Describing the Consumption of Chlortetracycline-Containing Supplement Offered Free-Choice to Commercial Beef Cows on Pasture

Abstract Extensive beef cattle production systems depend on forage as the predominant nutrient source. However, mineral requirements are not always met solely through forage consumption. To maintain optimum animal productivity, beef cattle producers must provide mineral supplementation and free-choice supplements are most common. Free-choice supplement intake by grazing cattle is highly variable and is dictated by season, individual animal preference, supplement palatability and other factors. Variation in supplement intake is problematic for delivering medication such as chlortetracycline (CTC), which is aimed to mitigate the risk of bovine anaplasmosis in beef herds. Research investigating the relationship between individual animal attributes and supplement consumption in extensive settings is limited. Recently, adoption of technology such as a portable, self-contained individual feeding unit (SmartFeed; C-Lock Inc.) into supplementation programs has facilitated unprecedented data collection in grazing cattle. The study objectives were to describe the consumption patterns of a granular CTC-containing supplement and to evaluate the factors contributing to consumption variability by gestating cows on pasture. A total of 103 nonlactating, pregnant, crossbred cows were assigned to this experiment on d -28 and allocated to 1 of 3 pastures equipped with a SmartFeed unit to record individual supplement intake. Cows were offered dried distillers grains for 27 d of acclimation to the units (mean intake of 0.29 kg of supplement/cow daily; 74% of cows) before transition to a commercially available granular mineral supplement containing CTC for 46 d. Supplement was formulated such that a 630 kg cow must consume 113 g/d to receive the label CTC dose (1.1 mg/kg). Cow intake and unit visit frequency data were analyzed using a general linear model (PROC GLM) and differences evaluated using Least Squared Means. The runs test for serial randomness was used to evaluate clustering of feeding and non-feeding events. Feeding and non-feeding events during the 46-d observation period were not randomly distributed (P < 0.01), and the mean number of consecutive days a cow did or did not consume mineral supplement were 1.8 and 5.6 days, respectively. As cow age increased, visits to the feeding unit decreased and total supplement consumption also decreased (P < 0.01). According to supplement label instructions, each cow was expected to consume 5.2 kg of supplement in total over 46 d to receive the total recommended 32.2 g CTC dose. Mean supplement consumption was 40.5 g· cow-1· d-1, providing 0.25 g CTC· cow-1· d-1, and only 1.9% (2/103 cows) consumed enough supplement to receive the recommended dose. Collectively, results indicate that providing a granular free-choice supplement containing CTC is not an effective method of ensuring cows consume an adequate amount of supplement to receive CTC at the label-directed dose and frequency.