Genetic Analysis of First Lactation Monthly Test Day Milk Yields, Peak Yield and 305 Days Milk Yield in Murrah Buffaloes

Dairy animals have traditionally been evaluated in breed improvement programmes based on 305 days milk yield records which require data recording on daily basis. However, daily record maintenance is time-taking, expensive and difficult under field conditions. Data recording at intervals instead of daily recording could be an alternative proposition. Studies conducted in the past revealed that genetic analysis of production traits in dairy cattle could be enriched by utilizing test day models instead of aggregated 305 days lactational milk yield record (Singh et al., 2016; Chakraborty et al., 2010; Singh and Rana, 2008; Lidauer et al., 2003). Test day model is a statistical model that (a) accounts for all genetic and environmental influences, (b) maximizes the amount of information for each animal leading to more accurate and intense selection, (c) evaluates the animal at an early stage leading to reduced generation interval and higher genetic gain per unit of time, and (d) reduces the time and cost of recording (Singh et al., 2016; Kaygisiz, 2013; Patil et al., 2012; Bilal et al., 2008; Jensen, 2001). In addition to test day models, peak yield is often utilized to have an idea about the production potential of an animal under field conditions. Peak yield is the highest milk yield that appeared on a day in the whole lactation. The usefulness of test day milk yields and peak yield depends on the estimation of genetic parameters like heritability of these records, and genetic and phenotypic correlations amongst these records and with FL305DMY. The estimation of genetic parameters plays an important role in determining the appropriate method of selection to predict direct and correlated response to selection, and formulating optimum breeding strategies for future genetic improvement of dairy animals (Singh et al., 2020; Jamuna et al., 2015; Godara et al., 2015; Pareek and Narang, 2014). Information on genetic parameters estimates of monthly test day milk yields and peak yield records of buffaloes is scanty to date, therefore, the present investigation was conducted to assess the genetic parameters for monthly test day milk yields, peak yield and first lactation 305 days milk yield in Murrah buffaloes.

Data
 
The data considered for the study comprised of 4,209 monthly test day milk yield records and 408 peak yield records on 408 Murrah buffaloes, sired by 62 bulls. The data were recorded from history-cum-pedigree sheets and daily milk yield registers of Murrah buffalo (1993 to 2017) maintained at ICAR- National Dairy Research Institute, Karnal, India. The records with lactation length less than 100 days, FL305DMY less than 900 kg, culled in the middle of lactation, still-birth, abortion, or any other pathological causes were considered as abnormal and therefore, such records were excluded from the study. A total of 11 monthly test day milk yield records were taken at an interval of 30 days. 6^th (TD-1), 35^th (TD-2), 65^th (TD-3), 95^th (TD-4), 125^th (TD-5), 155^th (TD-6), 185^th (TD-7), 215^th (TD-8), 245^th (TD-9), 275^th (TD-10) and 305^th (TD-11) days were considered for monthly test day milk records. Peak yield (PY) and first lactation 305 days or less milk yield (FL305DMY) were also recorded for each animal. The data was normalized by excluding outliers beyond three standard deviations on both the tail ends of normal distribution. For genetic parameters estimation, the records on the daughters of sires with a minimum of three progenies per sire were utilized for analysis. The non-genetic factors considered for the study were season and period of calving and age groups at first calving. Each year was divided into four seasons, viz. Winter (December - March); Summer (April - June); Rainy (July - September) and Autumn (October - November) based on the geo-climatic conditions used to prevail in the region. Period (1993 - 2017) was classified into eight groups, each of 3 consecutive years (except the last group which comprised of 4 years). Data was also classified into eight groups based on age at first calving utilizing Sturges’s (1926) formula.  
where, N = No. of observations
 
Statistical analysis
 
Mixed model analysis of data was carried out by least-squares maximum likelihood method (Harvey, 1990) to adjust the non-genetic factors on monthly test day milk yields, peak yield and FL305DMY records of Murrah buffaloes. The period, season and age at first calving (fixed effect) and sire (random effect) were considered as the non-genetic factors influencing the lactation traits.
 
The following model was used for 305 days milk yield:
 
                         Y_ijklm = μ + S_i + P_j + A_k + B_l + e_ijklm
 
where, Y_ijklm, FL305DMY of the m^th individual of l^th sire in k^th age group of j^th period and i^th season; μ, population mean; Si, fixed effect of i^th season (i=1 to 4); P_j, fixed effect of j^th period (j=1 to 8); A_k, fixed effect of k^th age group (k=1 to 8); B_l, random effect of l^th sire; e_ijklm, random error~NID (0,σ²e). For significant effects, the differences between pairs of levels of effects were tested by Duncan’s multiple range test as modified by Kramer (1957).
 
Estimation of heritability
 
Paternal half-sib correlation method (Becker, 1975) was used to estimate the heritability of different yields and their genetic correlations. The sires with three or more number of progenies were considered for the estimation of heritability. The data adjusted for significant effects of non-genetic factors was used for the estimation of heritability. The standard error of heritability was estimated as per the procedure given by Swiger et al., (1964).
 
Genetic and phenotypic correlations
 
The genetic and phenotypic correlations among different monthly test day milk yields, peak yield and FL305DMY were calculated from the analysis of variance and covariance among sire groups (Becker, 1975). The standard error of gentic and phenotypic correlations was computed according to Panse and Sukhatme (1967). The statistical significance of correlations was tested by ‘t’ test (Snedecor and Cochran, 1967).

First lactation 305 days or less milk yield (FL305DMY)
 
The effect of season of calving and age at first calving groups on FL305DMY was not significant whereas the effect of period of calving on FL305DMY was found to be significant (P≤0.05) (Table 1). Chakraborty et al., (2010) reported similar effect of season and period of calving and age at first calving groups on standard 305 days milk yield. The highest first lactation 305 days milk yield was observed in winter (2000±59 kg) followed by summer (1974±67 kg), rainy (1866±50 kg) and autumn season (1846±62 kg) (Table 1). The FL305DMY exhibited a declining trend from period I to period III and an increasing trend from period VI onwards (Table 1).
 

 
Heritability (h²)
 
The heritability estimated for monthly test day milk yields ranged from 0.22±0.15 (TD-1) to 0.51±0.19 (TD-9) (Table 2), which is in consonance with the results obtained by Geetha et al., (2007) and Madad et al., (2013) in Murrah and Iranian buffaloes, respectively. The heritability estimates for mid-lactation test day milk yields were found to be moderate. Similar findings have been reported by Tailor and Singh (2011), Bilal et al., (2008) and Lidauer et al., (2003). The heritability was observed as 0.33±0.16 and 0.35±0.17 for peak yield and first lactation 305 days or less milk yield, respectively (Table 2). Similar heritability estimates for FL305DMY were reported by Chitra et al., (2018), Sahoo et al., (2014) and Pareek and Narang (2014). However, Singh et al., (2016) reported comparatively lower estimates of heritability for FL305DMY (0.18±0.08). Pareek and Narang (2014) reported heritability estimates for peak yield as 0.48±0.17 in graded Murrah buffaloes. Low to moderate heritability estimation for these production traits is suggestive of the fact that these traits could be improved based on progeny performance and/or family selection coupled with better managemental practices (Godara et al., 2015; Tailor and Singh; 2011; Saha et al., 2010).
 

 
Genetic correlation
 
The genetic correlations among monthly test day milk yields, peak yield and with first lactation 305 days or less milk yield are presented in Table 2. The genetic correlations among monthly test day milk yields and first lactation 305 days milk yield were found to be positive and highly significant (P ≤ 0.01), except for TD-1. The genetic correlation of 305 days milk yield was found to be higher with TD-4 to TD-9 and TD-11. No consistent trend was observed for genetic correlations among monthly test day milk yields. The observed trends are in close confirmation with the results obtained by Singh et al., (2016), Tailor and Singh (2011) and Rekaya et al., (1999). The genetic correlation of peak yield with 305 days milk yield was 0.98±0.10. High genetic correlations between monthly test day milk yields (TD-4 to TD-9) and FL305DMY suggested that these test day milk yields could be exploited and considered for early prediction of overall production of the animal. The selection based on early test day milk yield records would reduce the cost of record maintenance substantially.
 
Phenotypic correlation
 
The phenotypic correlations among monthly test day milk yields, peak yield and with first lactation 305 days or less milk yield are presented in Table 2. The phenotypic correlation between peak yield and 305 days milk yield was 0.73±0.03. The phenotypic correlations between monthly test day milk yield and first lactation 305 days milk yield ranged from 0.38±0.04 (TD-1) to 0.79±0.03 (TD-7). The phenotypic correlations among different monthly test day milk yields ranged from 0.14 (TD-1 and TD-11) to 0.83 (TD-9 and TD-10). Singh et al., (2016) reported that the range of phenotypic correlation between test day milk yields varied from 0.23 (TD-1 and TD-10) to 0.79 (TD-6 and TD-7) in Murrah buffaloes. However, Chakraborty et al., (2010) reported that the phenotypic correlation between test day milk yields ranged between 0.05 (TD-1 and TD-7) and 0.65 (TD-9 and TD-10) in Murrah buffaloes. Similar trends were also reported by Tailor and Singh (2011), Geetha et al., (2007) and Lidauer et al., (2003). A decreasing trend was observed with respect to phenotypic correlation as the test day progressed. Singh et al., (2016) also reported that the value of phenotypic correlation among monthly test day milk yields showed a decreasing trend with late test day milk yields. The consecutive test day milk yield records showed higher phenotypic correlations throughout, which is in consonance with the results observed by Sangwan et al., (2015), Tailor and Singh (2011) and Geetha et al., (2007). The coefficients for mid-lactation test day milk yields were positive and significant with FL305DMY suggestive of the fact that the selection based on these test day records would indirectly be effective for FL305DMY. Similar results were observed by Sangwan et al., (2015), Kokate et al., (2013) and Tailor and Singh (2011).

Based on the study, it can be concluded that the heritability estimated for FL305DMY was moderate (0.35±0.17). In addition, the heritability estimates for monthly test day milk yields and peak yield were low to moderate which suggested that these traits could be improved through progeny testing and/or family selection coupled with better managemental practices. Peak yield and monthly test day milk yield (TD-4 to TD-9 and TD-11) showed high genetic and phenotypic correlations with FL305DMY. Therefore, these lactation yields offer ample scope to access the production potential in early stage of lactation and for the selection of high genetic merit animals.

The authors are thankful to the Director, ICAR-National Dairy Research Institute, Karnal for providing necessary facilities for conducting this study. The help rendered by Livestock Record Unit of ICAR-NDRI, Karnal is deeply acknowledged for providing the required data for the analysis.