humidity, wind speed and precipitation) are interrelated to determine presence, nutrition, and growth of plants (Fernandez-Illescas and Rodríguez-Iturbe, 2004). These factors directly establish the vegetation type and its chemical composition. Additionaly, those variables have a bearing on the available nutrients for grazing goats (Skarpe et al., 2007; Yayneshet et al., 2008). The previous is expressed in physiological processes, such as milk production and composition, determining variability in the biochemical characteristics in the same year and between seasons (Mellado et al., 2006; Richardson and Hahn, 2007; Guevara et al., 2009).
The goat milk industry requires to determine milk quality in the local conditions in which the animals are raised. We hypothesis that milk quality of creole goats under a traditional extensive grazing system differs according to seasonality and is higher compared to milk from Saanen and French-Alpine goats reared in a stabled system. Moreover, this study is of practical importance for goat producers since knowing when protein and fat content is higher throughout the year would allow them to add value to milk and cheese. Therefore, the objective of this study was to compare milk quality between creole goats in a traditional extensive grazing system and stabled Saanen and French-Alpine goats feed alfalfa and commercial concentrate during three seasons of the year (drought, rain, and transition).
Materials and methods
Study site and management systems
The study was conducted in two sites. Site 1) is located in the buffer zone of Sierra de La Laguna Biosphere Reserve, Mexico, in the plot named El Mezquitillo at 23° 27' 33.264"N and 110° 5' 18.7794"W, and Site 2) at 24° 5' 45.1932"N and 110° 20' 20.3598"W. Both sites are located in the Municipality of La Paz, Baja California Sur, Mexico.
Site 1 consisted of creole goats in an extensive grazing system. Goats were kept confined at night in corrals with no feed, but water ad-libitum and set free during the day to graze vegetation (ad libitum) in the zone. According to León de la Luz et al. (2012), the predominant vegetation in the area is characterized by low deciduous forest with presence of tepeguaje [desert fern] (Lysiloma microphylla), lomboy (Jatropha cinerea), palo colorado (Colubrina viridis), palo blanco (Lysiloma candidum), uña de gato [Mexican mimosa] (Mimosa distachya), pino pinto (Chloroleucon mangense), copal (Bursera epinnata), palo adán (Fouquieria diguetii), palo de arco (Tecoma stans), and lomboy colorado (Jatropha vernicosa).
Site 2 was used for Saanen and French-Alpine goats in a stabled system with goats confined in corrals 24 h and fed alfalfa hay and a commercial concentrate (sorghum, wheat husk, molasses, wheat bran and soymeal produced by a local enterprise).
Animals
In Site 1, Creole goats selected had normal body condition of 3/5 and 36 kg average weight. In Site 2, Saanen and French-Alpine goats were used. Similarly, the animals selected had a body condition of 3/5 and 36 kg average weight. The study started in September 2018 (rainy). On that date, all the goats in study (n = 15) were in 80 ± 10 days of lactation. Subsequently, the goats were dried-up and started another milking period. In May 2019 (transition) all the goats were in 60 ± 10 milking days and by June 2019 (drought) in 150 ± 10 days. In both production systems, the females used were approximately in their fourth milking stage.
Seasons and milk sampling
Three seasons of the year were classified as rainy (September 2018), transition rain-drought (March 2019) and drought (June 2019).
Duplicate milk samples were taken from five creole goats in extensive grazing for five days (replicates in time), and similarly from five Saanen and five French-Alpine goats (five days, replicates in time). In both sites, milk was obtained by manual milking, stored in 100-mL jars and frozen at -20 °C for subsequent analysis.
Milk quality
Milk samples were analyzed according to Kljajevic et al (2018) using a Lactoscan® (ultrasonic milk analyzer device Model LW, Milkotronic Ltd., Nova Zagora, Bulgaria). The 50-mL milk samples were placed in duplicates and the device automatically determined the values for protein, fat, lactose, non-fat solids, density, salt contents (%), freezing point (°C), conductivity (mS cm-1), and pH.
Experimental design
A complete randomized nested design within season (fixed effects) with three levels (rain, transition, and drought) and breed factor (fixed effects) with three levels (Creole -extensive grazing-, Saanen and French-Alpine -stabled). Thus, the breed factor was considered nested in seasons and non-balanced. Five goats of each breed were considered for each season and breed, each goat representing one replicate. In each season and for five consecutive days per season milk sampling was performed (replicates in time) and each sampling per day represented a subsample.
Statistical analyses
To comply with normality assumptions, data of the variables expressed in percentages were arcsine transformed (Little and Hills, 1989; Steel and Torrie, 1995). One-way analyses of variance were performed considering a complete and non-balanced randomized design with seasonality factor with three levels (rain, transition and drought), and breed factor with three levels (Creole, Saneen and French Alpine). Differences within dependent variables (protein, salt, fat, lactose, non-fat solid, density, freezing point, conductivity and pH contents) were determined by Tukey’s test (p≤0.05). The analyses were performed in the General Linear Model (GLM) and Nested Design ANOVA with the statistical program Statistica® v. 13.3 (TIBCO®, 2018).
Results
Seasonality effect on milk quality
The effects of season on milk quality of grazing creole goats and stabled pure breeds (Saanen and French-Alpine) are shown in Table 1. Fat (p=0.00), non-fat solids (p=0.0020), protein (p=0.0086), lactose (p=0.0022), and salt (p=0.0134) were higher (p≤0.05) in milk from the rainy season followed by drought and transition, respectively. Milk freezing point was lower (p≤0.05) during the rainy season. Milk conductivity was higher (p≤0.05) in the drought season. Milk density and pH did not differ (p≥0.05) among seasons.
Breed effect (nested in seasonality) on milk quality
The variables related to milk quality by the effect of breed in extensive grazing and stabled systems, respectively, nested in seasonality are shown in Table 2. Fat content was greater (p≤0.05) in creole goats during the drought season followed by both Saanen and creole breeds in the rainy season. Fat content was lower (p≤0.05) in Saanen goats in drought. The non-fat solid content was also higher (p≤0.05) in creole goats in the three seasons -transition, rain and drought, respectively- while milk of French-Alpine goats during drought showed the lowest content (p≤0.05) of non-fat solids. Milk density increased significantly (p≤0.05) in creole goats during transition and rainy seasons, respectively, while the lowest values were recorded for French-Alpine goats in drought and transition. Protein content increased (p≤0.05) in creole goat milk during transition and rainy seasons, while that of French-Alpine goats decreased during drought. Lactose content was higher (p≤0.05) in creole goats during drought and rainy season, while that of French-Alpine was reduced during drought. The lowest freezing point (p≤0.05) was recorded for creole goats sampled in transition, while the opposite was observed for French-Alpine goats in drought. Conductivity showed the highest value (p≤0.05) in French-Alpine goats during drought and the lowest one in creole goats in transition. Milk pH of the three breeds in the three seasons showed values close to neutrality. However, milk from creole goats had a slightly higher (p≤0.05) pH in the rainy season, while the opposite was observed in Saanen goat milk in the same season. Salt content was higher (p≤0.05) in creole goats in rainy, transition and drought, respectively, while salt content in French-Alpine goats slightly decreased in drought.
Discussion
Seasonality effect on milk quality
Goat milk composition depends on several factors, such as breed, milking stage, feed, differences among animals, climate and locality (Guo et al., 2001). According to our results, seasonality has an effect on milk quality in grazing and stabled goats, with the rainy season allowing for increased quality. The milk fat content was higher (p≤0.05) during the rainy season. This result may be attributed to the diet composition- particularly the amount and composition of fat that has an effect on milk fat content (Vega et al., 2007). The protein content in milk was greater (p≤0.05) in the rainy season with an average of 3.04%. Similar results have been reported in French-Alpine during summer (Milewski et al., 2018), with higher protein values in the rainy season. Moreover, Vega et al. (2007) found that protein content was lower in the rainy season compared to the dry season. Protein increase in the rainy season is attributed to an increase in feed consumption. According to Maldonado et al. (2017), lactose is the most stable component in ruminant milk. In the present study, lactose was higher (p≤0.05) in the rainy season. In this respect, Vega et al. (2007) reported 4.45% lactose in Saanen goats during the rainy season and 4.2% in French-Alpine in the summer. Lactose values fluctuate from 4.46 in winter to 4.45% in summer for French-Alpine goats (Milewski et al., 2018).
Breed effect (nested in seasonality) on milk quality
Different feed sources exist, such as forage, pasture and concentrates that are used in animal diets, as well as different feeding strategies, which have a direct bearing on goat milk quality (Tsiplakou et al., 2010). The results in this study showed significant differences in milk quality of Saanen and French-Alpine stabled goats with respect to creole grazing goats, of which the latter were those that showed the greatest milk quality. Fat content was the highest in creole grazing goats with respect to stabled Saanen and French-Alpine. These results agreed with those of Tsiplakou el al. (2010), who reported that fat was greater in milk of goats raised in a conventional system with respect to those in an organic one, which is attributed to the type of food consumed by the animals.
Moreover, Kondyli et al. (2012) showed similar results when fat content was evaluated in grazing goats in the mountains in spring and summer season. The non-fat solid content showed the highest values in creole grazing goats with respect to stabled Saanen and French-Alpine. Likewise, milk of creole grazing goats showed the greatest content in non-fat solids in transition and rainy seasons.
Table 1 Seasonality effect on milk quality of creole goats in traditional extensive grazing system and two breeds, Saanen and French-Alpine goats in a stabled system.
| Seasons of the year | Fat (%) | NFS (%) | Density (g/cm3) | Protein (%) | Lactose (%) | Freezing point (°C) | Conductivity (mS cm-1) | pH | Salts (%) |
|---|---|---|---|---|---|---|---|---|---|
| Drought | 3.79±0.20b* | 7.96±0.08b | 1.02687±0.22a | 2.90±0.02b | 4.38±0.04b | -0.50±0.006b | 5.85±0.05a | 7.03±0.01a | 0.653±0.006b |
| Transition | 3.73±0.17b | 8.09±0.12ab | 1.02751±0.40a | 2.98±0.04ab | 4.50±0.07ab | -0.51±0.01ab | 5.35±0.06b | 6.99±0.03a | 0.659±0.01ab |
| Rain | 4.92±0.15a | 8.37±0.09a | 1.02736±0.37a | 3.04±0.03a | 4.64±0.06a | -0.54±0.007a | 5.48±0.04b | 7.06±0.04a | 0.682±0.008a |
NFS = non-fat solids. Values correspond to the average ± standard error. *Average values within the same column with different letters (a, b, c, d) differ statistically (Tukey’s HSD p≤0.05).
Table 2 Breed effect (on creole goats in a traditional extensive grazing system and Saanen and French-Alpine goats in a stabled system) considering seasonality.
| Seasons of the year | Breeds | Fat (%) | NFS (%) | Density (g/cm3) | Protein (%) | Lactose (%) | Freezing point (°C) | Conductivity (mS cm-1) | pH | Salts (%) |
|---|---|---|---|---|---|---|---|---|---|---|
| Drought | Creole | 5.50±0.32a | 8.55±0.13ab | 1.0276±0.44bc | 3.11±0.04ab | 4.70±0.07abc | -0.56±0.01cde | 5.49±0.10cd | 7.12±0.02ab | 0.702±0.01ab |
| Drought | F-Alpine | 2.77±0.12e | 7.38±0.06d | 1.02555±0.22c | 2.69±0.02d | 4.06±0.03e | -0.46±0.004a | 6.22±0.06a | 6.97±0.02ab | 0.604±0.005d |
| Drought | Saanen | 2.76±0.13e | 7.83±0.08cd | 1.0272±0.29bc | 2.86±0.03cd | 4.31±0.04cde | -0.49±0.005ab | 5.92±0.04ab | 7.00±0.01ab | 0.642±0.006cd |
| Transition | Creole | 4.25±0.39bcd | 8.91±0.19a | 1.03022±0.53a | 3.32±0.08a | 5.04±0.12a | -0.58±0.02e | 5.15±0.08d | 7.00±0.01ab | 0.726±0.01a |
| Transition | F-Alpine | 3.69±0.25cde | 7.58±0.17cd | 1.02567±0.77c | 2.76±0.06cd | 4.18±0.0.10de | -0.48±0.01ab | 5.38±0.16cd | 7.05±0.12ab | 0.623±0.01cd |
| Transition | Saanen | 3.14±0.11de | 7.60±0.10cd | 1.02608±0.39c | 2.77±0.03cd | 4.18±0.05de | -0.47±0.007ab | 5.57±0.08cd | 6.93±0.02b | 0.614±0.01cd |
| Rain | Creole | 5.03±0.22ab | 8.85±0.18a | 1.0290±0.76ab | 3.22±0.06a | 4.87±0.10ab | -0.57±0.01de | 5.22±0.07cd | 7.22±0.09a | 0.726±0.01a |
| Rain | F-Alpine | 4.66±0.28abc | 8.01±0.15bcd | 1.02627±0.51c | 2.91±0.05bcd | 4.55±0.15bcd | -0.51±0.01abc | 5.64±0.06c | 7.02±0.03ab | 0.646±0.01bcd |
| Rain | Saanen | 5.04±0.30ab | 8.14±0.10bc | 1.02647±0.37c | 2.96±0.03bc | 4.46±0.05cde | -0.52±0.008bcd | 5.64±0.09c | 6.91±0.02b | 0.667±0.009bc |
NFS = non-fat solids. Values correspond to the average ± standard error. *Average values within the same column with different letters (a, b, c, d) differ statistically (Tukey’s HSD p≤0.05)
Protein content in creole grazing goats was also the highest compared with stabled Saanen and French-Alpine. The previous results can be attributed to the increase in nutritional value of the species consumed by grazing goats compared with hay forage and balanced feed, which agrees with that reported by Morand-Fehr et al. (2007). Lactose content in the three seasons was the highest in creole grazing goats. In this respect, Kondyli et al. (2012) reported that no significant differences were observed in lactose in two seasons of the year. Milk conductivity was higher in Saanen and French-Alpine goats in the drought season, while salt content was higher in creole goats in the three seasons, which is attributed to the nutritional content of the species consumed in grazing. The previous results agree with that reported by Zervas et al. (2011), who showed that grazing goat milk showed the highest values of vitamins, lipid compounds, and fatty acids than those in milk of animals fed concentrate or hay forage. A recent study described a model that highlights the complexity of the factors that influence diet selection of grazing goats because goats are highly selective, not only by resource seasonality and availability, but also the origin of each nutrient source (Manousidisa et al., 2018). In the same context, Armenta et al. (2011) reported that grazing goats in northwestern Mexico varied diet selection throughout the year, only were some rangeland species highly preferred and used. This finding explains the fact that although the northwestern region of Mexico experiences long drought periods, goat milk quality is not affected since they are selective, search, and prefer consuming rangeland species that provide more nutrients.
In conclusion, goat milk quality increased in the rain period (seasonality effect). Creole grazing goats showed that their milk quality was superior with respect to both stabled Saanen and French-Alpine. Creole grazing goats showed superior milk quality in rainy season compared with stabled Saanen and French-Alpine.
