1. Introduction
The Murciano-Granadina Goat is the most important Spanish dairy goat, both in the number of individuals and production levels. There are more than 500,000 animals, of which 104,010 registered to the official herd book. They are spread across Spain, especially in Andalusia, Murcia and Castilla la Mancha, but an increasing number of animals are now reared abroad, mainly in other European countries, North Africa, Latin America and the United States [
1].
Murciano-Granadina dairy biotype [
2] is eumetric and characterizes by a subrectilinear or rectilinear profile. The coat is uniformly black or mahogany (stains of any color are unacceptable), with black or pink mucous membranes, respectively. Males’ average height and weight are 77 cm and 65 kg, while females’ average height and weight are 70 cm and 50 kg, respectively. Horns are not normally present, although they can appear in certain animals. Additionally, its continuous polyestrous nature makes the breed enjoy great prolificacy and longevity (6 births) [
3,
4].
Murciano-Granadina breeds’ average milk production is 584.4 kg, with an average lactation length of 287 days. Average milk fat and protein contents are 5.3% and 3.6%, respectively [
1,
4]. Despite its traditional semi-intensive handling, a tendency to intensification has progressively arisen until recent years. This newly implemented system uses close-to-farm natural resources and byproducts generated by agriculture.
Its important social-cultural and environmental roles are key for the maintenance and expansion of the population in rural areas. In these regards, the breed benefits from resources that cannot be used by other species. For instance, Murciano-Granadina goats feed on pastures from uncultivated areas and agricultural byproducts. Its great adaptability makes it especially suitable for cleaning arid or semi-arid scrubland zones, with its direct repercussions in fire prevention [
5].
Quantitative and qualitative milk production are multifactorially affected. These factors can be grouped into animal-intrinsic (breed, size and body weight, age at parity and lactation number, litter size, physiological state, etc.) and extrinsic factors (Feeding, breeding system, kidding season, milking, dry period length, among others) [
6].
The study of the lactation curve traces the evolution of populational or individual variations occurring in milk production through lactation. It enables detecting unexpected deviations at herd level as consequences of a deficient or erroneous diet or underlying pathologies in herds. According to Wood [
7], knowledge of the lactation curve is necessary to determine the nutritional and reproductive management of lactating animals. This can be done by estimating the total production per lactation and curve shape parameters (lactation peak and persistence).
Conditioning factors not only influence milk production but also affect curve shape. Generally, the lactation peak is reached between the fourth and seventh weeks, subsequently decreasing afterward until the end of lactation. First-lactation goats usually present average lower productions and shorter lactation lengths when compared to goats at their second lactation or over.
The lactation curve can be described through several mathematical functions [
3]. Such functions normally comprise an ascending phase, a production peak and a descending phase. By fitting these mathematical functions, the data obtained from different milk recording schemes can be used to predict milk yield and trace the evolution of total production. One of the most efficient functions, thus most frequently dealt with, is the equation proposed by Wood in 1967 [
8].
The objective of this paper was to evaluate the effects of the number of kids, season, year and farm on total milk yield, daily milk yield, lactation length, total fat and protein production and percentages in primiparous goats. Additionally, lactation curve shape parameters (peak and persistence) were estimated and evaluated to determine whether these may be conditioned by the number of kids and season, using the linearized version of the model of Wood in random regression analyses.
4. Discussion
Average production values in first lactation goats in the present study were lower than the values reported in other studies. For instance, León et al. [
10] reported productions of 372.58 kg for the first lactation of goats of the same breed. However, the findings in this study were similar to those by other authors. In these regards, Martínez Navalón and Peris Ribera [
11] conducted a study between 1999 and 2002 and reported average production of 309 kg for first-parity goats with lactations of 216 days on average. Similar values were also reported by Pérez [
12] between 2002 and 2012. Our results are in line with those reported by the Union of Farm Associations for Milk Control in Castilla y León [
13] from controls first- natural lactation goats collected from 2015, 2016 and 2017, reporting average productions of 425.65, 401.10 and 453.60 kg, respectively in lactations with an average length of 254.47, 248.16 and 255.02 days, respectively.
Differences between Murciano-Granadina goats milk production and that from other Spanish native breeds have been reported in the literature. For instance, Florida breed primiparous goats milk productions of 412.83 kg in 225 days lactations have been reported [
14]. However, García et al. [
15] reported higher average yields of 441.07 kg and longer average lactation lengths (276.74 days) for the Florida breed. Malagueña primiparous goats presented comparatively higher average milk productions of 380.9 kg of lactations with average lengths of 256 days [
16]. Contrastingly, lower milk productions of 286.85 kg with lactations of 210 days have been reported for the Palmera dairy goat [
17]. Among all the Spanish breeds, Payoya production values were the closest ones to those in our study, with values of 314.2 kg and average lactation lengths of 234 days [
18].
The values for fat and protein percentages reached the upper margin of the results found in the bibliography for Murciano-Granadina goats. Hence, similar results were found for Murciano-Granadina goats officially controlled in Castilla León. Fat and protein percentages were 4.98 and 3.56% in 2017, 5.33 and 3.55% in 2016 and 4.94 and 3.66% in 2015. Pérez [
12] obtained similar results as well for their 10-year-long study involving farms of CAPRIGRAN Y ACRIMUR, results which were also similar to those reported by Martínez Navalón and Peris Ribera [
11] who reported a protein percentage of 3.55% and a fat percentage of 4.76% in the same breed.
Compared to first birth goats from other native breeds such as Florida, Malagueña or Palmera, fairly similar fat and protein percentages were found, with small variations occurring in regards to fat percentage. For instance, in Florida, goat fat percentages of 4.9% and protein percentages of 3.54% were found [
14]. In Malagueña goats, lower fat percentages with 4.27% and higher protein percentages of 3.64% were reported [
16], while the primiparous Palmera breed goats presented similar fat percentages (5.04%) to those in the present study and higher than 4.44% protein percentages for standardized lactations at 210 days [
17].
Compared to the productions reported in the literature for other highly productive European breeds, great differences can be found. For example, in controls carried out in the Alpine breed caprine breed in 2016 in France, much higher milk productions were found for primiparous goats, with 856 kg of milk in lactations with 308 days on average. The milk from these goats also presented lower fat and protein percentages (3.89 and 3.35%, respectively). In the Saanen breed in France, first-kidding goats reached productions of 1000 kg of milk, in lactations with an average length of 337 days and with fat and protein percentages of 3.69 and 3.23%, respectively [
19].
Prolificity has been reported to be responsible for remarkable alterations in milk yield and composition. In fact, the tendency to present higher numbers of kids may be the result of selection with adequate reproductive management. In the present paper, higher dairy productions for multiparous goats were found, which was also found in publications by other authors such as Hayden et al. [
20]; Salvador and Martínez [
21]; Martí Vicent [
22]; Pérez [
12], which suggests litter size has a positive correlation with milk production. This may be due to a higher volume of the placenta in animals with more than one kid, which determines a higher production of placental lactogen, which is the primarily responsible hormone for the development of the udder in mammals. This influence of the number of goats on production is independent of other factors such as the number of lactations, lactation length, age, body weight, among others.
Additionally, Byatt et al. [
23] studied the positive effect of placental lactogen by stimulating milk production in cattle using increasing doses of recombinant placental lactogen. In turn, authors such as Delouis et al. [
24], Salvador and Martínez [
21], or Martí Vicent [
22] identified the pivotal role of lactogen but also suggested it may not be the only hormone responsible for the development of the mammary gland during pregnancy. In these regards, other hormones such as prolactin and fetus-placental and ovarian steroids and other hormones involved in general metabolism may be involved as well.
The number of kids did not affect fat and protein percentages in Murciano-Granadina goats’ milk in our study, which agrees with the results by other authors such as Olechnowicz and Sobek [
25], Ibnelbachyr et al. [
26] and others [
21,
27]. Oppositely, some authors such as Gómez et al. [
28], Ciappesoni et al. [
29] and Bagnicka et al. [
30] have reported significant differences across the different possibilities within the number of kids factor. For instance, fat and protein levels have been reported to be lower in goats with a higher number of kids as an indirect effect of the increase in milk production. Still, the results of different studies are influenced by different factors, for example, feeding, body condition; hence, it is not easy to distinguish which influences may indeed have a genetic origin from those which may not.
Although component percentages were not significantly different as the number of kids increased, significantly higher means were found for total protein and fat production. These trends were also described for multiple births, Florida goats [
31], when a predominant number of female kids was present in litters. As a result, females kidding multiple female kids presented not only higher milk yields, which was also supported by our results, but also higher fat, protein and lactose production. Parity season is one of the most commonly reported conditioning factors to influence milk production and composition. This may be due to the different climatic factors which can affect the animals during each season. The effect of the different factors may become accentuated in animals, which present a marked seasonality, for which temperature, photoperiod, humidity may have a greater impact. Many of these variations are related to pasture curve and food availability during each season, which will influence not only milk production but also lactation length and the shapes which lactation curve describes [
32]. Pizarro et al. [
33] found similar results to those in the present study on Murciano-Granadina goats and ascribed the lack of significance in literature found for protein and fat contents to studies only recording the information from animals with single and twin-parities, hence the influence of higher numbers of kids may be disregarded and responsible for our findings.
Summer lactations were more productive than autumn lactations, and these, in turn, were better than those of winter, with the worst lactations occurring after the births in spring. This is because late summer births often result in high and longer-lasting initial productions. Furthermore, as seen in
Figure 4, even if this occurs, lactation peaks do not reach higher levels above the initial productions and are reached within a few days after the lactation started.
Likewise, the slope of the curve, and therefore the decline of production, is not so sharp. On the contrary, births in spring are followed by lactations characterized by higher peaks, which is due to the positive effect of the abundance of food at this time, which coincides with the higher demands of the animals. Nevertheless, the descending phase of the curve is sharper since it is simultaneous to the summer. During summer months, the quality and quantity of pastures are the lowest of the year, which will also determine these lactations may present the shortest average lactation lengths. Lactations after the births occurring in winter and autumn have intermediate characteristics. These results are very similar to those obtained by Deroide et al. [
34]; Pérez [
12]; León et al. [
10]; Fernández et al. [
35]; Verdejo et al. [
36] and Carrizosa et al. [
37] in Murciano-Granadina goats and Sánchez et al. [
38] in Florida goats.
In terms of protein, the highest percentage was obtained in summer, followed by autumn, with the lowest percentages being reported for winter and spring. These results are similar to those obtained by other authors, such as those published by Carrizosa et al. [
37], Gómez et al. [
28], Fernández et al. [
35], Pérez [
12] and Deroide et al. [
34] with Murciano-Granadina goats. For these authors, the lowest percentages were found for parities occurring during the spring months, with the highest percentages being reported after the parities taking place around the month of September.
The highest fat percentages are reached in summer and autumn, with the lowest ones being reported for winter and spring. Similar results were found in the studies by Gómez et al. [
28] and Deroide et al. [
34] in which the highest fat percentages were found for births taking place between August and December, while the lowest were reported for the months between January and July.
Such fluctuations are in line with the results by other authors such as Martínez Navalón and Peris Ribera [
11] who reported similar fluctuations in average milk productions during the period 1995–2001, also reported by CAPRIGRAN and ACRIMUR associations between 2002 and 2012. Similar results, such as those obtained by Lôbo et al. [
39], also reported oscillations in average milk productions over the years that their study took place.
In this context, milk production and average daily production describe fluctuations. In recent years, milk production peaks close to 350 kg have been reached, and then declined. This has translated into maximum production differences of around 80 kg. On the other hand, the fat percentage described an upward trend from 1999 on, reporting an average of 5.35%. For protein percentage, average values of around 3.68% were found in recent years.
Protein percentages also presented an upward trend. However, oscillations were reported for the period of the study. Other authors, such as Byatt et al. [
23], found significant differences per year of lactation for fat and protein percentage in milk. Avilés et al. [
40] and Martínez Navalón and Peris Ribera [
11] also reported an upward trend for fat and protein percentages, both in primiparous and multiparous goats. Likewise, Lôbo et al. [
39] reported fluctuations in fat and protein percentages, describing upward trends and reporting the highest percentages found in the literature for the last years comprised in the study. This finding may be ascribed to the effects of the selection of individuals by breeders based on dams’ fat and protein indices.
The significant effect of livestock on the variables considered in the present study may be a sign of the underlying variability. Variability found may be ascribed to the large difference of conditions across populations, among other factors, such as operating system, reproductive, food and/or health management or infrastructure. Even microclimatic conditions to which each population is exposed [
21] may be responsible for a certain degree of the variability that can be observed. Among the studies in literature, the best representation of interfarm variability was reported by Pérez [
12], who concluded that 20% of interfarm variations among the herds comprising CAPRIGRAN and ACRIMUR associations were due to the effect of livestock. Authors such as Palma [
41] found variability may depend on the conditions to which each livestock unit is exposed or to the ones which are present where herds are located. In this context, food management may be one of the pivotal factors to explain the greatest interherd variability. Simultaneously, according to Kučević et al. [
42] and Sandrucci et al. [
43], these significant differences may also occur in milk composition when different herds are compared.