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The association of birth type (single vs. double) and pregnancy type (single vs. double) with lactation performance and lactation curve was investigated in Holstein cows in Iran. Data of 243298 lactations on 138021 cows in 261 herds collected during January 2000 to December 2012 were used. The average twinning rate was 3.64% and ranged from 1.44% (primiparous) to 5.41% (multiparous). Factors associated with twining were calving season, calving year, herd, and parity. The rate of twinning increased from 2.49% in 2000 to 3.42% in 2012. The probability of twinning was higher in multiparous cows than that in primiparous [Odds ratio (OR) (95% confidence interval (CI))= 3.64 (3.44-3.84) for multiparous vs. primiparous cows]. The probability of twin pregnancy was higher in cows previously delivering twins than that in those delivering singles [OR (95% CI)= 3.17 (3.02-3.34) for cows delivering twins vs. cows delivering singles]. The lactation curve for cows delivering singles in previous gestation and pregnant with twins in subsequent gestation tended to be lower and flatter. The incidence of twin pregnancy was increased as the cow reached its peak yield earlier, and produced more milk at peak lactation. Primiparous cows delivering singletons in their previous gestation and having twins in their subsequent gestation produced more 100-d, 200-d and 305-d milk than those pregnant with singletons. Multiparous cows that delivered singleton in previous gestation and were pregnant with twin in the subsequent gestation, produced more milk during the first 100-d and 200-d of lactation than those pregnant with singles. [ABSTRACT FROM AUTHOR]

The objectives were to estimate (co)variance components and genetic parameters for lactation curve traits in Holstein dairy cows in Iran. The used data were records on Holstein cows collected during January 2003 to December 2012 by the Animal Breeding Center of Iran (Karaj, Iran). In order to describe the lactation curve, an incomplete gamma function was fitted to 691200 test-day records on 96263 lactations, 377696 test-day records on 52168 lactations, and 182143 test-day records on 24951 lactations for the first three parities, respectively. Lactation curve traits that were analyzed included a scaling factor associated with initial yield, the inclining and declining slopes of the curve, peak time, peak yield, 305-d milk yield and a lactation persistency measure derived from the incomplete gamma function. The estimated heritability of lactation curve traits for the first, second and third lactation ranged from 0.012 to 0.29, 0.017 to 0.15, and 0.02 to 0.13, respectively. Genetic correlations among lactation curve traits for the first, second and third lactation ranged from -0.68 to 0.99, -0.84 to 0.98 and -0.90 to 0.94, respectively. The estimated repeatability of lactation curve traits ranged from 0.07 to 0.40. The moderate to large positive genetic correlations of 305-d milk yield with initial yield, peak yield and lactation persistency suggest that one of these traits could be used as a selection criterion to improve all four traits. However, the peak yield and 305-d milk yield were more heritable than lactation were persistency and initial yield. [ABSTRACT FROM AUTHOR]

In this study, 65,971 lactations on 41,842 cows in 64 herds were used to determine the association between dry period length (DPL) and lactation performance, lactation curve, calf birth weight, and the incidence of calving difficulty during the subsequent parity in Holstein dairy cows in Iran. The length of the dry period was classified into 7 categories: 0 to 35 d, 36 to 50 d, 51 to 60 d, 61 to 70 d, 71 to 85 d, 86 to 110 d, and 111 to 160 d. Cows with the standard DPL (51 to 60 d) produced more 305-d milk, fat, and protein over the next lactation compared with those with shorter dry periods. Shorter dry periods (0 to 35 d and 36 to 50 d) were associated with lower initial milk yield, steeper inclining and declining slopes of the lactation curve, and higher milk persistency compared with dry period length of 51 to 60 d. Peak lactation was achieved later in cows with 0- to 35-d and 36- to 50-d dry period length than in those with dry period length of 51 to 60 d. We also observed a relationship between DPL and calf birth weight: smaller calf birth weight was recorded with a dry period of 51 to 60 d compared with longer dry periods. The incidence of calving difficulty did not differ in cows with 51- to 60-d dry period compared with cows with 0- to 35-d and 36- to 50-d dry periods. In conclusion, the results of this study did not support previous findings that suggested a shorter dry period could be beneficial to dairy production. [ABSTRACT FROM AUTHOR]

This study was carried out to investigate the relationship between early lactation curve parameters and calving interval in Holstein cows of Iran. In order to describe the lactation curve, the incomplete gamma function was fitted to 5,754,428 test-day milk records corresponding to 766,108 lactations on 315,634 cows in 2,448 herds. Cows with higher milk yield during early lactation had shorter calving intervals; however, longer calving intervals increased the 305-d milk production (P<0.01). Cows with higher yield at the beginning of lactation, steeper ascending, and steeper descending slope had shorter calving intervals (P<0.01). Calving interval was increased by 2.73 (±0.12) d for every extra kg of milk at peak lactation (P<0.01). The calving interval was directly impacted by the persistency of milk yield, but milk yield persistency was reduced in cows with shorter calving intervals (P<0.01). [ABSTRACT FROM AUTHOR]