Emir. J. Food Agric ● Vol 33 ● Issue 2 ● 2021 113 Impact of frequency of milking on milk yield and fertility of Holstein cows undergoing extended lactations due to failure to conceive Jesús Mellado 1 , Jessica Flores 2 , Francisco G. Véliz 2 , Ángeles de Santiago 2 , José E. García 1 , Hugo L. Gutierrez 1 , Miguel Mellado 1 * 1 Autonomous Agrarian University Antonio Narro, Department of Animal Nutrition, Saltillo, Mexico, 2 Autonomous Agrarian University Antonio Narro, Department of Veterinary Science, Torreon, Mexico INTRODUCTION In temperate zones with limited impact of heat stress, a 305-d lactation is considered an optimum lactation length. However, in zones where dairy cows undergo thermal stress for most of the year, heat stress negatively impacts the ability of a cow to become pregnant before 100 days post- calving and this infertility leads to involuntary extended lactations (Mellado et al., 2016). In Australia and New Zealand the potential role of extended lactations with a 24-mo calving interval on pasture-based systems has been studied (Auldist et al., 2007; Kolver et al., 2007). This management practice is the result of the marked transformation that the modern dairy cow had gone through during the past decades. These effects included reproductive failure derived from the steady and continued selection for increased milk yield (Walsh et al., 2011; Berry et al., 2016), and today it is not uncommon for cows to be dried off with very high milk yields (Lehmann et al., 2017; Rajala-Schultz et al., 2018). Drawbacks of extended lactations are alteration of herd composition as it results in fewer calvings per year, and hence fewer replacement calves, heifers, and dry cows if the number of lactating cows is maintained for prolonged periods of lactation. This may reduce herd feed use, and it thereby should increase the forage to concentrate ratio because cows will spend a greater period of the year on the downward slope of the lactation curve. Additionally, cows with extended calving intervals have longer periods in late lactation, which leads to a greater partitioning of nutrients away from the udder and toward body energy reserves (Marett et al., 2015). 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 ≥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° 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 frst lactation, total milk yield did not differ between 3x and 2x cows (19796 ± 3354 vs. 19269 ± 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 ± 3920 vs. 18910 ± 2632 kg; p < 0.01) with greater (p < 0.01) DIM for 3x (685 ± 117 days) than 2x (631 ± 88 days) cows. Lactation persistence was greater (p < 0.05) in 2x (62 ± 9%) than 3x (60 ± 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 ± 2.0 kg) and 3x (29.3 ± 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 frst-service CR and services per pregnancy was negatively affected by three-times-a-day milking. Keywords: Conception rate; Dairy cattle; Extended lactations; Heat stress; Lactation length ABSTRACT Emirates Journal of Food and Agriculture. 2021. 33(2): 113-119 doi: 10.9755/ejfa.2021.v33.i2.257 http://www.ejfa.me/ RESEARCH ARTICLE *Corresponding author: Miguel Mellado, Autonomous Agrarian University Antonio Narro, Department of Animal Nutrition. E-mail: melladomiguel07@gmail.com Received: 19 October 2020; Accepted: 12 February 2021