Abstract
Nutritional requirements and intake of beef cows vary across different growth and production stages, and the selection of replacement females with increased nutritional efficiency may improve the cow herd productivity. However, it remains unclear if animals exposed to ergot alkaloids maintain their nutritional efficiency metrics, such as residual feed intake (RFI) and weight gain (RADG), across different production stages. This two-year study aimed to evaluate changes in nutritional intake, and in various physiological and performance parameters of heifers across different production stages. In year 1, purebred Angus heifers (216.3±6.1 kg; 911 mo of age; n=36) were randomly separated and fed either an endophyte-infected (EI) or endophyte-free (EF) ration for 49 days. On day 1, temperature data loggers were inserted intravaginally to record vaginal temperature (VT) on a 20-minute-frequency throughout the entire study. Weekly measurements were collected to monitor growth and physiological responses of both treatment groups. Individual feed and water intakes were measured utilizing the Vytelle® Sense system. In year 2, all heifers were exposed to an ultrasonography assessment for pregnancy detection, and those confirmed as pregnant (431.47±9.1 kg; 17-19 mo of age; n=27) were sorted back into year 1 groups, reintroduced to the Vytelle® Sense system, and were all fed an EF ration for 56 days. The same physiological and performance measurements were collected on the same frequencies as on year 1. A post hoc analysis using the individual feed (FI) and water intake (WI) data evaluated RFI, RADG, total WI and WI per body weight (WI/BW). All physiological and performance data were analyzed using a MIXED procedure with repeated measures and a CORR procedure to estimate Pearson correlations for RFI and RADG in SAS. The effects of treatment, time, and its interactions were evaluated for BW, ADG, Body Condition Score (BCS), Hair Shedding Score (HSS), VT, FI and WI. In year 1, EF heifers had greater BW, ADG, and BCS, and lower VT and HSS over time (P <0.0001) than the EI heifers. The EF heifers also reported greater FI (P <0.0001) but lower WI (P=0.0144) and WI/BW (P=0.0132) than EI heifers over time. In year 2, no differences were observed in BW, ADG, BCS, HSS, and VT between groups. The EF heifers had a greater FI (P=0.0458) and tended to have a greater WI (P=0.0616) than EI heifers over time, but no differences were observed in WI/BW. In a longitudinal comparison, both groups had greater BW, ADG, and FI (P <0.0001) in year 2 than in year 1, while the WI/BW of both groups was greater (P <0.0001) in year 1 than in year 2. Additionally, no differences were observed in BCS, VT, and WI of both groups between years. A strong correlation was observed between years in the RFI of EI heifers (r=0.622), but a very weak correlation was found in their EF pairs (r=-0.150). Additionally, moderate correlations were observed during the same period in the RADG of the EI (r=0.439) and EF (r=0.374) groups. Moreover, 80% of the EI heifers that had a negative RFI after year 1 maintained a negative score after year 2, whereas 75% of the EI heifers with a positive RFI after year 1 retained a positive score after year 2. In contrast, no differences were observed for the RFI of EF heifers, as 50% of both the positive and negative scores in year 1 did not maintain their score after year 2. For the RADG, 60% of the EI heifers that had a positive score in year 1 maintained a positive score after year 2, whereas 63% of the EI heifers that had a negative score in year 1 retained a negative score after year 2. Similarly, 75% of the EF heifers that had a positive RADG after year 1 retained a positive score after year 2, whereas 67% of the EF heifers that had a negative RADG after year 1 maintained a negative score after year 2. Based on these data, exposure to an EI ration negatively impacts nutritional intake, which consequently decreases animal physiological and performance parameters. However, the EI heifers had a greater retention percentage of RFI scores than their EF pairs while maintaining similar retention rates of the RADG scores, suggesting that these efficiency metrics may not be severely impacted by ergot alkaloid exposure or pregnancy status. Future research focusing on long-term evaluations of the retention of nutritional efficiency parameters and their relationship with physiological stressors will improve the productivity of the beef cattle industry.