Impact of frequency of milking on milk yield and fertility of Holstein cows undergoing extended lactations due to failure to conceive

The objective of this observational study was to determine the effect of two (2x) compared to three (3x) times a day milking in Holstein cows undergoing lactations [greater than or equal to]600 d on milk production and reproductive performance. Two large adjacent commercial dairy herds with similar size, facilities, and management in a hot area of northern Mexico (25[degrees] N) were used. Cows in one herd (n=214) were milked two times a day (2x), the other herd was milked three times a day (3x; n = 245) and both groups were milked for at least 600 days. For cows in first lactation, total milk yield did not differ between 3x and 2x cows (19796 [+ or -] 3354 vs. 19269 [+ or -] 3652 kg; p > 0.10) in lactations with an average of 696 and 650 days in milk (DIM), respectively. Multiparous 3x cows produced more total milk days than 2x cows (20942 [+ or -] 3920 vs. 18910 [+ or -] 2632 kg; p < 0.01) with greater (p < 0.01) DIM for 3x (685 [+ or -] 117 days) than 2x (631 [+ or -] 88 days) cows. Lactation persistence was greater (p < 0.05) in 2x (62 [+ or -] 9%) than 3x (60 [+ or -] 10%) cows. Overall conception rate (CR) did not differ between 2x and 3x cows (53.3% vs. 49.8%) but 3x cows required one more service (p < 0.01) to get pregnant than 2x animals. Given that average milk yield throughout the complete lactation did not differ between 2x (29.8 [+ or -] 2.0 kg) and 3x (29.3 [+ or -] 2.9 kg) cows, it was concluded that two-times-a-day milking is equally effective as three-times-a-day milking to attain acceptable milk yield in lactations over 600 days. However, reproductive performance based on first-service CR and services per pregnancy was negatively affected by three-times-a-day milking.