Introduction

Goats are relevant worldwide, especially in developing countries, where they are bred with multifunction criteria¹. However, dairy goat breeds are especially important in the southern European livestock, and even though their census is low compared to the total global population, France and Spain have achieved considerable yield increases and more extended lactation periods. This productive efficiency responds to the need to provide large quantities of milk to the high-quality goat-cheese industry, linked to deep-rooted cultural traditions in some European countries, which continue to offer an optimistic perspective for the goat sector².

The Murciano-Granadina (MG) breed is part of a select group of specialized dairy goats, becoming one of the leading dairy breeds of Spain, both in census (104,000 breeding females included in the Breed Registry) and yields (584.4 kg of milk per lactation)³. This information positions the MG breed in the first order, next to the Florida and Malagueña breeds, due to its high potential yield compared to other native Spanish breeds like the Guadarrama, Majorera, Palmera, Payoya, and Tinerfeña goats.

The MG breed was officially recognized in the seventies, and since then, there are references on the Scheme for the Genetic-Functional Assessment of Reproductive Males (Resolution of March 28, 1979, of the Directorate General for Agricultural Products⁴. The current animal genealogical and functional records integrate a robust database, which allows breeders to perform genetic evaluations.

The current MG selection program has been consolidated after 29 yr of selection, emphasizing traits like milk yield, fat, protein, and dry extract yield. In this case, the estimation of genetic trends (TG) is important to evaluate the efficacy of the applied breeding schemes and to provide the breeders with information that will allow them to develop more efficient selection programs⁵.

Also, the evaluation of TG and phenotypic trends (TF) improves the understanding and helps transmit the selection effect compared to previous generations^6,7, as well as to contrast the obtained results depending on the proposed scheme, allowing to correct any deviation from the expected.

In reference studies performed in dairy species, researchers have generally focused on the TG and TF for milk yield (RL) and composition traits; however, the genetic behavior of other traits related to the lactation curve has not been reported ⁸.

Furthermore, this is the first study of the TGs for lactation curve parameters, peak yield (PP) and persistency (P) in particular, in MG goats. This study aims to justify the consolidation of these traits for their inclusion as selection criteria in the MG breeding program. Therefore, this study aimed to evaluate the TG and TF for the PP, RL, and P traits in the Murciano-Granadina (MG) breed.

Material and methods

Results

The descriptive statistics for the analyzed traits revealed a typical mean and deviation of 1.05 ± 0.32 and 1.21 ± 0.35 kg of milk for RL and PP, respectively, and 1.03 ± 0.35 for P.

Figures 1-6 show the TG and TF regarding the year of birth for the analyzed traits with their respective linear regression coefficient (b). These results suggest dynamic variations for the MG goat population, detecting a rebound stationary point in 1999, which coincides with the start of the modern selection scheme based on the BLUP evaluations. Subsequently, oscillations around the X-axis for these three traits reveal that for 22 year, the PP and RL traits showed an upward trend behavior and similar magnitudes; meanwhile, the P trait showed a stationary to a downward trend.

Figure 1 Peak yield genetic trend 

Figure 2 Yield genetic trend 

Figure 3 Persistency genetic trait 

Figure 4 Peak yield phenotypic trend 

Figure 5 Yield phenotypic trend 

Figure 6 Persistency phenotypic trend 

The TGs for PP and RL over the analyzed years (1990 to 2012) showed an irregular behavior with increasing and decreasing intervals (Figures 1 and 2) but following an upward trend line with coincident peaks in the years 1992, 1996, 2004, and 2009. For these same traits, the TFs (Figures 4 and 5), also show upward trend lines with regular and consistent fluctuations in the increasing/decreasing intervals (coincident prominent peaks in the years of 1992 and 1998).

The TG for the P trait shows an irregular behavior with increases, decreases, and prominent peaks in the years of 1993, 1995, 2005, 2010, and 2012, unlike the behavior of the other traits that show a stationary trend line (Figure 3).

Particularly, we observed a positive TG for PP and RL and irregular deviations from linearity (Figure 1 and 2). The increase for these treats was significant (P<0.05) with a b of 0.00071 kg/year and 0.00114 kg/year for PP and RL, respectively. Similarly, the TG for P was also positive with prominent peaks (1993, 1995, 2005, 2010, and 2012) and notorious decreases (1992, 1998, 2006 to 2009), showing irregular fluctuation between them with a no-significant increase (P>0.05) and a b of 0.0000219 units/year with a stationary trend line (Figure 3).

Although the TF direction for PP and RL shows an upward trend line, the fluctuations of the increasing/decreasing intervals are also regular and consistent due to the proximity between the points (prominent coincident peaks in 1992 and 1998) and the trend line (Figures 4 and 5). Moreover, the TF for P (Figure 6) decreased when the fluctuations of the increasing and decreasing intervals were regular and consistent in time, with peaks in the years of 1991, 1993, 2002, and 2012.

Consequently, the TFs were positive for PP and RL showing a significant (P<0.05) increase in the TF average for these traits, where b was 0.0069821 kg/year and 0.0111697 kg/year, respectively (Figures 4 and 5). However, although the TF for P was negative with regular fluctuations and a significant (P<0.05) decrease with a b value of -0.0007629 units/year (Figure 6), there is also a slight increase in this trend since the year of 2009, which stabilizes during 2010, with an increase in 2011. The TA analysis for P suggests a negative influence, although not significant (P>0.05 with b of -0.00000293 units/year).

Discussion

The TGs monitor and evaluate the selection program efficiency¹⁶. The TG evaluation of dairy traits indicates the direction of the breed selection vector, as well as the rate of genetic improvement due to the application of the breeding program¹⁷. It is also essential to provide information to the breeders for them to develop more efficient selection programs⁵.

In this study, the direct estimation of the TG indicated that there was a significant and positive genetic improvement in all the studied traits, except for the P trait, which indicates that selection was effective, both in the period based in bulk selection (1990-1999) and in familiar information (1999-2012). Therefore, this information could be useful to evaluate previous efforts for the improvement and preservation of the genetic potential of these traits and to determine the future strategies and work in the population of MG goats.

It is important to consider that this is the first study in which the PP, RL, and P traits are evaluated as a group, RL being a trait routinely used for breed selection, while PP and P are candidate traits for possible inclusion in the selection scheme. Our results show that, in the 22 analyzed years, the PP and RL traits showed a similar and upward trend behavior, while the P trait showed a stationary to a downward trend, which supports the idea that high yields are detrimental to the P.

The results show that both the TG and TF for the three traits (PP, RL, and P) have an irregular behavior with fluctuating increasing and decreasing intervals, most noticeable in the years before 1999; until that year, the selection was based on the phenotype (bulk selection). However, since 1999, when the annual mean VG trends for the PP and RL traits present a more constant-positive evolution, consistent with the year when the work guidelines for the selection scheme of dairy goat stallions of the MG breed were established⁴, based on family information and BLUP estimations, according to the guidelines published in the Resolution of May 12, 1999, of the Ministry of Agriculture, Fisheries, and Food.

Additionally, the reference points of the mean VGs are closer to the trend line, with prominent peaks in the years of 2004 and 2009, and noticeable decreases in the years of 2005, 2006, 2011, and 2012; according to MURCIGRAN ⁴, they could be related to the incorporation of animals to the selection schemes from livestock farms outside the selective nucleus in the mentioned years.

The TGs for PP and RL are parallel, prominent peaks are observed in years before 1999; the most noticeable decrease occurred in 2012. In this regard, in December 2011 it was officially agreed to merge the associations of CAPRIGRAN and ACRIMUR in MURCIGRAN⁴, leading to the management of a single Breed Book, and therefore the data and records thereof, which would justify the noticeable decreases between 2011 and 2012 in both the TG and TF.

Even though the TG for P has prominent peaks (1993, 1995, 2005, 2010, and 2012) and notorious decreases (1992, 1998, 2006 to 2009) in addition to alternating and irregular between them (sic), it shows a positive seasonal trend line. Meanwhile, the TF is a discrete decreasing trend line with less prominent peaks in the years 1991, 1993, 2002, and 2012.

This information supports the idea of paying attention to the P trait and its group behavior with others because of its direct impact on the lactation curves. Lengthening the P would allow for flattening the declining part of the lactation curve, or at least mitigate the crucial peaks that affect the immune system (the antagonistic relationship between milk production and disease resistance traits) promoting a more efficient lactation with considerable benefits, such as avoiding animal health risks and associated costs^18,19.

In this regard, specific studies in dairy goats conclude that animal selection using P as a criterium and linked to the peak yield value is possible without changing the amount of total milk²⁰, and therefore genetically modify the lactation curve¹⁹. Thus, early estimation of P in lactations in progress can represent a useful tool for both the breeding and management strategies²¹, especially in the implementation of strategies to reduce the production of greenhouse gases derived from the fight for climate change mitigation²².

The TG estimations evaluate the results of the program and inform the breeders about the selection decisions made, allowing the necessary adjustments to optimize the genetic progress of each population²³; therefore, the results of this evaluation allow to propose the incorporation on the P trait to the breeding program.

Finally, in general, the phenotypes and the breeding values for the PP and RL traits increased in the period of study; however, although the P trait was genetically stationary, its TF was decreasing, probably due to environmental factors, such as the change of management. Studies referred to in this regard indicate that the environment does not directly modify the genetic constitution of the individual. However, it does determine the extent of its expression and the genetic potential of animals, which will express to the extent that environmental conditions allow²⁴.

Conclusions and implications

The global analysis of the TG and TF for the analyzed traits demonstrated irregular dynamic variations with increasing and decreasing intervals, as well as peaks in determined points throughout the study. The genetic and phenotypic behaviors of these three traits indicate that, in 22 years, the PP and RL traits showed a similar upward trend and magnitude, while the P trait was stationary to decreasing. The latter supports the idea that high yields are detrimental to the P. In all cases, important events in the breeding program, like the federation of associations or the introduction of family selections, left their mark on trends. These findings will allow to inform breeders about the behavior of these traits and consider incorporating the P in the selection scheme.