Author: Celso Heinzen Junior

Publisher:

Published: 2024

Total Pages: 0

ISBN-13:

Adequate energy supply to lactating dairy cows is essential to maximize genetic potential of milk production and efficiency. Corn silage often accounts for more than 50% of diets and therefore, corn starch is one of the main sources of energy fed to lactating dairy cows. However, the energy supply found in form of starch is dependent on its availability to the animal. Several strategies exist to improve starch digestibility in corn silages, including hybrid selection, kernel processing and prolonged storage. However, different ways to assess starch digestibility are used in the dairy industry and research, while ruminal disappearance is the most common procedure, it differs among laboratories, and much is unknown about how the analytical results of starch digestibility assays relates to in vivo digestibility and energy supply. Moreover, high inclusion of corn, in general, in dairy cattle diets are often related to milk fat depression, due to a highly fermentable carbohydrate source and unsaturated fatty acids profile that corn provides. Therefore, a series of experiments were conducted to evaluate a novel technology of corn silage hybrids effects on silage fermentation and starch digestibility, to better understand the sources of variation on starch digestibility assays to improve sample comparison, and to assess the risks of feeding starch and fatty acids from corn silage in milk fat depression.Two experiments were conducted to evaluate the effects of a genetically-modified corn hybrid with alpha-amylase expressed in the kernel (AMY) on fermentation profile, aerobic stability, nutrient composition, and starch disappearance of whole-plant corn silage (WPCS) and earlage. Both hybrids, AMY and its isogenic counterpart (ISO), were grown in 10 replicated plots (5 for WPCS and 5 for earlage). Samples of each plot were collected at harvest, homogenized, and divided into 5 subsamples which were randomly assigned to 5 storage lengths (0, 30, 60, 90 and 120 d). Minor differences on fermentation profile were observed between AMY and ISO for WPCS and earlage. Starch concentration was greater for AMY than ISO in WPCS and earlage and greater starch disappearances at 0 h and 6 h were observed for ISO in WPCS and earlage. Minor effects on fermentation profile, microbial counts, aerobic stability, and nutrient composition suggests that AMY can be ensiled for prolonged periods with no concerns for undesirable fermentation or nutrient losses. However, in situ starch disappearance was lower for AMY compared to ISO. A series of experiments was conducted to investigate variation across time-points, sample grinding size procedures and bag pore sizes used in ruminal starch digestibility assays. Experiment 1: samples of different starch sources submitted three times to multiple laboratories to evaluate variability of starch digestibility assays and the use of different incubation time-points on feedstuff ranking. Greater variation was observed for shorter incubation time-points, while different submission of samples did not influence starch digestibility for corn forage and corn grain samples. Moreover, different incubation time-points can change the ranking of samples. Experiment 2: samples of different starch sources were used to investigate how different grinding sizes and incubation time-point procedures affect starch digestibility assays variation and ranking of samples. Longer incubation time-points and finely ground samples reduced variation of in vitro starch digestibility assays; however, smaller grinding size increased starch digestibility in all starch sources and affected sample ranking. Experiment 3: samples from different starch sources were incubated in situ at 0 h to investigate the loss of particles and their influence on ruminal kinetics. Our results suggest that secondary loss of particles on ruminal incubations at 0 h might be a problem that limits the relationship between bag disappearance and degradation in forage and corn grain samples. Experiment 4: dry ground corn and pure starch samples were washed in water at different temperatures and using different filter types (Dacron polyester in situ bags, Ankom Technology; DPB, F57 bags, Ankom Technology; F57 or filter papers Whatman G3; WG3) to understand their effects on DM recovery. Samples incubated in room temperature water had greater DM recovery when compared to samples incubated in water at 39℗ð C. Pure starch had greater DM recovery in F57 and WG3 when compared to DPB, however, dry ground corn samples DM recovery was similar across all filter types. Experiment 5: dry ground corn and pure starch samples were incubated in rumen fluid at 0 h, using different filter bags (F57 and DPB) to investigate the effect of sample type and filter bags on DM and starch recovery. F57 bags were able to retain 100% of the starch incubated in pure starch, however, no difference between bags were found for dry ground corn samples for DM or starch recovery. These experiments highlighted the importance of the consideration of specific procedures in starch digestibility assays when ranking samples or generating information for ration formulation. The objective of the last study was to investigate the effects of dietary fatty acids and starch on milk fat secretion, the fatty acid profile changes in whole-plant corn silage (WPCS) and high-moisture corn (HMC) with different silage management practices as well as the contributions of these nutrients derived from WPCS and HMC to milk fat secretion. Twenty-three published studies that evaluated lipids supplementation to dairy cows were included in the dataset and meta-regressions analysis were conducted to model the effects of: 1) dietary fatty acids, starch and forage concentrations (expressed as % of DM) on milk fatty acids concentration (expressed as g/100g); 2) dietary fatty acids, starch and forage concentrations (expressed as % of DM) on milk fatty acids yield (expressed as g/d); 3) intake of fatty acids (expressed as g/d), starch, and forage (expressed as kg/d) on milk fatty acids concentration (expressed as g/100g); and 4) intake of fatty acids (expressed as g/d), starch and forage (expressed as kg/d) on milk fatty acids yield (expressed as g/d). While predictions of milk fatty acids concentrations are useful, models that predict milk fatty acids yield had greater performance. In general, starch concentration and starch intake had a positive effect on de novo and total milk fatty acids yield, while unsaturated fatty acids had a negative effect. Samples of WPCS and HMC from previous studies from our laboratory were analyzed for fatty acids profile to evaluate the effects of cutting height and storage length on fatty acids profile of WPCS and the effects of DM concentration and storage length on fatty acids profile of HMC. Increasing cutting height increased unsaturated fatty acids in WPCS, however, storage length and DM concentration effects did not follow a clear pattern of changes in fatty acids profile of WPCS and HMC. Simulations using the generated models were conducted to visualize the contributions of different inclusions of WPCS and HMC in the diet as well as different WPCS and HMC management practices to milk fat secretion. Our results suggest that energy contributions from starch and saturated fatty acids are important in milk fat synthesis predictions, while supporting the general role of unsaturated fatty acids on milk fat depression described in the literature. Management practices like cutting height, dry matter concentration and storage length affect fatty acids profile of WPCS and HMC and it may affect rumen bacteria metabolism and milk fat secretion.