Effects of stage of pregnancy on variance components, daily milk yields and 305-day milk yield in Holstein cows, as estimated by using a test-day model

Abstract

Pregnancy and calving are elements indispensable for dairy production, but the daily milk yield of cows decline as pregnancy progresses, especially during the late stages. Therefore, the effect of stage of pregnancy on daily milk yield must be clarified to accurately estimate the breeding values and lifetime productivity of cows. To improve the genetic evaluation model for daily milk yield and determine the effect of the timing of pregnancy on productivity, we used a test-day model to assess the effects of stage of pregnancy on variance component estimates, daily milk yields and 305-day milk yield during the first three lactations of Holstein cows. Data were 10 646 333 test-day records for the first lactation; 8 222 661 records for the second; and 5 513 039 records for the third. The data were analyzed within each lactation by using three single-trait random regression animal models: one model that did not account for the stage of pregnancy effect and two models that did. The effect of stage of pregnancy on test-day milk yield was included in the model by applying a regression on days pregnant or fitting a separate lactation curve for each days open (days from calving to pregnancy) class (eight levels). Stage of pregnancy did not affect the heritability estimates of daily milk yield, although the additive genetic and permanent environmental variances in late lactation were decreased by accounting for the stage of pregnancy effect. The effects of days pregnant on daily milk yield during late lactation were larger in the second and third lactations than in the first lactation. The rates of reduction of the 305-day milk yield of cows that conceived fewer than 90 days after the second or third calving were significantly ( P < 0.05) greater than that after the first calving. Therefore, we conclude that differences between the negative effects of early pregnancy in the first, compared with later, lactations should be included when determining the optimal number of days open to maximize lifetime productivity in dairy cows. Keywords: dairy cattle, days open, days pregnant, test-day model ImplicationsDaily milk yields of cows decline as pregnancy progresses. By using a test-day model, we revealed that stage of pregnancy did not affect the estimated heritability of daily milk yield. However, 305-day milk yields were decreased in cows with fewer days open, and the rates of reduction associated with decreased days open (⩽90) in the second and third lactations were larger than that in the first lactation. These differences between the effects of stage of pregnancy in the first compared with later lactations should be included when determining the optimal number of days open for lifetime productivity in dairy cows.

The economic value of days open in Holstein cows in Japan based on simulated changes in conception rate

“…The length of DO and calving interval changed with changing CR because CR affected the required number of AI services per conception. The total milk yield of a lactation was affected by CR because the shape of the lactation curve was affected by DO (Brotherstone et al, 2004;Olori et al, 1997;Yamazaki et al, 2016). DO evidently affected the calving interval and milking length.…”

“…Therefore, the total milk yield of a lactation period and the annual milk yield may decrease. The milk yield in the late lactation period would decrease if DO is shortened (Brotherstone, Thompson, & White, ; Olori, Brotherstone, Hill, & McGuirk, ; Yamazaki et al, ).…”

“…Therefore, the total milk yield of a lactation period and the annual milk yield may decrease. The milk yield in the late lactation period would decrease if DO is shortened (Brotherstone, Thompson, & White, 2004;Olori, Brotherstone, Hill, & McGuirk, 1997;Yamazaki et al, 2016). Byrne et al (2016) suggested that the economic value of fertility was affected by changes in replacement cost and profit from milk yield, which are affected by calving interval, AI cost that is affected by CR, and profit from calf sales that is affected by number of calves.…”

Environmental factors affecting the conception rates of nulliparous and primiparous dairy cattle

“…Therefore, setting a sufficient voluntary waiting period regardless of milk yield will improve the uterine condition in early lactation, and careful management in the subsequent peak lactation period can provide a stable conception rate throughout the lactation period. Moreover, cows that conceived 90 d after calving have shown increased 305d milk yield compared with cows that conceived before 90 d (Yamazaki et al, 2016). Therefore, the extension of the voluntary waiting period to 90 d may lead to the maintenance of milk yield in addition to an increased conception rate.…”

Suitable indicator of heat stress for genetic evaluation of heat tolerance in Holstein cows in Japan

“…The Holstein—Japan's main dairy cow—excels as a dairy breed, producing 8016 to 9531 kg of average milk yield over 305 days during the first to third lactation periods (Yamazaki et al, 2016). However, many studies have shown that Holstein cows are very sensitive to heat stress and suffer negative effects of heat stress on their milk yield and reproduction (e.g., Atagi et al, 2019; Bernabucci et al, 2014; Boonkum et al, 2011; Fuquay, 1981; Hagiya et al, 2020; Negri et al, 2021; Ravagnolo et al, 2000).…”