LAUSR

Sixteen Quarter Horse mares (mean±SEM; 12±5 yr; 5.1±1.0 BCS; 579±9 kg BW) were used to evaluate the time course of the nutrient profile of colostrum and milk following parturition. Mares received a commercial concentrate designed to meet NRC (2007) requirements for gestating mares and were fed to maintain a BCS between 5 and 7. Horses were group housed in dry lots with ad libitium access to coastal Bermudagrass hay and water. As soon as parturition was observed, colostrum was collected by hand pre-suckle (0 h post-parturition), and at 6, 12, 24, and 36 h post-parturition. Samples were evaluated for concentrations of protein, fat, lactose, somatic cell count (SCC), urea nitrogen, and non-fat solids (NFS) by a commercial laboratory. Differences between means were determined using PROC MIXED in SAS (v9.4) with time as a repeated measure. Following parturition, colostrum protein and NFS decreased from h 0 to 6 (protein: 13.1±0.7 to 9.7±0.8%; NFS: 20.9±0.8 to 16.8±0.9%) and from h 6 to 12 (P0.04). Protein continued to show a trend for a decrease from h 12 to 24 (P = 0.08), plateauing around 3%. NFS remained around 10% from h 12 to 36. Conversely, lactose did not change from h 0 to 6 (3.6±0.2 to 3.9±0.2%) but increased from 6 to 12 h (5.5±0.2%) post-parturition (P < 0.0001). Fat, SCC, and urea nitrogen remained unchanged through 36 h post-parturition. These results indicate colostrum protein concentration decreases as early as 6 h post-parturition, and the non-fat solid portion of milk shifts from primarily proteins to lactose. However, the impact of these changes on foal health should be more clearly defined.