structural and metabolic responses of holstein heifers to rumen undegradable protein supply

This study evaluated the effects of various levels of rumen-undegradable protein (rup) on performance, blood metabolites, and structural growth of holstein dairy heifers. seventy-two holstein heifers (initial body weight = 128.7 ± 15.5 kg; age = 4 ± 0.31 months) were allocated into 12 pens, with 6 heifers per pen. the pens were randomly assigned to 1 of 3 dietary treatments over a period of 60 days. the dietary treatments consisted of three percentages of rup (28%, 33%, and 38% of crude protein). feed intake was recorded from each pen daily. the heifers were weighed every two weeks. blood samples were taken on day 1, 30, and 60. body condition score, back fat thickness, and skeletal growth were recorded on day 1 and 60 of the experiment. increasing dietary rup levels reduced dry matter, crude protein, and rdp intakes (p < 0.05), whereas it enhanced rup and mp intakes (p < 0.01). feeding different levels of rup had no effect on final body weight, body condition score (bcs(, and back fat thickness (p > 0.05). serum total protein and cholesterol concentrations increased (p < 0.05) in response to higher dietary rup, whereas serum urea concentration decreased (p < 0.05). the rup supplementation positively influenced wither height and body length growth (p < 0.01), but it did not affect other growth parameters (p > 0.10). overall, supplementing prepubertal heifer diets with increased rup levels enhanced the supply of metabolizable protein, improving structural growth and metabolic status without increasing fat deposition.