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The effect of milk intake on forage intake and growth of nursing calves

Department of Animal Science, Cornell University, Ithaca, NY 14853

	Abstract

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Thirty-nine Holstein steer calves were assigned to one of five treatments at birth and individually fed for 200 d with milk replacer reconstituted to equal the fat and protein concentration of beef cow milk. Treatment levels were the quantities of reconstituted milk fed per day based on lactation curves, which were based on peak milk levels (PML) of 2.72, 5.44, 8.16, 10.88, and 13.6 kg/d, respectively. In addition to reconstituted milk, chopped alfalfa hay was offered ad libitum to allow for maximal voluntary forage consumption. All calves were fed a high-energy diet postweaning until they reached a similar degree of fatness in the 12th rib (4 to 5% chemical fat) as determined by ultrasound. There were differences (P < 0.05) among groups in weaning weight, preweaning ADG, age, and weight at slaughter. During the preweaning phase, there was a linear relationship (P < 0.01) for daily milk and forage DE intake; however, DE intake per unit of BW did not differ across treatments (P = 0.06). Increasing PML resulted in a linear (P < 0.01) decrease in alfalfa hay intake in the preweaning phase, and G:F increased quadratically (P < 0.01). During the postweaning phase, preweaning milk intake had no meaningful effect on postweaning ADG, but overall ADG had a linear relationship (P < 0.01) with preweaning milk level. There was no effect of PML on the 12th-rib lipid percent, marbling score, or quality grade, but protein and fat concentration in the carcass and empty BW increased linearly (P < 0.01) with PML. The group fed at 2.72 kg/d PML was 58 kg lighter (P = 0.03) and required 34 d more (P < 0.01) to reach the predetermined degree of fatness at slaughter than the group fed at 13.6 kg/d PML, suggesting that increased milk production by the dam can decrease the number of days to the slaughter weight at which a similar rib lipid concentration is reached.

Key Words: Dry Matter Intake • Forage Intake • Milk Intake • Nursing Calf Growth

	Introduction

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Forage intake by nursing calves is influenced by amount of suckled milk (Baker and Barker, 1978) and by availability and quality of forage (Wright and Russel, 1987). The range of correlation between level of milk consumption and weight gain among calves varies from 0.4 to 0.88 (Totusek and Arnett, 1965; Wyatt et al., 1977). Forage intake by nursing calves increases as lactation declines, and subsequently, applies greater pressure on the forage management program (Fox et al., 1988). It has also been shown that forage intake per unit of BW before weaning is consistently increased for calves receiving low quantities of milk (Le Du et al., 1976; Broesder et al., 1990), and that the consumption of milk decreases herbage intake (Baker et al., 1976).

Few experiments have evaluated the performance by calves nursing controlled amounts of milk, while consuming forage, and the subsequent effect on the feedlot finishing phase. Therefore, the objectives of this study were to evaluate the effect of milk intake on forage intake, pre- and postweaning growth, and body composition of calves. Due to their availability at birth, Holstein bull calves were used as a model.

	Materials and Methods

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Preweaning Phase

Humane animal care and use procedures were followed (Cornell University Institutional Animal Care and Use Committee Protocol 86–209). Thirty-nine Holstein bull calves from the Cornell University dairy herd were individually-fed with nurse pails one of five levels of milk based on Eq. [1] (Wood, 1967), with the adjustments proposed by George (1984) and Fox et al. (1988) for mature beef cows, assuming five peak milk levels (PML: 2.72, 5.44, 8.16, 10.88, and 13.66 kg/d):

[1]

where MY_t is milk yield at time t, kg/d; t is days after parturition; and A, b, and c are parameters of the equation proposed by Wood (1967). The coefficient A is a general scaling factor associated with the average daily milk yield at the start of lactation.

Calves were fed reconstituted milk substitute (18% DM; Table 1) and ad libitum chopped alfalfa hay from birth for 200 d. This approach avoided technical problems associated with establishing and maintaining a range of known milk and forage intakes over an extended period in a group of naturally suckled beef breed calves. The composition of the milk replacer and chopped alfalfa hay is shown in Table 1. Within 24 h after parturition, as they became available from the Cornell dairy herd, the calves were weighed, fed 7.3 kg/d of colostrum from the Cornell dairy herd, and assigned randomly to one of five PML.

Postweaning Phase

Calves were weaned as they reached 200 d of age and were switched to a TMR high-energy diet (Table 2) containing 33 ppm of monensin formulated for three stages of growth (136 to 227, 227 to 318, and 318 kg of BW until market) with the Cornell Net Carbohydrate and Protein System (CNCPS; Fox et al., 1992). Diet CP (DM basis) level at weaning was 16% and was decreased to 14 and 12% at 227 and 318 kg BW, respectively. Calves were fed individually in the same pens described previously and were weighed at 2-wk intervals until slaughter. Animals were slaughtered as they reached an ultrasonic lipid concentration of 4 to 5% in the 12th rib. Based on the results of Abdalla et al. (1988), a sound reflectance or attenuation reading, which is the loss of signal energy as a function of reduced tissue penetration as fat concentration increases, of 0.4 to 0.5 was taken to be indicative of the 4 to 5% chemical fat in the LM. The attenuation reading to determine degree of lipid was measured with an ultrasonic scanner (GE Co. Medical Systems Group, Milwaukee, WI). Empty body weight (EBW) was estimated from chilled carcass weight (CCW) based on Eq. [2] (Fox et al., 1976):

[2]

where EBW and chilled carcass weight are expressed in kilograms.

The 9th- to 11th-rib section was removed as described by Hankins and Howe (1946) using their skeletal reference points. The 9th- to 11th-rib section was weighed and separated into soft tissue and bone with ligaments. The soft tissue was chopped with a knife and ground in an Alexanderwerk Schneidmeister (model M-40, Horsham, PA) silent cutter and two 0.5-kg homogenized samples were frozen. The samples were then ground with dry ice to pass a 2-mm screen of Wiley mill (Thomas Scientific, Swedesboro, NJ) as described by Adballa (1986). Protein was determined using the standard macro-Kjeldahl method (AOAC, 1990), and fat was analyzed using the chloroform-refluxing Soxhlet apparatus. The percentages of protein and lipid in the rib section were used to calculate carcass protein and carcass fat as shown in Eq. [3] and [4], which were developed from Holstein calves (Nour and Thonney, 1994) similar to those used in the present study:

[3]

[4]

where Fat_Rib and Protein_Rib are the fat and protein concentration of the 9th- to 11th-rib section.

The predicted carcass fat and protein values were used to predict empty body fat (EBF, % of EBW) and empty body protein (EBP, % of EBW) using Eq. [5] and [6], respectively (Garrett and Hinman, 1969):

[5]

[6]

Initial fat and protein composition at the beginning of the trial was assumed to be similar to those reported by Diaz et al. (2001). They reported fat and protein values for Holstein calves at birth of 4.7 and 20.9% of EBW, respectively. Initial EBW was assumed to be 40.4 kg (Diaz et al., 2001). Fat and protein deposition was then determined from the difference between predicted initial and final concentration of fat and protein in the EBW. Retained energy (RE, Mcal) was computed based on fat and protein deposition multiplied by the caloric value of fat and protein, respectively (Lofgreen, 1965), as indicated in Eq. [7] to [9]:

[7]

[8]

[9]

where FG is fat gain (kg); PG is protein gain (kg); DOF is days on feed; and RE is expressed in megacalories per day.

The ME value of diets and alfalfa hay were assumed to be 82% of the DE values (NRC, 1996, 2000). From the energy and protein gained, the total efficiency of ME (k_ME) was determined by Eq. [10]:

[10]

where RE is as defined above; k_ME is efficiency of ME to RE; and MEI is ME intake (Mcal/d).

Digestibility and Blood Measures

Acid detergent insoluble ash (ADIA) was used as an internal marker (Porter, 1987) by collecting a fecal sample from each steer over 5 d near the end of the preweaning period to determine the influence of milk on forage digestibility. Samples were analyzed in duplicate for ADIA, as described by Van Soest (1994). The five fecal samples were composited and a 1- to 4-g sample of feed or feces (sample size was increased as necessary to insure a 10 mg of ash residue) was refluxed in 100 to 250 mL acid detergent solution (Goering and Van Soest, 1970) for 1 h, filtered through a 50-mL Gooch crucible (coarse porosity), and ashed at 500°C for 12 h. This was repeated during the postweaning period to establish the DM digestibility of the high-energy diets. Diet digestibility percentage was calculated as shown in Eq. [11]:

[11]

The preweaning forage digestibility was determined by assuming that the ADIA concentration in milk was zero because there is no fiber in milk with which ADIA is associated. We assumed the milk digestibility was 95% (Diaz et al., 2001). Feed samples (milk powder, alfalfa hay, and postweaning dietary ingredients) were analyzed for CP using the standard macro-Kjeldahl method (AOAC, 1990). Protein solubility and degradability were determined according to Krishnamoorthy et al. (1983). The NDF and nonsequential ADF concentration were determined according to Goering and Van Soest (1970). The residual N in the NDF samples was measured by the standard Kjeldahl method (AOAC, 1990).

Serum samples were obtained at 1 mo of age, weaning, 1 mo after weaning, and before slaughter and analyzed for insulin and triiodothyronine (T3). Insulin and T3 concentrations were determined by RIA insulin and T3 kits (Micromedic Systems, Inc., Horshem, PA).

Statistical Analyses

All statistical analyses were conducted with PROC GLM of SAS (SAS Inst., Inc., Cary, NC). A one-way analysis of variance was used to analyze the data. The experimental units were steers. Orthogonal polynomials (linear, quadratic, cubic, and quartic) were used to assess the effect of PML on response variables. Residual plots showed that the residuals were normally distributed.

	Results and Discussion

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Preweaning Phase

Data for animal growth and consumption of reconstituted milk and alfalfa hay are shown in Table 3. Calves had quadratic increases in ADG and weaning BW due to milk level (P < 0.01; Table 3). The average daily milk DMI for 2.72, 5.44, and 8.16 kg/d PML (0.38, 0.79, and 1.16 kg of DM/d, respectively) were similar to the target amount based on Eq. [1] (0.39, 0.79, 1.19 kg of DM/d, respectively); however, for 10.88 and 13.66 kg/d PML (1.49 and 1.77 kg DM/d, respectively), they were lower (P < 0.01) than the target amount (1.59 and 2.01, respectively). Some calves in 10.88 and 13.66 kg/d PML rejected a portion of the milk offered daily, and some calves had diarrhea for few days, indicating those milk levels at early ages exceeded calf milk intake capacity. The target ratio of milk DMI was 1:2:3:4:5, and the observed ratio was 1:2:3:3.9:4.6 for 2.72, 5.44, 8.16, 10.88, and 13.66 kg/d PML, respectively.

Overall, calves fed 13.66 kg/d PML consumed 1.39 kg more milk DM/d, grew 0.48 kg/d faster, and had 96 kg greater weaning BW (Table 3) than calves fed 2.72 kg/d PML (P < 0.01). Calves fed 8.16 kg/d PML averaged 0.78 kg more milk DM/d, and had 0.378 kg/d and 75 kg greater ADG and weaning weight, respectively, than calves fed 2.72 kg/d PML (P < 0.01). The increased growth rates of calves with increased milk intake observed in this study agree with data reported in the literature on calf growth (Baker et al., 1976; Le Du et al., 1976; Wyatt et al., 1977). Appleman and Owen (1975) reported that higher milk levels produce faster initial gains but not after 12 to 16 wk (84 to 112 d) of age.

The G:F value increased quadratically as milk level increased (P < 0.01; Table 3), suggesting the average amount of energy available from milk and alfalfa was higher in the high milk level intake groups. This was expected because milk DE was greater than the average alfalfa DE concentration (4.87 vs. 3.18 Mcal/kg, respectively; Table 3) and, although milk intake increased, alfalfa intake decreased linearly (P < 0.01; Table 3). The G:F did not improve at the same rate as milk intake, however. Milk DMI as a percentage of the total DMI increased linearly through 10.88 kg/d PML; however, the rate of increase in ADG averaged five percentage units per PML increase after 8.16 kg/d PML compared with a higher rate of change from 2.72 to 5.44 kg/d PML (0.56 to 0.785 kg/d; 40%) and from 5.44 to 8.16 kg/d PML (0.785 to 0.938 kg/d; 19%). This finding suggests that when PML was increased above 8.16 kg/d, it resulted in a decreasing rate of increase in ADG. The decreased rate of improvement in ADG and G:F to 200 d at high milk intake is at least partly due to a heavier average weight for calves fed at higher levels of milk.

The forage DM digestibility was 73.1% for 2.72 kg/d PML compared with 69.9% for 13.66 kg/d PML. This change with increasing PML, however, was not significant (P = 0.15). Baker et al. (1976) and Le Du et al. (1976) also found the level of milk in the diet did not affect the digestibility of the herbage consumed by calves. There were linear (P < 0.01) effects of PML on milk DE intake (DEI), alfalfa DEI, and total DEI, and quadratic effects on milk DEI (P < 0.01) and total DEI (P = 0.02). Calves fed the 13.66 kg/d PML consumed 462% more milk DEI, 150% more total DEI, and 46% less daily alfalfa DEI than the calves fed at the 2.72 kg/d PML at the same age.

The quantity of daily DEI per kilogram of BW was similar among PML (Table 3), averaging 78.3 kcal/kg. This finding indicates that across PML, calves had the same intake of DE/kg of BW, but those consuming a higher proportion of DEI as milk gained faster (Table 3). Feed composition data (NRC, 1978) indicate the efficiency of DE to NE_g is 35 and 23% for milk and alfalfa hay, respectively. The NRC (2001) indicated that in milk-fed calves, the efficiency for DE to ME for milk and calf starter are 96 and 88%, respectively; the efficiency for ME to NE_g is 69% for milk and 57% for calf starter. This would be expected due to less methane and heat increment loss per unit of DE in milk.

Treatment mean values were used for further analysis of the preweaning performance with the equations of Guiroy et al. (2001) to allow comparisons accounting for differences in DMI, diet DEI, BW, and composition of gain. The weight at 28% EBF was computed with the data in Table 4, and the diet DE values in Table 3 were used to predict diet NE_m and NE_g; NE_m required was assumed to be 77 kcal/kg^0.75 of shrunk BW for Holstein calves fed in this facility, based on Ainslie et al. (1993). Observed/expected G:F are shown in Table 3. Calves fed the lowest PML had lower than expected G:F; all other groups had 5 to 11% higher G:F than predicted. Metabolizable protein requirements for each treatment were computed with the NRC (2000) equations. Diaz et al. (2001) found that the NRC (2000) equations accurately predict net protein retained in milk-fed calves during early growth. The MP supplied by the diet was computed by assuming that digested alfalfa carbohydrate and protein fractions are degraded in the rumen and provide microbial protein at 13% of TDN, but based on the studies of Diaz et al. (2001), all the milk protein escapes ruminal degradation and is 89% MP. Metabolizable protein required for the observed ADG appeared to be deficient in all diets; however, the ideal AA pattern in milk protein may have allowed for more efficient growth as milk intake increased. Infusion of casein increased N retention 22% in Holstein steer calves (Houseknecht et al., 1992).

Growth and feed intake data for the postweaning period and over the entire experiment are summarized in Table 5. Although the quartic effect was significant, milk intake had no consistent effect on postweaning ADG. Nonetheless, as PML increased, calves started the postweaning period at increasing weights so that days required to reach 4 to 5% lipid concentration in the LM decreased (P < 0.01), even though final BW increased (P = 0.03). Postweaning DMI increased quadratically (P = 0.03) and DEI increased linearly (P = 0.01) with increasing PML due to higher mean BW, but there was no effect on DEI/BW.

The CNCPS (Fox et al., 2004) was used to evaluate the postweaning performance to account for differences in initial and final BW, composition of ADG, and final EBF. The G:F was 12% higher than expected for those fed the low PML, whereas the other groups were within 1 to 5% of expected G:F, suggesting the low PML group had some compensatory growth during the finishing period. These results agree with those of Abdalla et al. (1988), who found that Holstein calves fed a low-protein diet during early growth exhibited compensatory growth during the finishing period.

Composition

Predicted carcass and empty body composition data based on actual 9th- to 11th-rib section composition are summarized in Table 4, along with predicted efficiencies of protein and energy utilization. There was no significant effect of PML on 12th-rib lipid percent, marbling score, or quality grade, indicating that the use of ultrasound resulted in harvesting the steers at the same rib lipid and quality grade endpoint; however, carcass weight and EBW increased linearly (P = 0.04) as PML increased. In addition, fat thickness and numerical yield grade increased linearly (P = 0.03), whereas carcass and EBP concentrations decreased and fat concentration increased (P = 0.01) as PML increased. In the empty body gain, concentration of protein decreased (P < 0.05), whereas the concentration of fat increased (P < 0.05) linearly as PML increased. Guiroy et al. (2001) indicated that EBF increases 1% for each 14.26-kg increase in EBW as growing cattle approach or exceed 28% EBF. When their adjustment is applied to the current data, those on the highest PML had lighter weights at 28% EBF (Table 3), indicating the higher concentrations of fat in the empty body gain resulted in lighter EBW at 28% EBF. Thus, the higher EBF at the target quality grade for the three highest PML was a result of increased BW, as well as a higher concentration of fat in the gain. It is possible that the calves fed higher PML had higher EBF at the same quality grade due to a younger age at slaughter.

Although there was a linear increase (P < 0.01) in RE and MEI with increasing preweaning milk intake, the total efficiency of RE to ME was similar among PML at approximately 23%. These results are consistent with those of Abdalla et al. (1988), who reported overall efficiency of energy use was not affected by previous plane of nutrition in Holstein calves.

Serum insulin concentrations increased linearly (P = 0.01) with PML at the weaning sampling time, but there was no significant effect due to preweaning milk level during postweaning growth (Table 6). Serum T₃ concentration increased with PML during the first month (P = 0.01; Table 4) and the first month postweaning (P = 0.02; Table 6). The increasing serum insulin and T3 concentrations with increased milk intake during the first month of life suggest increased release of those hormones in response to increased daily nutrient intake and levels of absorbed propionate and glucose.

	Implications

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¹ Correspondence—phone: 607–255–7712; fax: 607–255–9829; e-mail: dgf4{at}cornell.edu.

Received for publication August 13, 2004. Accepted for publication January 6, 2005.

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