الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
A total of 632 first lactation records for British Friesian cows which belonging to the dairy unit of the milk and meat project, Faculty of Agriculture, Alexandria University. The records were collected between 1985 and 2007.
Traits under study:
The productive traits studied were total milk yild (TMY), 305day milk yield (305DMY), lactation length (LL), daily peak yield (PY),dry period (DP) and persistency (P). The reproductive traits studied were first service period (FSP), age at first calving (AFC) and days open (DO). The environmental effects such as effect of season of calving, year of calving and age at first calving on each trait were studied.
The results obtained could be summarized in the following:
1. Season of calving effects: It was observed that cows calving in winter gave the highest total milk yield (5037.2±305.9 kg). whereas, those calving in autumn gave the lowest one (4773.2±322.2 kg). but, during autumn, they had the highest 305 day milk yield (4450.70±219.0 kg) and during spring, they had the lowest one (4246.60±205.5 kg). Summer calvers cows had the longest LL (371.90±22.5 d). While, autumn calvers recorded the shortest LL (345.90±23.1 d). PY ranged from (19.03±0.30 kg) for summer calvers to (20.65±0.29 kg) for winter calvers. The longest DP was obtained in winter calvers being (70.8±7.19 d) and the shortest one was obtained for autumn calvers being (67.3±7.70 d). The highest P was obtained in summer (217.30±3.63) and the lowest was obtained in spring (203.77±3.60). In generally, season of calving had no significant effect on total milk yield, 305day milk yield, dry period and persistency. While, lactation length and peak yield were significantly affected by season of calving.
The longest first service period was found in spring (98.03±13.5 d) and the shortest one was in autumn (86.80±14.5 d). The highest age at first calving was in autumn (30.82±0.26 mo) and the lowest one was in summer (29.44±0.27 mo). The days open was ranged from (127.9±24.00 d) in autumn to (166.9 d) in winter. The effect of season of calving on first service period was non-significant. But, it had significant effect on both age at first calving and days open.
2. Year of calving effects: Least square means and standard errors of productive traits in different years of calving showed that the highest average of total milk yield was during the year 2004 being (7491.10±856.90 kg) and the lowest one was during the year 1987 being (3033.66±1059.80 kg). The 305days milk yield ranged from (2776.30±720.30 kg) in year 1987 to (6241.60±582.40 kg) in year 2004. The lactation length was the highest (470.3±58.60 d) in year 2001 and the lowest (218.6±58.56 d) was in year 1989. The daily peak milk yield was ranged from (15.90±0.69 kg) in the year 1987 to (25.40±0.70 kg) in the year 2005. The dry period was the longest (119.30±25.8 d) in year 1986 and the lowest one (50.60±22.8 d) in year 2007. The persistency was ranged from (169.10±9.60) in the year 1997 to (277.60±8.50) in the year 2000. The year of calving had a highly significant effect on total milk yield, 305day milk yield , peak yield and persistency. While, lactation length and dry period weren’t affected by the year of calving.
The shortest first service period was found in year 1994 (30.50±28.24 d) and the longest period was in year 2003 (172.50±36.90 d). The highest age at first calving (35.88±0.65 mo) was in year 2000 and the lowest (25.14±0.60 mo) was in year 2006. The days open ranged from (53.45±59.50 d) in the year 1989 to (230.90±88.26 d) in the year 2005. The year of calving had significant effect on first service period and highly significant effect on age at first calving. But, days open wasn’t affected significantly by the year of calving.
3. Effect of age at first calving: The cows calved at >36 months gave the highest total milk yield (5026.22±338.60 kg) but those calved between 32-36 months had the lowest (4711.45±310.87 kg). 305day milk yield ranged from (4206.6±217.0 kg) when the cows calved at 24-28 months to (4587.9±230.0 kg) for those calved at >36 months. The lactation length was ranged from (352.6±24.26 d) for the cows calved at >36 to (380.7±22.90 d) for those calved at 24-28 months. The daily peak milk yield was increased with increasing of age at first calving as (19.10±0.30 kg) recorded in an age at first calving of 24-28 months and (20.80±0.52 kg) in an age at first calving of >36 months. The dry period was in a range from (63.00±7.50 d) at age at first calving 24-28 months to (72.37±7.30 d) at age at first calving 32-36 months. The persistency was as high as (216.1±7.40) for age at first calving below 24 months and was as low as (205.4±3.90) for age at first calving 32-36 months. Age at first calving had a highly significant effect on peak yield. Also, it had a significant effect on 305day milk yield and dry period and it had no significant effect on total milk yield and lactation length.
Least square means and standard errors of reproductive traits in different ages at first calving showed that the first service period and days open weren’t affected by age at first calving. The cows of first calving at <24 months age had the shortest first service period (85.60±16.63 d) and the longest one was (97.57±14.30 d) for those calved at 24-28 months age. The cows of first calving at >36 months had the shortest days open (138.65±24.90 d) and those calved at 24-28 months had the longest one (164.63±23.65 d).
Genetic parameters:
1. Heritability estimates (h²): The heritability of total milk yield was (0.37). For 305day milk yield, it was (0.06). Heritability estimate of lactation length was (0.38). The estimates of heritability of daily peak yield, dry period and persistency were (0.32), (0.06), (0.06), respectively. The heritability estimates of first service period, age at first calving and days open were (0.06), (0.06) and (0.42), respectively.
2. Genetic correlation: The genetic correlations among total milk yield, lactation length, age at first calving and days open were positive and most of them were high. These results indicated that genetic correlations between age at first calving and each of total milk yield, lactation length and days open were (0.69), (0.83) and (0.87), respectively. These results indicated that increasing age at first calving of cows will be associated with an increase in total milk yield and lactation length. Also, genetic correlations of days open with each of total milk yield and lactation length were (0.23) and (0.44), respectively. Genetic correlation between total milk yield with lactation length was (0.98).
The present results indicated that selection for high milk yield will be associated with genetic improvement in lactation length.