| In the present dissertation, three experiments were conducted to evaluate the effect of starch, neutral detergent fiber (NDF) source, fiber digestibility, and rate of passage in precision feeding dairy heifers.;The first experiment had two objectives: evaluate effects of starch concentration on digestibility and rumen fermentation and compare two sources of trace minerals (TM; inorganic, ITM, and organic, OTM, form) on digestibility and rumen fermentation. Eight rumen cannulated dairy heifers (15.4 +/- 0.8 mo of age and 438.31 +/- 18.08 kg of body weight) were subject to a split-plot, 4 x 4 Latin Square design with 19-d periods; 15 d adaptation and 4 d sampling. The whole-plot factor was type of TM; organic as proteinates (OTM) or inorganic sulfates (ITM), and the subplot was starch level (3.5, 12.9, 22.3, and 31.7%). Results of this experiment supported the hypothesis that the type of TM affects rumen bacteria populations and produces responses in ruminal fermentation. Digestibility of dry matter (DM), NDF, acid detergent fiber (ADF), hemicellulose, and starch was not affected by treatments.;The objective of the second experiment was to evaluate sorghum silage (SS), including digestibility and fermentation parameters, in precision-fed dairy heifers. Eight Holstein heifers (13.7 +/- 0.6 mo of age and 364.8 +/- 17.64 kg of body weight) fitted with rumen cannulas were used in a replicated 4 x 4 Latin Square design; treatments were 4 levels of F:C (85:15, 75:25, 65:35, 55:45). When the concentrate proportion of the diet increased, heifers tended to improve feed efficiency, primarily due to lower DM intake (DMI) with the same average daily gain (ADG) over diets with a high proportion of forage. Rumen pH was affected by F:C, decreasing as the proportion of concentrate increased in the diet since heifers spent less time consuming feed. However, pH was never lower than 5.7 in diets with F:C 55:45, and fiber digestibility was not affected. Volatile fatty acid proportion was slightly influenced by treatment, where butyrate increased as concentrate increased in the diet. Dry matter and starch digestibility were affected by F:C and were improved in diets with more concentrate. Neutral detergent fiber, ADF, and hemicellulose digestibility were not affected by F:C. Wet and dry feces were reduced linearly as F:C decreased, but total manure was not affected by treatment due to increased urine production on high concentrate diets. In the in situ analysis, corn silage had a faster rate of digestion for DM and NDF than SS. Brown mid-rib SS can effectively be fed in precision diets for dairy heifers.;The third experiment was conducted with the objective to compare ad-libitum vs. precision feeding diets with two forages and different levels of NDF to evaluate rumen fermentation, diet digestibility, feed efficiency, and digesta passage rate. Eight Holstein heifers (18.4 +/- 0.6 mo and 457.2 +/- 27.29 kg BW) fitted with rumen cannulas were used in a two-factor, split-plot, Latin Square design with 19-d periods, 14 d of adaptation and 5 d of sampling. The whole-plot factor was feeding system with ad-libitum or precision feeding and 4 heifers in each plot. The subplot included 2 factors: forage quality (low quality: grass hay, LFQ; high quality: corn silage, HFQ) and NDF content (high NDF, 48 % HNDF; low NDF, 39.8 %, LNDF). In this study we showed that the reduction in DMI for precision feeding diets improved feed efficiency in comparison with ad-libitum diets for dairy heifers. We observed that HFQ diets increased DMI, resulting in altered feed efficiency due to changes in intake based on fiber intake. Precision-fed diets resulted in a lower minimum rumen pH than ad-libitum diets, but the amount of time spent at minimum pH was not great enough to reduce fiber digestion or rumen fermentation. Ad-libitum diets resulted in lower mean pH than precision-fed diets, but the rumen pH was more consistent throughout the day than in precision feeding, where rapid fermentation resulted when heifers ate much of their daily diet within a small amount of time. HNDF diets presented higher minimum pH, suggesting that the presence of additional fiber stimulates rumination and buffers the rumen. Overall, VFA proportions were not affected by the type of diet but were modified by forage quality. Overall, apparent total tract digestibility was not affected by the type of diet. In situ digestibility was affected by forage quality and NDF level. Rate of passage was not affected by type of diet 22 h after feeding, but it was highly affected with the rumen at maximum capacity. Ad-libitum diets had higher passage rate than precision diets for the nutrients analyzed. (Abstract shortened by ProQuest.). |
