Estimation of genetic parameters of weekly test-day milk yields and first lactation 305-day milk yield in Murrah buffaloes

Aim: The aim was to estimate genetic parameters of weekly test-day (TD) milk yields and first lactation 305-day milk yield (FL305DY) in Murrah buffaloes. Materials and Methods: The data on 39059 weekly test-day milk yield (WTDY) records during first lactation of 961 Murrah buffaloes calved from 1977 to 2012 and sired by 101 bulls maintained in an organized farm at National Dairy Research Institute, Karnal was analyzed to study the effect of genetic and non-genetic factors. Least squares maximum likelihood program was used to estimate genetic and non-genetic parameters affecting WTDY and FL305DY. Heritability was estimated using paternal half-sib correlation method. The genetic and phenotypic correlations among WTDY and 305day milk yield was calculated from the analysis of variance and covariance among sire groups. Results: The least squares means for FL305DY was found to be 1853.49±15.88 Kg. The least squares means of overall WTDY ranged from 2.44±0.07 kg (TD-43) to 7.95±0.06 kg (TD-8). Effect of period, season and age at first calving groups was found to be highly significant (p<0.01), significant (p<0.05) and non-significant on FL305DY, respectively. The h2 estimate of FL305DY was 0.25±0.09. The estimates of phenotypic and genetic correlations between 305-day milk yield and different WTDY ranged from 0.52 to 0.84 and from 0.19 to 0.98, respectively. Conclusions: Our study showed that the effect of period of calving was highly significant (p<0.01) on FL305DY as well as all the WTDY. The estimates of phenotypic and genetic correlations were generally higher in the middle segment of lactation suggesting that these TD yields could be used as the selection criteria for early evaluation and selection of animals.


Introduction
Today in many countries of different continents, test-day (TD) milk yield records are employed in genetic evaluations instead of 305-day total lactation milk yield of animals.India has about 51 million milch buffaloes [1] contributing about 51% [2] of the total milk produced in the country.In order to find out an alternative to daily milk yield recording, which is a costly and time consuming proposition under field conditions, some studies have been made in the past in buffaloes on TD milk yields [3,4].Various advantages of using TD milk yield records are individual test date effects and the number of records per animal as well as the interval between records can be accounted for reducing the generation interval, better adjustment of non-genetic factors influencing the milk yield leading to more accurate genetic evaluation.
Murrah is the most important buffalo breed with superior genetic potential for milk production.Although TD milk yield records offer greater advantage compared to 305-day milk yield in selection schemes, information on estimation of genetic parameters based on TD records particularly, weekly records are limited.
The present investigation was undertaken with the objective to study the influence of various non-genetic and genetic factors on weekly TD and first lactation 305 day milk yields (FL305DY) in Murrah buffalo.

Data
A total of 39059 weekly TD milk yield (WTDY) records during first lactation of 961 Murrah buffaloes calved during 1977-2012 at the National Dairy Research Institute, Karnal were collected from the history-cum-pedigree sheets and daily milk yield recording registers.The traits considered were WTDY and FL305DY.Culling in the middle of lactation, abortion, still-birth or any other pathological causes affecting the lactation yield were considered as abnormalities and thus, such records were not taken for the study.Records of buffaloes with <800 kg of milk production and <100 days lactation length were not considered in the present investigation.To ensure the normal distribution, the outliers were removed, and data within the range of mean ± 3 standard deviation were only considered for the study.The data were analyzed to study the effect of non-genetic factors (season, period and age at first calving [AFC] groups) on 43 WTDY records (from 6 th day, 13 th , 20 th , and 300 th day of lactation) and FL305DY records.The data were classified into different seasons, periods and AFC groups.Each year was divided into 4 seasons on the basis of rainfall, temperature and humidity over the years -winter (December -March); summer (April -June); rainy (July-September); and autumn (October -November).The data spread over 36 years  were classified into 12 periods.The data was classified into 5 AFC sub-groups (<1120, 1120-1280, 1281-1440, 1441-1600 and >1600 days) using Sturges formula [5].

Statistical methods
Least squares maximum likelihood program of Harvey [6] was used to estimate and study the effect of genetic and non-genetic factors on WTDY and FL305DY records of Murrah buffaloes: Where, Y ijklm = Observation on the m th individual in i th season, j th period, k th AFC group and sired by l th bull.μ=Overall population mean S i =Effect of i th season P j =Effect of j th period A k =Effect of k th AFC group B l =Effect of l th bull (Sire) e ijklm =Random error, NID (0, σ 2 e ) Duncan's multiple range test as modified by Kramer [7] was used for testing differences among least squares means.

Estimation of heritability
Paternal half-sib correlation method [8] was used to estimate the heritability of different characters and their genetic correlations.The sires with five or more no of progeny were included for the estimation of heritability.The data adjusted for significant effects of non-genetic factors was used for estimation of heritability.The standard error of heritability was estimated as per Swiger et al. [9].

Genetic and phenotypic correlations
The genetic and phenotypic correlations among WTDY and 305-day milk yield was calculated from the analysis of variance and covariance among sire groups as given by Becker [8].The standard error of genetic and phenotypic correlations was obtained according to formula given by Panse and Sukhatme [10].

Results and Discussion
The least squares mean along with their standard errors for WTDY and FL305DY are presented in Table-1.The highest WTDY was observed in WTDY-8 (7.95 kg) and the lowest was observed in WTDY-43 (2.44 kg).In general, WTDY increased till WTDY-8 and thereafter a gradual decline was noticed till the end of lactation.The average 305-day or less milk yield was 1853.49±15.88kg.Chakraborty et al. [3] also observed that the average 305-day milk yield was 1818.06±22.46kg in Murrah buffalo.However, Patil et al. [11] reported the average 305-day milk yield being 1750.91±28.62kg in Murrah buffalo.

Season
The effect of season of calving was highly significant (p<0.01) up to WTDY-12; significant (p<0.05) for FL305DY, WTDY-13, WTDY-14 and WTDY-15.Non-significant effect of season of calving was observed afterwards (Table-2).Kumar et al. [12] reported significant effect and Chakraborty et al. [3] reported non-significant effect season of calving on monthly TD milk yields in Murrah buffaloes.

Period
Highly significant effect (p<0.01) of the period of calving was observed for FL305DY as well as different weekly TD yields (Table-2).Rana [13], Kumar et al. [12] and Chakraborty et al. [3] also reported highly significant effect of period of calving on the monthly TD milk yields in Murrah buffaloes.

AFC groups
The effect of AFC groups on WTDY presented in Table-2.The effect was significant up to the middle of the lactation after which the effect was non-significant.Non-significant effect of AFC groups was also observed on FL305DY.Rana [13] reported the effect of AFC on monthly TD milk yields to be significant whereas Katneni [14] observed that the effect was not significant on all weekly TD yields except WTDY-43 in first lactation of Murrah buffaloes.

Genetic and phenotypic correlations
The estimates of genetic and phenotypic correlations among 305-day milk yield and different WTDY are presented in Table-4.The phenotypic correlations between 305-day milk yield and WTDY were highly significant (p<0.01).The estimates of genetic and phenotypic correlations between 305-day milk yield and WTDY ranged from 0.19 to 0.98 and from 0.52 to 0.84, respectively and the estimates were higher in the middle segment of lactation.Rana [13] reported that among monthly TD milk yields and 305-day milk yield, the genetic correlations were close to unity and phenotypic correlations were quite high and positive (0.60-0.99) in Murrah buffaloes.Chakraborty et al. [3] reported that among monthly TD milk yields and 305day milk yield, the genetic correlations ranged from 0.11 to 0.89 and corresponding phenotypic correlations ranged from −0.05 to −0.65 in Murrah buffaloes.

Conclusions
Our study showed that the effect of period of calving was highly significant (p<0.01) on FL305DY as well as all the WTDY.Season and AFC groups have significant effect up to middle of lactation.The h² estimate of FL305DY was 0.25±0.09and it ranged from 0.02±0.07(WTDY-43) to 0.30±0.1 (WTDY-18) for weekly TD yields.The estimates of phenotypic and genetic correlations of weekly TD yields with 305-day milk yield was generally higher in the middle segment of lactation suggesting that these TD yields could be used as the selection criteria for early evaluation and selection of animals.

Author's Contribution
AS, AKG and AKC designed the experiment.MS and GSA assisted SKS in data recording and literature collection.SKS did the data analysis and prepared the manuscript.All authors read and approved the final manuscript.

Table 1 :
Least squares means of FL305DY and different WTDY (in kg).

Table 2 :
Mixed model ANOVA showing mean sum of squares for factors affecting WTDY and FL305DY.

Table - 3
: Heritability estimates along with their standard error for FL305DY and different WTDY.Genetic and phenotypic correlations among FL305DY and different WTDY.