**4. Discussion**

In this study, means of 305 days milk yield was 5807.83±78.27 kg. The findings of present study were in accordance with those of Ajili et al. [12], Boujenane [13], Erdem et al. [14], Inci et al. [15], Bilgiç & Aliç [16], Duru & Tuncel [21] and Kaya et al. [22]. As reported previously [7, 20, 26], the effect of calving season on milk yield was significant and milk yield was the highest in cows calving in winter. Similar finding have been reported by Javed et al. [27] and Tekerli et al. [28] in Holstein Friesian cows. Thorpe et al. [29] showed the effects of season of calving on production performance of dairy cattle in Kenya. Cows calving in winter have high milk yields, due probably to good feeding levels in the first 3 or 4 months of lactation. 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. On the contrary many workers [30 - 32] observed that the season of calving had a non-significant effect on lactation milk yield in Holstein Friesian cows.

Analysis of variance revealed that 305-days milk yield is significantly (P<0.05) affected by age at calving. The result is closely in accordance with the results of Kaya et al. [22] and Catillo et al. [33]. 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. The negative effect of early calving on milk yield could have been due to different factors, such as higher body weight gain before puberty. Milk yield decreased after 7 years of age. As reported in the literature [7, 20], this confirms that milk yield increases with age up to maturity and decreases thereafter.

305-days milk yield differed significantly (P<0.05) with calving year (Table 2). The lowest milk yield was obtained in 1998. After 1998, milk yield increased up to 2003. 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. The variation in milk yield observed in different years reflected the level of

letter are significant (p<0.05).

**4. Discussion** 

Friesian cows.

thereafter.

**Table 5.** Least square means of milk yield traits by parity

113.28±3.25 days and shortest in 3rd lactation at 87.23±2.17 days (Table 5).

dry period were statistically significant (P<0.001). Dry period was longest in 5th lactation at

n Mean±SE n Mean±SE n Mean±SE

**Lactation number 305 days milk yield (kg) Lactation length (day) Dry period (day)** 

1 1457 5412.27±52.41a 1457 311.7±3.12a 1123 93.14±2.34a 2 1353 5721.35±54.74b 1353 307.1±2.78ab 1099 89.27±2.41b 3 1246 5614.23±47.13bc 1246 303.3±3.27c 987 87.23±2.17b 4 1127 5417.58±46.51a 1127 317.5±4.17b 956 95.16±3.04a 5 978 5123.47±48.45c 978 297.8±3.04ac 979 113.28±3.25c a, b, c: differences between groups with same letter in the same column are non-significant, differences with different

In this study, means of 305 days milk yield was 5807.83±78.27 kg. The findings of present study were in accordance with those of Ajili et al. [12], Boujenane [13], Erdem et al. [14], Inci et al. [15], Bilgiç & Aliç [16], Duru & Tuncel [21] and Kaya et al. [22]. As reported previously [7, 20, 26], the effect of calving season on milk yield was significant and milk yield was the highest in cows calving in winter. Similar finding have been reported by Javed et al. [27] and Tekerli et al. [28] in Holstein Friesian cows. Thorpe et al. [29] showed the effects of season of calving on production performance of dairy cattle in Kenya. Cows calving in winter have high milk yields, due probably to good feeding levels in the first 3 or 4 months of lactation. 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. On the contrary many workers [30 - 32] observed that the season of calving had a non-significant effect on lactation milk yield in Holstein

Analysis of variance revealed that 305-days milk yield is significantly (P<0.05) affected by age at calving. The result is closely in accordance with the results of Kaya et al. [22] and Catillo et al. [33]. 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. The negative effect of early calving on milk yield could have been due to different factors, such as higher body weight gain before puberty. Milk yield decreased after 7 years of age. As reported in the literature [7, 20], this confirms that milk yield increases with age up to maturity and decreases

305-days milk yield differed significantly (P<0.05) with calving year (Table 2). The lowest milk yield was obtained in 1998. After 1998, milk yield increased up to 2003. 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. The variation in milk yield observed in different years reflected the level of management as well as environmental effects. Between 1996 and 2001, short lactation duration and consequently low milk yield may result from the deficiency of attention and feeding conditions. The significant effect of year of calving productive performance of dairy cows could be attributed to the changes in feeding and managerial systems and environmental conditions which occurred from year to another as well as to differences between years in the quantity and quality of forage available. 305-days milk yield differed significantly (P<0.05) with lactation order (Table 5). The 305-days milk yield in second lactation was significantly higher than in first lactation. This result is consistent with Munim et al. [34] who found significant (P<0.05) effect of parity on milk yield. Nevertheless, the result differed from that of Habib et al. [35] who found non-significant (P>0.05) effect of lactation number on milk yield. The significant effect of parity on productive performance may be due to the changes in managerial systems and environmental conditions among parties. The average lactation length calculated in this study was 309.6 days. This was very close to the ideal value (305 days). This length of lactation was longer than results reported by Sattar et al. [36] and Alim [37] who reported a lactation length of 293 ± 3 and 291.86 ± 6.55 days in Friesian cows in Libya and Pakistan, respectively. The lactation duration of Holstein cows was between 284.7 and 333.9 days in previous studies [14-16, 18, 21, 22]. The shorter lactation duration is 127.56 days it may be related to incomplete lactations when 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 97.7±2.25 days. Dry period was higher than the ideal value (80 days) but shorter than funding of Sattar et al. [36] who reported a longer (224.99 days) dry period. 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 increases 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 management. It can be said that differences of management among years was the most important factor affecting milk yield traits.

Dairy cows are usually dried-off for two months prior to the next calving. This rest period is necessary to maximize milk production in subsequent lactation. It was reported that the dry period is required for the renewal of the udder glandular tissue [38, 39]. Nevertheless, the optimal dry period was established as 60 days. A significant increase in milk yield of the dairy cows caused a new attention in creating the optimum dry period [40]. Two months were accepted as a sufficient dry period for high-productive cows [41]. A research done in Poland by Borkowska et al. [42] and Winnicki et al. [43] indicated that in practice the dry period is extended or excessively shortened, which leads to a reduction in milk production as compared to the recommended optimum. Milk yield is usually reduced when the dry period is less than 40-60 days (25-40% less milk). Dry period longer than 60 days in length does not result in a significant increase in milk production. Long dry periods decrease the average annual production of the cow by extending the calving interval beyond the normal 13-14 month interval and causing a decrease in the lifetime production of the dairy cow.
