THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Post-weaning Nutrition and Puberty Induction Protocol for Beef Heifers in Florida
In this report, we will provide you with a summary of a 3-year study conducted at Range Cattle REC that evaluated the effects of different post-weaning growth rates and the use of a protocol for puberty induction on reproductive performance of beef heifers.
A major determinant of pregnancy success, and lifetime productivity of replacement heifers, is the age at puberty attainment relative to the beginning of her first breeding season. In Florida, the beef industry is dominated by bos indicus-influenced cattle because of their high tolerance to heat and parasites, which allows these animals to be managed extensively on pasture and reduce production cost. However, age at attainment of puberty for these heifers decreases reproductive efficiency. Bos indicus-crossbred heifers need to reach a greater proportion of their mature size before becoming pubertal compared to Bos taurus heifers. In a previous study done at Ona, we observed greater pregnancy rates (89% versus 70%) when heifers achieved 73% instead of 64% of mature body weight at the start of breeding season. These results indicate that bos indicus-crossbred heifers may need to be closer to 70% of mature weight at the start of breeding season to obtain acceptable pregnancy rates. The use of estrous cycle control protocols consisting of exogenous hormone administrations is another strategy that can initiate estrous cycles in prepubertal heifers (heifers that did not ovulate and attained puberty). Therefore, we evaluated the effectiveness of a puberty induction protocol in Brangus crossbred heifers fed to achieve low, medium, or high growth rates during a 168-day heifer development period.
The 3-year experiment described herein was conducted from September 2013 to February 2016. We utilized Angus × Brahman crossbred heifers (60 heifers per year) of approximately 550 lb and 10 months of age at the start of the study. Heifers remained on bahiagrass pastures and were offered molasses-based supplementation to achieve a low (LOW; 1.0 lb/day), medium (MED; 1.6 lb/day), or high (HIGH; 2.2 lb/day) average daily gain from September to February (total of 168 days). Supplements were formulated to allow heifers to achieve 55, 63, or 70% of their mature body weight at the initiation of breeding season (assuming a mature cow body weight of 1100 lb). Supplements were offered 3 times weekly at amounts to provide 3.9, 6.4, and 9.7 lb of supplement dry matter per heifer daily. In mid-November, heifers were randomly enrolled (SYNC) or not (NOSYNC) in a puberty induction protocol. The puberty induction protocol consisted of inserting an intravaginal controlled internal drug release (CIDR; Zoetis Animal Health, Florham Park, NJ) on day 0. Then, CIDR removal on day 7, followed by 100-µg injection of GnRH (2 mL Factrel; Zoetis Animal Health) on day 9, and 25-mg injection of prostaglandin (5 mL Lutalyse; Zoetis Animal Health) on day 19. Immediately after estrus synchronization, all heifers were exposed to yearling Angus × Brahman bulls from December to February.
Our most important finding was that the success of the current puberty induction protocol did not depend on post-weaning heifer growth rate. In other words, puberty induction had the same impact on heifer reproductive performance irrespective of growth rate assignment. The use of puberty induction protocol decreased age at puberty by 24 days, numerically increased overall calving rate (64% versus 57%), and percentage of heifers that calved during the first 8 weeks of the calving season (98% versus 82%) compared to not using a puberty induction. Thus, puberty induction protocol was an effective strategy to significantly increase reproductive success of Bos indicus-influenced heifers.
It is important to highlight that overall average daily gain and mature body weight of heifers at the beginning of breeding season were significantly less than we expected. Heifers fed to achieve LOW, MED, and HIGH growth rates had an overall average daily gain of 0.55, 0.90, and 1.12 lb/day, respectively. The poorer growth performance can be explained by the impact of high environmental temperatures and moisture on energy requirements of heifers. It is possible that different outcomes of the puberty induction protocol used in this study would have been observed if heifers had achieved the predicted growth performance. However, additional studies are required to validate this statement.
Although heifer growth performance was below the expected, approximately 27, 45, and 46% of LOW, MED, and HIGH heifers, respectively, achieved puberty at the start of breeding season, which explains the greater percentage of HIGH and MED heifers calving during the first 7 weeks of the calving season compared to LOW heifers. Despite these differences on puberty attainment and calving distribution, overall calving rates were not affected by diet (61, 57, and 62% for LOW, MED, and HIGH heifers, respectively). So, in this study, calving rates greater than the observed in this study did not occur because: (1) growth performance of all heifers was less than the expected; and (2) a high percentage of heifers did not ovulate before the end of the breeding season (25, 22, and 38% of HIG, MED, and LOW heifers, respectively).
Beef heifers that calved within the first 21-day period of the calving season had greater pregnancy rates and calf weaning weights for the first six parturitions, and remained in the herd longer compared to heifers that calved during the second and third 21-day period of calving season (Cushman et al., 2013). In agreement with those results, heifers that achieved puberty before the start of breeding season had greater calving rates (72% versus 51%, respectively), and calved 15 days earlier than heifers that did not achieve puberty before the start of breeding season. In addition, 90% of heifers that achieved puberty before the initiation of breeding season calved within 5 weeks of the calving season compared to only 60% of heifers that were not cycling before the initiation of breeding season.
Our results indicate that the major limiting factor for reproductive success of Bos indicus-influenced heifers is delayed attainment of puberty. It also indicates that successful calving rates and early-calving can occur if Brangus crossbred heifers become pubertal before the start of breeding season.