عنوان مقاله [English]
Introduction Ingestion of solid feed is necessary to stimulate rumen development in the young calf and facilitate the transition from a pre-ruminant to a functioning ruminant. Lipogenic nutrients in ruminants originate from fermentation of fiber to acetate and butyrate, dietary fat or are derived from body reserves. Glucogenic nutrients originate from starch that has escaped rumen degradation or gluconeogenesis. The use of low-starch starters is prevalent in dairy calf industry worldwide. These starters differ greatly in their composition of both fiber and fat. The NRC (2001) suggested that adequate digestible fiber should be included in starter diets but limited data exist to substantiate this statement. Digestible fiber sources such as sugar beet pulp, soybean hulls, and wheat bran have a lower ME value than corn and barley grains which are starch rich. A meta-analysis of 6 studies showed positive linear relationships between starch concentration of starter feed with ADG. However meta-regression analysis indicated that growth responses to starch concentration were influenced by ME concentration in dry feed fed to the calves. One possible way to enhance energy intake is to increase the energy density of the diet with fat. Convenient starter diets are typically low in fat. Most previous studies have either compared high-starch to high-fiber starter diets or investigated effects of fat supplementation on rumen environment and growth performance whereas our interest is in growth responses to diets differing in starch, NDF, and fat in calves. We hypothesize that a low-starch starter feed in which starch is substituted by fiber and fat will support growth performance similar to a traditional high-starch starter concentrate and high-fiber, high-fat diet might also improve ruminal environment in calves. We also hypothesized that preference for certain type of starter would become apparent, providing an indication of which type of energy sources may be considered more palatable for calves and therefore recommended to formulate starter feed mixture. The objectives of our study was to investigate the effects of lipogenic starter (low-starch with high-fiber and high-fat content) in comparison with a glucogenic (high-starch) or mixture of glucogenic and lipogenic starters on growth performance, ruminal fermentation, as well as blood metabolites of dairy calves during the first 70 d of life.
Materials and Methods In first experiment, thirty Holstein female calves (4 d of age; 41.0 ± 4.0 kg of BW), randomly assigned by BW to one of the three dietary treatments. Treatments consisted of (1) a high starch starter feed containing corn and barley grain (glucogenic); (2) a high fiber and fat starter feed containing soybean hulls, corn germ meal, sugar beet pulp, wheat bran, full fat soybean, and vegetable oil (lipogenic); and (3) a starter feed containing mixture of diets 1 and 2 (MIX). Starter intakes were measured daily and all the calves were weighed at birth and subsequently every 7 d until the end of the experiment. Feed efficiency was calculated as the weight gain to feed intake ratio (kg/kg). In addition, the structural growth indices were measured on d 4, at weaning and at the end of the study. Blood samples from each calf were collected at 35 and 70 d to determine glucose, BUN, and β-hydroxybutyrate. Ruminal fluid was obtained 2 h after offering the morning feed at 35 and 70 d to determine the rumen pH and concentration of volatile fatty acids. Data were analyzed as a completely randomized design using a repeated-measures mixed model (PROC MIXED) of SAS software. In second experiment, 20 Holstein female calves fed a standard ration until 70 days of age. Three days after weaning, each calf was involved in a pairwise preference test between glucogenic and lipogenic diets. For each calf in each pairwise preference test, preference ratio was calculated as the consumption of one feed as a percentage of the consumption of both feeds in the test. Preference ratios were compared for a difference from 0.5 (lack of preference) using t-tests. The feed type was assigned +1 if it was preferred, −1 if it was not preferred, and 0 if the preference ratio in a pairwise preference test did not differ from 0.5. Total preference scores were used as an indicator of overall preference rank of individual feed type.
Results and Discussion Results of the first experiment showed that dry matter intake was higher in the MIX diet group in the post weaning period than the other groups and a tendency to increase dry matter intake was observed throughout the study period. Average daily gain, feed efficiency, body weight, and skeletal growth were not different between treatments. Blood glucose (day 35 and 70) and plasma β-hydroxybutyric acid concentration (day 70) were higher in calves fed the glucogenic and lipogenic diet, respectively, compared to the other treatments. Rumen fluid pH and acetate concentration in calves fed the lipogenic diet were higher than those fed the glucogenic and MIX diets. The second experiment showed that calves preferred the lipogenic diet over the glucogenic diet.
Conclusion The results from this study showed that providing MIX diet rather than glucogenic and lipogenic diets increased post weaning intakes of starter without improving ADG and final BW. The results of the preference test clearly indicate that lipogenic diet is a highly palatable feed for recently weaned dairy calves, whereas glucogenic diet is less preferred. Given the positive trend observed in MIX feed diet, this diet seems to be recommendable for farm application. Whether the effects of energy source in early life could have long-term consequences on growth and milk yield are not known and warrant further research.