Deuterium oxide dilution can be used to determine the net energy content of feeds for dairy cattle and goats ()
1. INTRODUCTION
Accurate net energy values for feed ingredients are a critical part of the information that dairy producers need to feed their cows for optimal production efficiency. The industry has available a much broader selection of byproduct feeds than it once had, but estimation of the energy concentration in those special feeds is a problem. Equations that predict net energy for lactation (NEL) from fiber, protein, near infrared reflectance, and other rapid feed analyses were based on a limited number of large animal calorimetric determinations. These NEL determinations have been performed for alfalfa, grains, and other traditional dairy feeds, but not for the diverse, currently available byproducts. In most cases, a general formula is used to create tabular feed NEL values from total digestible nutrients (TDN) or digestible energy values. Often the TDN or digestible energy values themselves were predicted from fiber content rather than directly measured. When the relationship between fiber content and NEL for a new feed is different from those described by NEL prediction equations, errors may occur in the formulation of diets that include the new feed. Errors in estimation of feed energy and prediction of animal performance are costly to dairy producers because their profits depend on minimizing feed costs and maintenance of proper body condition for efficient rebreeding and preparation for the next lactation.
The three main goals for these trials were 1) to explore a technique for measuring the NEL value of feeds without respiration calorimetry or slaughter, 2) to compare results obtained from lactating dairy goats with those from lactating dairy cows, and 3) to describe factors that limited the precision of these NEL estimates and thereby enable other workers to improve this technique.
2. MATERIALS AND METHODS
The following procedures were reviewed and approved by the University of California, Davis Animal Use and Care Committee as Approved Protocol #3429. This work was performed at the University of California when the author was a member of that faculty.
2.1. Body Composition and Energy
The body composition and energy content of the cows and does were determined from live body weight and deuterium oxide (D2O) dilution space before and after each 56-d feeding period according to the methods of Brown et al. [1] for cows and Brown and Taylor [2] for goats. Animals of the same sizes and types, diets of the same physical form and approximately same chemical composition, and the same animal care facilities were used in this trial as were used to establish these methods. Intrajugular injections of about 75 and 7.5 g of D2O were administered to cows and goats, respectively. The syringes used to administer the doses were weighed to the nearest 0.01 g before and after injections. The concentrations of D2O in milk samples from the following eight milkings were determined by an automated modification of the Byers infrared absorption assay [3]. Cow body fat, protein, and energy and goat body fat and protein were calculated as previously described [1,2] with equations that yield body composition estimates unbiased by water turnover rates. Goat body energy was calculated as the sum of [body protein (kg) × 24.794 MJ/kg] + [body fat (kg) × 38.614 MJ/kg].
2.2. Estimated Fasting Heat Production
The average body weight for any given test period was used to predict fasting heat production (HP) by the following equations:
Cow: HP (kJ/d) = 260.96(body weight)0.775
This equation is from Thonney et al. [4] based on reevaluation of data reported by Flatt and Coppock [5] and Goat: HP (kJ/d) = 428.86(body weight)0.66 [6].
These equations were based on measurements of heat production from published studies of animals most similar to those used in this study.
2.3. Feed and Milk Analyses
Milk samples proportional to production (1% of total) were taken at each milking and composited by week. Milk energy was determined by bomb calorimetry (A. Gallenkamp & Co., Ltd., London, England) of freezedried samples. Daily subsamples of feed offered and refused were taken and composited by period. Feeding cows required more than one batch of feed per period from the feed mill. Therefore, cattle feed samples were composited for analysis by batch, and period averages of appropriate batches were used to characterize the feeds for calculation of NEL. All feeds were analyzed for ether extract (crude fat), crude protein, dry matter, ash, Ca, and P by standard AOAC [7] procedures. The neutral detergent fiber, acid detergent fiber, and acid detergent insoluble N were determined by the methods of Goering and Van Soest [8]. The nutrient compositions of the three feeds for cows and goats are shown in Table 1. Each observation was the average of a pair of duplicate composite samples from each batch of diet or byproduct.
2.4. Treatments and Animal Care
Beginning 5 wk after freshening, all 36 subjects (18 Holstein cows and 18 French Alpine does) were offered the basal diet (Table 1) for ad libitum intake in a preliminary 14-d period. For each of the three feeds tested, one lactating Alpine doe and one lactating Holstein cow were assigned to each of six different sequences of three 56-d feeding treatments consisting of low, medium, and high doses of the basal diet; a medium and high dose of rice bran; or a medium and high dose of hominy feed. Table 2 shows feeding treatment sequences and exact
Table 1. Feed nutrient analyses for basal diet, rice bran and hominy feed consumed by cows and goats.
Table 2. Feeding treatment sequences1.
doses. Assignments to sequences were balanced to cancel out period effects and to reduce bias caused by stage of lactation or by possible fluctuations in alfalfa quality over time. For each species, treatment assignments resulted in 30 observations of the basal diet: 18 at the low, 6 at the intermediate, and 6 at the highest dose. For each species, there were 12 observations for each byproduct tested, 6 at each dose in addition to the basal rations.
Cows were housed in individual paddocks, which provided separate areas for eating, rest, exercise, and defecation. Cows were milked twice daily beginning at about 0700 and 1800 h. The 18 goats were housed together outdoors in a large exercise yard, except when they were confined to individual feeding stations (elevated dairy calf pens modified to prevent access to the adjacent goat’s feed for 2 h after the morning feedings (900 h) and for 10 h between evening feedings (2100 h) and morning milkings. Cows and goats were provided water free choice at all times.
Increments of byproduct feed were mixed thoroughly by hand into the basal portions of each offering. Because dry matter offered was set as less than the estimated maximum voluntary intake, refusal of feed was infrequent. When refusal occurred, chemical analysis of orts showed that selection of diet components was not significant in either species.
2.5. Calculations and Statistical Analysis
The net energy used by each individual for each feeding period was calculated as the sum of 1) energy in milk secreted during the feeding period, 2) the change in estimated body energy that feeding period, and 3) the HP estimated from the average body weight during the feeding period.
The net energy content of the basal diet for each individual was calculated as the net energy used divided by the dry matter consumed. The mean of 30 basal diet net energy determinations for each species was used to calculate the net energy of the byproducts by difference. To determine the net energy of rice bran and hominy feed, the net energy attributable to the basal diet was subtracted from the total net energy used, and the remainder was divided by the amount of byproduct dry matter consumed. These net energy values are found in Table 3. Student’s t test was used to contrast means for goats and cows [9]. Null hypotheses of similar species means were accepted at P > 0.05 for the basal diet and both commodities.
3. RESULTS AND DISCUSSION
The net energy means for the basal diet were similar among cows (5.73 MJ/kg), does (5.98 MJ/kg), and a proportional combination of NRC [10] estimates for ingredients (6.11 MJ/kg) (Table 3). The measured values for the rice bran were only about 6% greater than the NRC tabular value (7.11 MJ/kg for cows and 7.07 MJ/kg for does vs. 6.69 MJ/kg from NRC). Although all rice bran came from the same supplier at the same time, the test rice bran fed to cows was richer in fat than the NRC [10] value (17.4 vs. 15.1%), and the rice bran fed to the goats contained a little less fat (13.9%). This small difference in fat content may be contributed to the lower NEL estimated for goats. The NEL for hominy feed (8.20 MJ/kg) for goats compared well with the NRC value (8.41 MJ/kg), but the NEL concentration for hominy feed (6.99 MJ/kg) measured in cows was lower than the NRC value or that measured in goats, but that difference was not statistically significant.
Table 4 contains data showing that dry matter intake and body weight were similar for each period. As expected, milk energy production decreased with time.