Study sites with geographical location and climate: The study was conducted from four different sites/herds located at Anwara (22010' to 22014' N and 91052' to 91056’ E) and Chandanaish ( 22012' to 22014' N and 9200' to 92006' E) Upazila in Chittagong district and Bangladesh Agricultural University (BAU) nucleus herd ( 24030' to 25010' N and 90015' to 9105' E) and Char Jailkhan community herd ( 24077' to 24078' N and 90039' to 90041' E) in Mymensingh district having a pronounced tropical monsoon-type climate has warm temperatures throughout the year, with a hot and rainy summer and a dry winter with relatively little variation from month to month. January tends to be the coolest month with temperatures averaging near 260C (780F) and April/May the warmest with temperatures from 33 to 360C (91 to 960F). The climate is one of the wettest in the world. Most places receive more than 1,525 mm of rain a year, and areas near the hills receive 5,080 mm. Most rains occur during the monsoon (June-September) and little in winter (November-February) (Anonymous, 2012).
Management of animals in the herds: The feeding and management of RCC by small holders was semi-intensive. The animals were grazed most of the time in a day (6-8 hrs/day). So, land side grasses are the basal diet of the animals. Rice straws were the second main basal diets of the animals specially during cropping seasons. Most of the farmers provided rice bran and common salt to their animals fed with drinking water daily. Housing system provided to the animals were traditional in a shabby house made of bamboo with thatched roofs. The feeding and management of RCC at BAU Nucleus herd was intensive. The animals were housed in a paddock with a faced out open house system. The animals of the Nucleus Herd were stall fed throughout the year. The animals were provided concentrate, green grass and straw where straw was the basal diet supplemented with urea and/molasses. The animals were allowed to graze due to lack of facility. Some times, the animals were given a mixture of molasses and straw only twice a day adlibitum throughout the year. Green forages like German grass, Sorghum grass, and Maize fodder were provided with limited amo unt due to scarcity of their availability round the year. Concentrate mixture was supplied once a day in the morning at the rate of 600g/lactating cow, 500g/pregnant cow and 250g/dry cow and heifer. The pregnant cows were transferred to a separate house a few days prior to calving and returned to cow barn few days after calving. The calves were allowed to suckle their dam for few hours after milking and again few hours before evening and it continued up to 3-4 months. Afterwards, calves were allowed to suckle once a day after milking until weaning. In the herds of Chittagong district of Bangladesh, farmers seldom vaccinate d and deworm ed their animals. In the BAU nucleus and community herd in Mymensingh, the animals were dewormed and vaccinated (against FMD a nd Anthrax disease) at regular interval for close monitoring .
Animals and data preparation: The data consisted of records of 101 animals included from two generations raised in the above mentioned herds collected from 2005 to 2011.
After collecting the necessary data for analysis, preliminary editing was performed to exclude abnormal or biased data. Then all data were plotted in a normal distribution curve and those fell beyond the normal curve were eliminated. About 2 to 10 % records were rejected as they fell outside the range of normal distribution fitted for statistical models.
Data analysis and statistical model: Animals were arranged in contemporary groups of calving parity, herd, year and season. The general linear model (GLM) procedure of SPSS 11.5 was used to test the main fixed effects as well as interactions effects .