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Ten calves from each feeding location were later transported to RMSTC for harvest: (1) at the conclusion of the backgrounding/stocker phase, (2) 60d into their feeding period, and (3) upon reaching market weight. On each harvest day, left and right subiliac and superficial cervical lymph nodes were collected from each carcass, pooled by type within animal (n = 238 total samples), and subjected to an initial screening for Salmonella prevalence using a Hygiena BAX Q7 unit. Samples returning a positive BAX result were subjected to cultural analyses and confirmation per the USDA-FSIS Microbiology Laboratory Guidebook (MLG). Three isolates from confirmed positive samples were submitted to the USDA-APHIS National Veterinary Service Laboratory for serotyping. Sampling of environmental components (water, pen soil, individual feed ingredients, prepared rations, and fecal drop samples) were conducted at each of the three feeding locations. Samples of each component were collected in triplicate prior to initial placement of calves in preconditioning pens at the start of the backgrounding/stocker feeding stage and at the conclusion of their time in the preconditioning (approximately 45 d). After preconditioning, calves at the commercial feeding locations spent 120 to 130 d on pasture before being placed in feedlot pens; feed samples were obtained on d0 and upon conclusion of the stocker phase. Collection of all environmental components then were repeated prior to initial placement in feedlot pens, and again every 30d until market weight is reached. Environmental samples were subjected to Salmonella procedures as previously described. Additionally, pen soil samples were submitted to the Texas A&M AgriLife Extension Soil, Water and Forage Testing Laboratory for routine analysis including micronutrients (pH, NO3-N, Conductivity and Mehlich III by ICP P, K, Ca, Mg, Na, S, DTPA ZN, FE, CU, and Mn).
Overall, rate of Salmonella-positive environmental samples differed by location (P < 0.017). McGregor returned the lowest rate (11.5%), followed by Location A (23.6%), and Location B at 41.0%. At McGregor, Salmonella-positive samples were most often collected from pen soil (33.3%) and trough water (42.4%), while Salmonella was rarely (3.7 and 0.8% for feces and individual feed ingredients, respectively) or never (0%; prepared ration from the bunk) recovered from samples in the other environmental component categories. Salmonella-positive samples were recovered for all sample types at both Locations A and B. The largest (P < 0.017) quantity of Salmonella-positive pen soil and freshly voided fecal samples were recovered from Location B. Salmonella presence in individual feed components collected from each location prior to ration mixing varied (P < 0.017) based on location. Salmonella-positive individual feed ingredients were least common at McGregor (0.8%), occasional at Location B (12.1%), and recovered most frequently from Location A (28.2%). This trend does not translate directly to prepared rations as sampled from the feed bunk. The highest percentage discrepancy between these two categories is seen for Location B, as prepared ration samples returned Salmonella-positive results 46.2% of the time. This differs (P < 0.017) from the 0% Salmonella-positive sample recovery documented for McGregor, but not (P > 0.05) from Location A (24.2%).
In total, 36 unique serovars were identified from environmental component samples. Six serovars comprised 61.83% (128/207) of total serovars reported, these were: Anatum (18.36 %), 6,7:g,m,s:e,n,z15 (11.59%), Montevideo (11.11%), Muenchen (8.21%), Mbandaka (7.25%) and Cerro (5.31%). Notably however, is the proportion of these data that are represented by serovars isolated from environmental component samples at Location B. The same six serovars comprise nearly 81% (97/120) of serovars reported for environmental samples from this Location B. This equates to over 75% (97/128) of the total for these six serovars isolated from environmental samples across all locations. From McGregor, Meleagridis is the only serovar recovered from both a LN and environmental components (1 LN; 1 feces; 7 pen soil). For Location B, three serovars were isolated from both LNs and environmental samples: Anatum (11 LN; 26 environmental), Muenchen (1 LN; 16 environmental), and Cerro (1 LN; 11 environmental). Overall, serovars varied by location and sample type with minimal overlap between LN and environmental samples.