Data source
A total of 821 lactation records on 340 Deoni cattle maintained at Livestock Research Centre, ICAR-National Dairy Research Institute, Southern Regional station, Bengaluru pertaining to the period between 2002 to 2017 (15 years) were used for the present study.
Data normalization
The milk performance traits like total lactation milk yield and its corresponding lactation length were studied. Data for milk yield traits were classified on basis of parity into six group 1
st to 5
th parity as one distinct group and greater than 5
th parity were pooled into a single group. Based on Season of calving data were classified into three groups of animals calved during rainy (July to October), summer (March to June) and winter season (November to February). Based on age at first calving, data were classified into three groups as animals calved within 40 months, between 40-50 months and exceeding 50 months. Animals having incomplete records due to abortion and still birth were excluded from the study. The data were standardized by taking the records of the performances within the range of two standard deviation from the mean value.
Least squares analysis technique
(Harvey, 1966) was used for non-orthogonal data to find the effect of parity, season of calving and age at first calving on lactation milk yield (LMY) and lactation length (LL) of the cattle as per the following formula.
Yij = μ + Fi + eij
Where,
Y
ij = Performance record (LMY and LL) of the j
th cow under i
th subclass of the non-genetic factor.
μ = Overall population mean.
F
i = ith subclass of the non-genetic factor.
i = 1, 2, 3, 4,5 and >5 for Parity.
i = 1, 2 and 3 for season of calving.
i = 1, 2 and 3 for age at first calving).
e
ij = Random error which is normally independently distributed.
Pairwise comparison of the least squares means was analysed by Duncan’s Multiple Range Test (DMRT), as modified by
Kramer (1957) and its significance were estimated by one-way ANOVA using SPSS 16.0 software.