Abstract: The present research was carried out to determine milk yield traits and effects of environmental factors on these traits in Holstein cows raised at Polatli State Farm located in Ankara province. A total of 3011 production records between 1993 and 2006 were constituted the research material. Environmental factors were analyzed by Least Squares Method. Effects of parity (lactation number) on 305 days milk yield, lactation duration and dry period were significant (p<0.001-p<0.05). Effect of calving year on 305 days milk yield was significant at the p<0.001 level, nevertheless, not significant on both lactation length and dry period. Effect of calving year was significant statisticaly for all milk yield traits (p<0.001). Effect of calving age on 305 days milk yield and dry period were significant (p<0.001), nevertheless, not significant for lactation length. The least square means of 305 dayss milk yield, lactation duration and dry period were 5606.92±75.49 kg, 303.40±3.04 days and 98.15±2.25 days, respectively. Under Turkish conditions and steppe climate of middle Anatolia, it can be concluded that Holstein cattle are raised successfully for milk yield on Polatli state farm.

INTRODUCTION

Holstein cows were considered the most dairy cows in World. As number of Holstein cows in Turkey is more than other breeds, breeding of Hostein cows is very impotant in Turkey. In breeding of dairy cows, the most important aims are to obtain a calf in a year and high milk yield from cows. To obtain a calf in a year from cows dependents on some parameters in ideal limits (60 days dry period, 305 days lactation duration etc.). For these aims, good managment in herd of dairy cows is very important. Profitable breeding could be achieved by keeping lactation duration, dry period and service period between optimal limits (Alpan, 1994; Kumuk et al., 1999; Çilek and Takin, 2005; Koçak et al., 2007).

The yields of farm animals are the result of the combined effects of genotype and environmental conditions. In order to increase the yield level, it is necessary to optimise the environmental conditions and to improve the genetic structure of the animals. Environmental factors can be classified as factors with measurable effects (age, year, season, milking frequency, etc.) and factors with unmeasurable effects (infectious diseases, parasitic infestations, etc.). The measurable effects can be determined and used in the management of the farm (Yalçin, 1975; Yaylak and Kumlu, 2005; Çilek and Tekin, 2007b).

The 305 days milk yield of Holstein cows was between 4597 and 6464 kg in different studies in Turkey (Duru and Tuncel, 2002, 2004; Kaya et al., 2003; Bilgiç and Aliç, 2005; Erdem et al., 2007; Inci et al., 2007). Milk yields of Holstein cows were 5353 kg in Morocco (Boujenane, 2002) and 5905 kg in Tunisia (Ajili et al., 2007).

Many researchers reported that the effect of calving season on 305 days milk yield was as significant and indicated that milk yield was higher in autumn and winter (Kaya and Kaya, 2003; Yaylak and Kumlu, 2005; Çilek and Tekin, 2007; Erdem et al., 2007; Koçak et al., 2007). However, some authors reported that effect of calving season on 305 days milk yield was non-significant in commercial farm c onditions (Pelister et al., 2000b; Bilgiç and Aliç, 2005).

Although, effect of lactation number on 305 days milk yield was reported as significant (Özbeyaz and Küçük, 1999; Özçelik and Arpacik, 2000; Erdem et al., 2007; Inci et al., 20007). Some authors reported effect of lactation number on 305 days milk yield as non-significant (Bilgiç and Aliç, 2005; Koçak et al., 2007). Effects of calving age on 305 days milk yield have been reported as significant (Özbeyaz and Küçük, 1999; Pelister et al., 2000a; Çilek, 2002).

The lactation duration of Holstein cows was between 284.7 and 333.9 days in previous studies (Pelister et al., 2000a; Duru and Tuncel, 2002, 2004; Kaya et al., 2003; Bilgiç and Aliç 2005; Erdem et al., 2007; Inci et al., 2007). The effect of calving season on lactation duration was reported as non-significant (Pelister et al., 2000a; Bilgiç and Aliç, 2005; Koçak et al., 2007). The effect of lactation number on lactation duration was reported as non-significant (Özçelik and Arpacik, 2000; Bilgiç and Aliç, 2005; Erdem et al., 2007). However, the effect of calving age on lactation duration was reported as significant (Pelister et al., 2000a; Inci et al., 2007). Zambrano et al. (2006) reported that the effect of calving year on lactation duration was significant.

The dry period of Holstein cows was between 73.34 and 82.1 days in previous studies (Pelister et al., 2000a; Kaya et al., 2003; Erdem et al., 2007; Inci et al., 2007). The effect of calving season on dry period was reported as non-significant (Pelister et al., 2000a; Koçak et al., 2007). However, the effect of calving age on lactation duration was reported as significant (Erdem et al., 2007). The effect of lactation number on dry period was reported as non-significant (Erdem et al., 2007; Inci et al., 2007). However, the effect of calving age on lactation duration was reported as significant (Özçelik and Arpacik, 2000). Effects of calving age on dry period have been reported as significant (Pelister et al., 2000a).

Many researchers were found that effect of calving year on all milk yield traits (305 days milk yield, lactation duration, dry period) was significant (Pelister et al., 2000a; Bilgiç and Aliç, 2005; Zambarano et al., 2006; Erdem et al., 2007). Inci et al. (2007) reported that effects of calving year on 305 days milk yield and lactation duration were significant but non-significant on dry period.

This study was conducted to investigate the environmental factors affecting milk yield traits of Holstein cows raised betwen 1993 and 2006 at the Polatli state farm.

MATERIALS AND METHODS

At the Polatli state farm located in Ankara province, a total of 3011 milk yield records of Holstein cows raised between 1993 and 2006 were used. Seven age groups were formed beginning from 2 years and ending at 8 years and older for calving age; 4 groups for calving season (Spring, Summer, Autumn and Winter), 5 groups for parity (lactation number) and 14 groups for calving year, between 1993 and 2006. The 305 dayss milk yield was estimated from test milk yields collected once a month during all lactation periods using the Holland method (Çilek, 2002; Çilek and Tekin, 2006a, b). Lactations with less than 7 tests were not used in calculation. Milk yields were standardized to 305 days by using adjustment factors estimated by Çilek (2008) for this herd. Environmental factors, which influenced lactation milk yield, lactation duration and dry period were investigated. The General Linear Model (GLM) was used for variance analyses of milk yield traits. Duncan’s multiple range test was used for multiple comparisons.

The statistical model was:

RESULTS AND DISCUSSION

Least square means for milk yield traits are presented in Table 1. Effects of all factors (calving year, calving age, parity and calving season) on 305 days milk yield were significant (p<0.001). The average 305 dayss milk yield was estimated as 5606.92±75.49 kg; 305 days milk yields were lowest in 1995 at 4216 kg and highest in 2004 at 7280 kg. The highest level in cows calving in winter was 5890 kg and the lowest level in cattle calving in summer was 5506 kg. Total 305 days milk yields were lowest in 2 years of age at 5278 kg and highest in 7 years of age at 5961 kg.

Average lactation duration was estimated as 303.40±3.04 days. The effects of parity and calving year on lactation duration were statistically significant (p<0.001-p<0.05). However, the effect of calving age and calving season were non-significant (p>0.05). Lactation duration was shortest in 2006 at 252.9 days and longest in 2004 at 363.2 days. Lactation duration was longest in 1st lactation at 315.3 days and shortest in 5th lactation at 285.1 days. Lactation duration decreased with increase of lactation number.

Average dry period was estimated as 98.15±2.25 days. Effects of parity, calving age and calving year on dry period were statistically significant (p<0.001), but the effect of calving season was non-significant (p>0.05). The dry period was lowest in 1998 at 75.46 days and highest in D1999 at 141.06 days. Dry period increased with increase of calving age. The lowest dry period was found in 2 years old age at 84.48 days and highest in 7 years old age at 101.97 days. Dry period was longest in 3rd lactation at 85.69 days and shortest in 5th lactation at 127.95 days.

Table 1:	Least square means of milk yield traits
*p<0.05; **p<0.01; ***p<0.001; ns: non-significant; a, b, c: Means without a common superscript within each variable and each factor differ (p<0.05)

In this study, means of 305 days milk yield was 5606.92 kg. This value is agreement with values in other countries (Boujenane, 2002; Ajili et al., 2007) and in Turkey (Duru and Tuncel, 2002, 2004; Kaya et al., 2003; Bilgiç and Aliç, 2005; Erdem et al., 2007; Inci et al., 2007). As reported previously (Çilek, 2002; Çilek and Tekin, 2005), the effect of calving season on milk yield was significant and milk yield was the highest in cows calving in winter. Cows calving in winter have high milk yields, due probably to good feeding levels in the first 3 or 4 months of lactation and increase milk yield due to feed containing alfalfa being given during the period when milk yield begins to decrease. On the other hand, cows calving in summer have low milk yields due to their being subject to high environmental temperatures in the first 3 or 4 months of lactation. The lowest milk yield was obtained from cows calving at 2 years of age and the highest from those calving at 7 years of age. Milk yield decreased after 7 years of age. As reported in the literature (Çilek, 2002; Çilek and Tekin, 2005), this confirms that milk yield increases with age up to maturity and decreases thereafter.

In this farm, the lowest milk yield was obtained in 1995. After 1995, milk yield increased up to 2004. The reasons for this increase could be the use of bulls with high genetic capacity, selection for milk yield and culling in the herd and especially improvement in management and feeding conditions. Uniform feeding was started with the importation of silage machine in 1995. This caused the cattle to reflect their genetic capability better. Lower milk yield in 2006 may result from incomplete lactation when data were collected and mastitis infectious observed in herd. Between 1993 and 1999, short lactation duration and consequently low milk yield may result from the deficiency of attention and feeding conditions.

In this study, average lactation duration was 303.40 days. This was very close to the ideal value (305 days). The shorter lactation duration as 252.9 days in may be related to some incomplete lactations when this study was started and data were collected. Lactation duration decreased with increase of lactation number. Short lactation duration in the oldest cows (5th lactation number) may be related to incomplete lactations because of culling.

The average dry period was 98.15±2.25 days. Dry period was higher than the ideal value (80 days). However, the dry period increased with calving age, as a result of increase of milk yield level with age in the herd. It can be said that if milk yield icreases with calving age, dry period would decrease. Effect of calving year on all milk yield traits was significant. Differences among years may be related to managment. It can be said that differences of managment among years was the most important factor affecting milk yield traits.

CONCLUSION

In this study, there was increase of milk yield level according to previous research (Bilgiç and Aliç, 2005) in this farm. This may result from improvement in breeding, feeding and management conditions (selection for milk yield and culling in the herd etc.). Although, lactation duration was found almost at ideal value, dry period was estimated as higher than the ideal value. In order to make animals more profitable, it is essential they be made pregnant as soon as possible during the service period in order to shorten the dry period. Under Turkish conditions and steppe climate of middle Anatolia. It can be concluded that Holstein cattle are raised successfully for milk yield on Polatli state farm.