The greatest challenge in this study was to assess a combined index that allowed us to simultaneously improve a group of criteria with the least possible penalization, and the responses obtained in others, under four different economic scenarios related to the main sources of income on goat farms (milk production exclusively; milk and cheese production; cheese production exclusively; and milk production, cheese extract and sale of breeding animals).
3.1. Phenotypic Parameters
The basic descriptive statistics for the traits studied in the Florida breed are presented in
Table 2.
The daily averages for milk traits were 2.82 ± 1.58, 0.23 ± 0.13 and 0.099 ± 0.051 for MY, FPY and CY, respectively. For MY, female Florida production was similar to figures for Alpine and Saanen breeds [
24], US goat breeds [
25] and Polish breeds [
26]. Our average of MY was much higher than those of the Spanish Payoya breed [
27] and Nigerian Dwarf goats [
28], which could be due to the differences in nutritional and management conditions. The means of FPY and PY were in agreement with that observed in Nigerian Dwarf goats [
28] and lower than the averages for Alpine and Saanen breeds [
24].
Average SCS values have been calculated in other studies using different logarithmic transformations, including log
2, log
10, or natural logarithms. In this study, the SCS was calculated using natural logarithm transformation with an average SCS of 6.6 ± 0.95 and similar to that obtained in Polish breeds by the natural logarithm for SCS (6.62 ± 1.25; [
26]). The average milk SCS in Florida was higher than that of other dairy breeds (New Zealand mixed-breed dairy goats [
29]; Hungarian breeds [
30]; and Nigerian Dwarf goats [
28] in).
Worldwide phenotypic values for RE are not available in dairy goats since this character has been analysed only in the Florida and Payoya breeds. The average RE value was lower in Payoya than in Florida, indicating that most Payoya females start their reproductive life late since they are reared in a more extensive production system [
3].
The average LS obtained in this study, 1.62, was higher than the values of 1.4 and 1.33 observed in Saanen and Alpine goats [
31] and in the local Tunisian breed [
32], respectively, and similar to that of French Alpine, Saanen, and Toggenburg, with an average litter size of 1.7, 1.7, and 1.6, respectively [
33].
The average of FS obtained in the Florida breed was similar to the values estimated in Saanen and Alpine goats (81.5 ± 4.7; [
31]), US dairy goats (83.79 ± 4.11; [
34]) and Nigerian Dwarf goats (84.1 ± 3.26; [
28]). Mellado et al. [
31] reported values of 1.8 ± 0.8 and 3.4 ± 1.1 for mammary system, body size, and capacity, respectively, using a 5-point scale.
Earlier studies with different definitions of longevity in dairy goats produced an average of 625 d for productive life at 72 months in US breeds [
34], 1726 d for length of true life in UK dairy goats [
35], 967 d for functional longevity in Saanen, and 1007 d in Alpine goats [
36].
The lowest coefficient of variation was found for FS (2.29%) and the highest for LPL (60.64%), indicating that, except for type traits, there is an important phenotypic variation for the analysed traits in the Florida population, especially for milk production and longevity.
3.2. Genetic Parameters
Estimates of the heritabilities (h
2) and genetic correlations (r
g) for the evaluated traits are summarized in
Table 3. Heritabilities ranged between 0.03 for LS and 0.58 for LPL. In general, the estimated h
2 were therefore considered suitable for genetic evaluations and selection for these traits.
Milk production traits have been widely studied in dairy goats and there is abundant information about their genetic parameters. Our heritabilities for MY, FPY, and CY are within the range of values estimated in other dairy goat populations (Saanen [
37]; Alpine and Saanen breeds [
24]; and New Zealand goats [
38,
39]) and lower than the values reported by Mucha et al. [
40] in their meta-analysis. The h
2 estimates for SCS reported in other breeds covered a wide range of values, from 0.09 ± 0.03 in the Alpine breed [
41] to 0.32 ± 0.095 in Nigerian Dwarf goats [
28]. Our estimate was in agreement with the 0.15 estimated in French Alpine goats [
42] and the 0.21 values reported in the meta-analysis by Mucha et al. [
40] that included twenty-six breeds worldwide, and Bagnicka et al. [
26] in Polish breeds.
The heritability estimate for RE in this study was moderate and clearly higher than the estimates for the classical fertility criteria in both Florida and Payoya breeds [
3], and in other dairy goat breeds in the world [
43]. It is difficult to compare this figure with other estimates in the literature because of the lack of studies about female fertility, as defined in this study and in our previous work in Florida and Payoya breeds [
3]. Litter size is not very heritable, which indicates a low possibility of achieving rapid genetic progress through direct selection. Our estimate was in agreement with the low previous estimations (close to zero) in different goat breeds (0.068 in Saanen goats [
44]; 0.08 in the local Tunisian breed [
32]; and 0.002 in Markhoz goats; [
45]).
The type traits recorded and their definitions differ between countries, which makes them difficult to compare. Our estimates were in the range of values reported by Massender et al. [
46] for Canadian Alpine and Saanen breeds. Estimations of h
2 of several type traits in Nigerian Dwarf goats ranged between 0.003 for dairyness and 0.71 for stature [
28].
Any comparison of the h
2 estimates of longevity from this study with other estimates from other studies of goat is further complicated by the different definitions of longevity and statistical models. To date, except for the Florida breed, the h
2 values for longevity in dairy goats are available using linear models and were clearly lower than ours, ranging between 0.07 and 0.46 (US breeds [
34]; UK dairy goats [
35]; Saanen and Alpine breeds [
36]; German breeds [
47]).
Regarding the genetic correlations between the traits analysed, these covered a wide range of magnitudes and values, between −0.21 for FPY-LPL and 0.90 for MY-CY. In this study, the values of genetic correlations between milk yield traits were high, as is typical in dairy goats (Nigerian Dwarf goats [
28] and New Zealand breeds [
38]).
In general, the information about r
g between milk production traits and SCS in dairy goats was limited, with considerable controversy over its sign and magnitude. Previous studies reported positive r
g between these traits, varying between 0.06 and 0.59 (Alpine breed [
24]; Polish breeds [
26]; Nigerian Dwarf goats [
28]; and New Zealand breeds [
38]). In contrast, Rupp et al. [
24] observed negative r
g between SCS and FY, and PY (−0.13 and −0.04, respectively) in the French Saanen breed.
In our study, the r
g between milk yields and reproductive efficiency were moderate to high, in contrast with the unfavourable estimations observed in other breeds and species (dairy sheep [
48]; dairy goat [
49]; dairy cattle [
50]). This fact could be due to the definition of reproductive efficiency used in this study. An increase in RE means that the female is kidding at the optimum age, and consequently producing higher milk yields.
A wide range of r
g values was observed between type traits in comparison with others in the literature because the studies used different conformation measures. Valencia-Posadas et al. [
28] calculated r
g among 14 type traits, and their estimations oscillated between −0.57 for rear udder height–medial suspensory ligament and 0.55 for stature–rump width. Our estimates of r
g in the Florida breed between milk traits and type traits were low (0.003 to 0.11,
Table 3). Manfredi et al. [
51] found negative correlations ranging from −0.51 to −0.19 when estimating correlations between udder traits and milk yield in Saanen and Alpine French breeds. The values estimated by Valencia-Posadas et al. [
28] ranged between 0.01 to 0.88 in Nigerian Dwarf goats.
Genetic correlations involving SCS and type traits were generally low and in agreement with our estimates. Rupp et al. [
24] observed low correlations ranging from −0.27 ± 0.034 to 0.34 ± 0.030 in Alpine and from −0.19 ± 0.036 to 0.15 ± 0.030 in Saanen.
Valencia-Posadas et al. [
28] also reported correlations between −0.16 and 0.22 in Nigerian Dwarf goats. Because of the negative or low r
g between SCS and type traits in our study, the options for indirect selection are limited; however, given the relatively high magnitude of h
2 for SCS (0.20), direct selection for this trait may be successful.
Palhiere et al. [
36] estimated r
g of functional longevity with production traits, SCC, and udder type traits in French dairy goats; they obtained low values from −0.06 to 0.28 for milk yield, close to zero for fat and protein yields, negative values from −0.29 to −0.35 for SCS, and positive values for udder floor position and rear udder attachment (r
g from 0.17 to 0.29). Castañeda-Bustos et al. [
52] in US dairy goats observed a positive r
g for longevity in milk yields ranging between 0.23 and 0.39 and a low r
g for female fertility. Wolber et al. [
47] also observed positive genetic correlation of 0.30 for LPL in milk yield in German goat breeds.
Estimation studies of r
g for LS for other traits are scarce. Mourad [
53] found a higher r
g than ours (0.029,
Table 3) between LS and MY for the last three months of lactation (0.23, 0.43, and 0.48, respectively), as well as total lactation milk (0.91) in Egyptian Alpine goats.
3.3. Expected Genetic Responses
Table 4,
Table 5,
Table 6 and
Table 7 showed the expected direct and correlated genetic responses for each of the four scenarios and three selection indices type in the Florida breed. The breeding objectives and selection criteria included actual traits used in the breeding program of the Florida breed, as well as possible traits which are not used at present but could be used for selection, for which genetic parameters are available.
In Scenario I, where the overall objective was the optimization of milk yield, the highest genetic response was obtained when the primary objective (milk yield) was included as a unique selection objective/criterion (Index 1,
Table 4). Regarding the correlated responses, a low, favourable response was observed in MS, BCI and RE. However, in somatic cell score and length of productive life, there was a small, unfavourable response (increase in RCS and decrease in LPL). By including secondary traits (MS, BCI, RE, SCS and LPL, Index 2,
Table 4) and assigning them the same economic weight as the primary ones, a small improvement can be observed in their responses, except for RE. However, this causes the response in the primary trait (MY) to be reduced by approximately six-fold. In the third index, which gave more weight to the primary trait and different weights to the secondary ones according to their relative importance in MS, RE, and BCI, there were hardly any changes. In SCS and LPL, there were favourable responses, but given their very low magnitude, the significant decrease in the response of MY may not be compensated (Index 3,
Table 4).
In Scenario II, as in the previous one, the improvement in the overall objective (milk and cheese production) could be achieved by selecting only for the MY and FPY primary traits. In the other criteria, there were no correlated responses, except a slight decrease in RE and LPL, respectively (Index 1,
Table 5). By incorporating secondary traits into the index, the responses in MY and FPY decreased to less than half (Indices 2 and 3,
Table 5), and no significant changes were observed in them, except with RCS and LPL, where slightly favourable responses were observed (Indices 2 and 3,
Table 5).
Scenario I has been the most applied in this breed until now. Although there are more farms where milk has been used for cheese extract (fat plus protein yield), farmers do not pay attention to this criterion because, generally, a higher level of milk production allows a greater production of fat plus protein yield. Also, taking into account the feeding management, it is possible to maintain the fat content relatively high even with high production levels.
As with the two previous scenarios, for Scenario III, the greatest response in the primary trait FPY, which is directly related to cheese production, was obtained when it was included as a single selection objective and criterion (Index 1,
Table 6). In addition, this index produced the best responses in MY and CY due to their high genetic correlations with FYP. In the other criteria, there were favourable correlated responses, except for LPL (Index 1,
Table 6). In the combined indices with primary and secondary criteria, there were favourable responses in all the secondary ones, except for RE. Nevertheless, there was a reduction of almost half in the primary traits (Indices 2 and 3,
Table 6). This scenario could be interesting for those farmers who transform their own milk, but they represent a minority within the ACRIFLOR association (in general, for the Spanish dairy goat breeds), and the majority of them do not even process their entire production.
A few years ago, there were proposals by the cheese sector to subsidize the protein yield and even its quality for cheese production, but the current circumstances of the market and the current strong competition for milk production between the different industries have left these aspects aside for now. Under the actual circumstances, and with a clear decreasing trend in milk production in the Florida breed, Scenario I will continue to be applied.
In the last scenario, the index including only the MY, FPY, and LS primary traits (index 1,
Table 7) allowed for greater responses. Indices 2 and 3 provided positive responses in the secondary traits, except for RE, although it cannot be guaranteed that this fact would be linked to maximum economic performance. Practically, all Florida farmers sell animals for life, including females that are not left for replacement, and also a significant number of reproductive males. This sale is an important part of the farm’s revenues since it can represent between 10 and 20% of the farm’s total incomes.
The theory of selection index has been used previously in the Florida breed to estimate direct and correlated genetic responses for classic female fertility traits [
2]. The highest genetic response was obtained when the interval between all kiddings was included in the index as a selection criterion. Nevertheless, given its low heritability and late expression in the female’s life, it cannot be recommended as early selection criteria. As an alternative, reproductive efficiency has been proposed, which has demonstrated a high h
2, in addition to being easily evaluated and understood by the breeders [
3].
In all the scenarios, the use of Index 1 led to the highest genetic responses in the primary traits, which will produce a faster improvement in the overall objectives than Indices 2 and 3. The inclusion of secondary traits in the selection index led to positive correlated responses in those traits but was accompanied by a decrease in the responses in the primary criteria. Here, special attention should be paid to the relationships between some of the traits and their economic weights, as any selection made with the aim of improving one particular group may adversely affect others.
The choice of suitable selection criteria is one of the most important decisions made by dairy goat breeders [
20] and varies between the different dairy goat systems [
54,
55]. In Spain, milk yield and cheese extract constitute the main source of revenues for dairy goat farms through the sale of milk or cheese, or both, in addition to subsidies granted by the government. In the particular case of the Florida breed, the main selection objective of its breeding program is milk production. Milk solids traits therefore showed higher economic values when compared to others, and Index 1 had the best results, as it included the highest weighting for these traits. Also, in the indices applied in other dairy breeds, a greater economic weight has been assigned to milk or its solids in dairy goats [
13] and in dairy cattle [
56]. Morphological traits are the second selection objectives after milk production in this breed. The MS is interesting for the facility and milking speed (in addition to the obvious relationship between the morphology of the udder and the amount of milk produced), and the possibility of installing automatic milking machines in the milking parlors, which is only useful if there is acceptable mammary conformation of the doe. This point is highly valued by farmers since it means saving many labour hours that would be used for milking. The BCI is related to good body capacity, especially abdominal capacity, and therefore a greater intake capacity of the animals, which will allow greater production, and especially the use of rations with a higher proportion of forages that are cheaper and much healthier for the animal. On the other hand, FS reflects the general dairy morphotype of the animal, closely related to the previous aspects since both weigh 60% in this final score. Furthermore, these morphological aspects are appreciated by dairy farmers, so they can ultimately be very interesting aspects for the issue of animals for sale, which will have a higher price. This last aspect is closely related to LS and RE which, if they increase, will allow farmers to have a greater number of young offspring for sale as breeding animals, a very important aspect for the ACRIFLOR dairy farms.
Nevertheless, apart from milk solids, morphology and reproductive aspects, there are other traits which have an economic impact on livestock production, such as udder health. Several authors have proposed the use of somatic cell count as an indicator of mastitis [
57,
58,
59]. The negative weight assigned to SCS can result in selection for animals resistant to mastitis, especially in highly productive breeds subjected to high productive stress. This trait is of utmost importance for the udder health in the Florida breed although farmers still do not give it the importance it has, since although they know that it is related to udder health, they generally do not yet have any subsidies or penalties in this regard in the price they receive for their milk, since there are no official limits in the EU for goat milk. In fact, Index 1 in Scenario III (
Table 6) and Indices 2 and 3 in all the scenarios (
Table 4,
Table 5,
Table 6 and
Table 7) showed a reduction in the genetic response of the somatic cell score, which means they could be suitable for the genetic improvement of resistance to mastitis in this breed.
Finally, selecting for productive life of a doe is very interesting, since with the current high feed prices and the costs of keeping females in the herd, the amortization cost per litre of milk produced can be of 15–25% of the total cost of production of a litre of milk. Furthermore, in this breed, a decrease in longevity has been observed, and the average longevity is a little more than two lactations.