Animals, housing, and AI
This experiment was conducted on a commercial dairy farm in Carambeí, PR, Brazil. Lactating dairy cows (n = 76) were housed in freestall facilities bedded with sand and had ad libitum access to fresh feed and water. Cows were milked 3 times daily. Cows were fed TMR diets using corn and alfalfa silage as forage and corn- and soybean-meal based concentrates and supplemented with minerals and vitamins. On all farms, TMR diets were balanced by a professional nutritionist using the NRC (2001) nutritional requirements for lactating dairy cows. All cows in the commercial farms received bST every 11 d beginning at 60 DIM (500 mg/dose; Posilac; Monsanto Co., St Louis, MO), until dry-off. Cows received first timed AI at 60 to 75 DIM using the farm-managed at estradiol-progesterone-GnRH based timed AI protocol. Cows received an intravaginal P4-releasing insert (Repro Sync®, GlobalGen Vet Science, Brasil), 10,5 µg i.m. of GnRH de (2.5 mL of Gonaxal®, Biogénesis Bagó, Argentina), 2 mg i.m. of estradiol benzoate (2.5 mL of Syncrogen®, GlobalGen Vet Science, Brasil) on Day 0; 0,5 mg i.m. of Cloprostenol (2 mL of Induscio®, GlobalGen Vet Science, Brasil) on Day 7; 0.5 mg i.m. of Cloprostenol (2mL de Induscio®, GlobalGen Vet Science, Brasil), 1 mg i.m. of estradiol cypionate (0.5mL of Cipion®, GlobalGen Vet Science, Brasil), and the withdrawal of the P4 insert on Day 8. All cows received 10,5 µg i.m. of GnRH (2.5 mL of Gonaxal®, Biogénesis Bagó, Argentina) and were inseminated on Day 10.
Pregnancy diagnosis was performed 30 d after TAI using a portable scanner (Ibex Pro; E. I. Medical Imaging, Loveland, CO) fitted with a 7.5-MHz linear-array transducer.
Reproductive data such as calving date, date of AI, pregnancy diagnosis, BCS, and milk production were collected from the DelPro Farm Manager.
BCS evaluation
The BCS of all cows was daily evaluated by BCS cameras (DeLaval body condition scoring BCS, DeLaval International AB, Tumba, Sweden), which were mounted on the sort-gate at the exit where cows passed through daily post-milking. As the cow passed under the mounted camera a continuous video (30 FPS, 32,000 captured reference points) was taken and a 3D image from the video was automatically created and saved by the BCS camera software. In a secondary step, the saved 3D images were processed through an algorithm and analyzed to locate the key physical characteristics (pins, tail head ligaments, thurl, sacral ligaments, short ribs, and hooks) of the cow to calculate the automated score, viewable in DelPro Farm Manager (DeLaval International AB, Tumba, Sweden). The algorithm is based on the BCS scoring proposed by earlier studies but was reported in 0.1 increments in this study (Ferguson et al., 1994; Spoliansky et al., 2016). All automated BCS data were recorded and downloaded from DelPro Farm Manager.
Experimental groups
Aiming to evaluate the effect of the DIM of the BCS nadir on the reproductive parameters, cows were separated into two groups: Early BCS loss (n = 42), cows that reached the nadir of BCS ≤ to 34 DIM, and Late BCS loss (n = 34), cows that reached the nadir of BCS > 34 DIM.
The optimal cut-off point for DIM of the nadir of BCS effects in pregnancy by 150 DIM (P150) was calculated using the receiver operating characteristic (ROC) curve and was determined based on the highest sensitivity (Se), and specificity (Sp). Area under the curve (AUC), Se, Sp, and P-value of the ROC test are reported. Therefore, the cut-off value that better establish a relationship between the DIM in which the nadir of BCS was observed and the P150 was 34 DIM. The P150 was chosen as a reference due to cows that became pregnant by 150 DIM presented economic advantages and adequate reproductive performance (calving intervals of ~14 months; adapted from Arbel et al., 2001).
Statistical analysis
Statistical analyses were performed using the SAS program (SAS Institute Inc., Cary, NC, USA), except for ROC calculations and Kaplan-Meier survival analysis, which were performed using GraphPad Prism 5 software (GraphPad Software Inc., La Jolla, CA, USA).
Optimal cut-off points for determining the relationship between the DIM of the nadir of BCS and P150 were calculated using the receiver operating characteristic (ROC) curve. Therefore, cows were separated according to the DIM of the nadir of BCS in Early and Late DIM, and the pregnancy per AI (P/AI) was compared between these groups by Chi-square test.
For the continuous variables such as BCS at calving, the nadir of BCS, DIM of the nadir of BCS, BCS loss, and calving interval, the initial analysis included the effect of sire, age, BCS, and days postpartum in the statistical model. Sire, age, BCS, and days postpartum had no significant effect and were, therefore, excluded from the final statistical model. Thus, these continuous variables were analyzed using ANOVA, and means were compared among groups using Tukey’s post hoc test.
All data obtained from the repeat measurements (BCS, and milk production) were compared between groups by ANOVA using the Mixed procedure to evaluate the main effects of group, time, and their interactions.
Kaplan-Meier survival analysis curves were constructed to illustrate the rate at which cows became pregnant using the Kaplan-Meier survival analysis features of GraphPad Prism 5 software.