|Abstract:||Digestibilities of crude protein (CP) and amino acids (AA) are crucial characteristics of milk replacers (MR) and calf starters (CS) in a calf-rearing program. Since AA are only absorbed in the small intestine and because the fermentation in the large intestine would alter AA profile in digesta, ileal digestibility is considered the most accurate estimation of AA bioavailability. Because the synthesis of endogenous proteins is dependent on many physiological and dietary factors, flows of AA and CP at the ileum need to be determined in order to obtain accurate estimates of nutrient ileal digestibilities. Soy protein remains one of the most studied alternatives to milk proteins. Microbial-treated soy protein has become a feed ingredient of interest lately because of its lower concentration of ANF, its greater proportion of AA and small size peptides, and the presence of other beneficial bioactive compounds and peptides. These novel products have shown interesting results in terms of gastrointestinal health and growth in pre- and post-weaned calves.
The objectives of the series of experiments included in this dissertation were to (1) evaluate our capacity to successfully place a duodenal and ileal cannula in young calves, (2) analyze the effect of the inclusion of an enzyme-treated soybean meal (SBM) in a MR on the dietary ileal digestibility in pre-weaned calves, (3) evaluate the effect on ruminal and intestinal digestion of the substitution of regular SBM with enzyme-treated SBM in weaned calves, and (4) estimate ileal endogenous losses of CP and AA in pre- and post-weaned calves.
For objective (1), 2 pilot experiments were performed to evaluate ileal cannulation alone, and duodenal plus ileal cannulation. In the first experiment, a T-cannula was surgically installed in the terminal ileum of 3 Holstein calves approximately 5 cm anterior to the ileocecal junction at 15 d of age, and 2 paired non-cannulated calves were used as controls. Cannulation did not affect mean BW, ADG, milk and water intakes, and body frame dimensions. However, final BW (89.2 vs 94.6 kg) was lower and starter intake (0.06 vs 0.21 kg/d) tended to be decreased in cannulated calves when compared with control calves. No effects on health scores, rectal temperature, or the odds of incurring diarrhea or being medicated were observed. Flow of digesta (46.4 ± 0.04 g/h) increased linearly after feeding, whereas there was a quadratic effect of time on digesta pH with the nadir at approximately 8.5 h post-feeding. In the second pilot study, 3 Holstein male calves were fitted at 7 wk of life with a T- cannula at the terminal ileum and another at the proximal duodenum. The mean weekly BW of the 3 calves from arrival (wk 3) until the beginning of the experimental period (wk 14) were fit to the best broken-line model as described elsewhere. The parameters of the selected model [BW= a + b1 ✕ min (wk, x1) + (b2 ✕ max (0, wk-x2)); P < 0.001, adj. R2 = 0.996) showed a plateau on BW gain of almost 2 wk from 6.6 wk (x1) until 8.5 wk (x2), which corresponds with the time of the surgery (wk 7). In addition, the model identified different slopes of growth before (b1 = 6.5 kg/wk) and after the plateau (b2 = 5.7 kg/wk), which may indicate a detrimental effect on growth due to the digesta collection or body tissue adaptation to the cannulas.
For objective (2), 9 Holstein ileo-cannulated calves were randomly assigned to a 3 ✕ 3 replicated Latin square with 5-d periods. Calves were fed twice daily either a MR containing all milk proteins (WPC) or a MR with 50% of the CP provided by an enzyme-treated SBM (ESBM). No starter was offered to minimize rumen development. Ileal digesta pH with the ESBM diet was lower than with WPC. According to the piecewise nonlinear model of pH fluctuation, digesta pH during ESBM decreased slower after feeding and reached its nadir later than with the WPC diet. Apparent (AID) and standard (SID) ileal digestibility of most AA, CP, and total AA were lower or tended to be lower with ESBM. However, true ileal digestibility (TID) did not differ between diets for CP and all AA except Ala and Ile, which were greater for WPC, and Arg which tended to be greater with ESBM. In agreement with the estimated differences in total endogenous losses (ENDtotal), we found that flows of digesta DNA and crude mucin were greater with ESBM. Substitution of 50% of the protein from whey with an enzymatically treated soybean meal did not affect major nutrient digestibility or calf growth and even improved fecal consistency.
For objective (3), 12 weaned Holstein calves cannulated in the duodenum and ileum were randomly assigned to a single 3 × 3 Latin square with 10-d periods at 50 d of life. The 3 diets were fed for ad libitum intake and consisted of a control CS with regular SBM as the main source of protein (CTRL), an isonitrogenous CS with an enzyme-treated SBM as the main source of protein (ESBM), and a low protein diet with no supplemental protein (LOCP). Relative to intakes, duodenal flows of CP and total AA as well as microbial-N efficiency were greater, and flow of non-protein N was lower, with ESBM than CTRL. Similarly, the AID of CP and total AA of the diet and the test ingredient were greater for ESBM. The forestomach and intestinal digestion parameters suggest that an optimal balance of CP and the inclusion of previously hydrolyzed proteins improved the efficiency of microbial digestion and increased AA absorption despite a lower proportion of microbial N relative to total duodenal flow. The latter indicates that even the rumen undegradable protein supply with ESBM was more digestible.
Lastly, for objective (3) basal endogenous losses (ENDbasal) of AA and CP were estimated by feeding a nitrogen-free MR to each calf during 1 period in the MR study. Total endogenous losses (basal + specific) were estimated by multivariate regression of the χ2 distances between digesta and reference protein AA profiles. The ENDbasal of AA and CP were 13.9 ± 1.1 and 22.4 ± 1.1 g/kg of DMI, respectively. The estimated ENDtotal of AA and CP were higher with ESBM than with WPC. Endogenous losses across the small intestine in weaned calves were estimated by feeding a CS with low CP content and by regression of total and digestible CP flows across diets. The basal losses per kg of duodenal DM flow were 47 ± 15 and 37 ± 12 g/d and therefore the TID were 86 ± 0.1 and 87 ± 0.1 % for CP and AA, respectively.
Overall, enzymatic treatment of soy proteins increased N digestibility and show promise as a feedstuff for weaned calves and even as an ingredient for milk replacers. The estimate of endogenous protein losses is crucial to determine accurately dietary N digestibilities.