NUTRITION, FEEDING, AND CALVES

Effect of Supplemental Dietary Fat or Protein on the Short-Term Milk Production Response to Bovine Somatotropin

J. L. VICINI, G. F. HARTNELL, J. J. VEENHUIZEN,
R. J. COLLIER, and L. MUNYAKAZI
Animal Sciences Division
Monsanto Agricultural Group
St. Louis, MO 63198

ABSTRACT

Effects of supplemental energy or protein on the milk production response to bST administration were examined in two separate trials. In trial 1, 40 cows were used in a 2 x 2 factorial, completely randomized design to determine the effects of bST and fat supplementation. The study consisted of a 7-d pretreatment period and a 42-d treatment period. Fat was top-dressed at 3.0 Mcal/d of NEL, and bST was administered. Supplemental fat had no effect on milk production, and NEL intakes were unaffected. Administration of bST increased milk production by 7.1 kgld, and the milk production response was unaffected by supplemental fat. In trial 2, 4 cows were used in four periods with a 2 x 2 factorial arrangement in which water or casein was infused into the abomasum of cows fed for ad libitum intake or at 80% of their requirements. Diets and infusions were initiated simultaneously and continued for 11 d. All cows were given bST during the last 5 d. Infusion of water or casein did not alter the milk production response to bST, but restricted feeding reduced the bST response (3.2 vs. 7.2 kg/d). Concentrations of IGF-I in plasma were increased by bST administration, and the increase was greatest for cows fed for ad libitum intake. The milk production response to bST was not increased by additional energy or protein offered to cows fed well-balanced diets.

(Key words: somatotropin, fat, bypass protein, nutrition)

INTRODUCTION

Administration of bST increases milk production of lactating cows with a concomitant increase in feed intake (3). Although factors affecting feed intake appear to be important for long-term bST responses, feed consumption increased with a variety of feeding management systems, including pasture (6, 23). The improvement in feed intake does not completely account for the increased nutrient requirements for additional milk until several weeks after the initiation of treatment. Therefore, mobilization of body nutrient stores or reduced accumulation of these stores is required to provide the additional nutrients during this period. Differences in adipose tissue stores were apparent in one study (4) in which treated cows had significantly less total body fat than control cows after 8 wk of bST treatment and prior to the increase in feed intake.

Some investigators have conducted studies to determine whether dietary changes might increase milk production responses to bST administration. These studies included changing the energy content of the diet (12, 13, 15, 24, 25) or changing the amount or quality of dietary protein (5, 12, 16). Addition of these nutrients could improve the early portion of the bST response or limit BW loss if cows actually consumed additional quantities of limiting nutrients. The addition of fat may supply more energy to increase milk production or to limit the change in body composition. Protein supplementation may be beneficial because of the limited supply of AA mobilized from body tissues.

This study was conducted as two separate trials. The objective of the first trial was to determine the effects of supplemental dietary fat at the time of bST administration. In the second trial, the effect of postruminal infusion of casein, a high quality protein was examined.

Fat Supplementation Study

Forty multiparous Holstein cows were blocked by DIM into two blocks of equal size, and cows were assigned randomly within blocks to one of four treatments. At the beginning of the treatment period, cows in the two blocks ranged from 50 to 134 DIM and from 142 to 208 DIM, respectively. Treatments were 1) basal diet, 2) basal diet plus fat, 3) basal diet plus bST, and 4) basal diet plus fat plus bST. The study consisted of a 7-d pretreatment period followed by a 42-d treatment period.

=======table1 page2==========

*The TMR offered in fat supplementation study based on milk production and body condition score at beginning of study.

2 All values expressed on a DM basis except for DM, which is a percentage of wet weight.

3 Purina Mills (St. Louis, MO).

4 Values in parentheses are nutrient composition for diets fed in casein infusion study; NT = not tested.

Throughout the study, cows were fed one of two TMR (Table 1). based on milk production of each cow at the start of the study (19). Diets allowed for approximately 10% orts. Fresh feed was offered twice daily after each milking, and fat, when fed, was top-dressed with each offering of feed at 114 g during the first six feedings (3 d) and at 227 g for all other feedings (39 d). This amount of supplemental fat supplied 1.5 or 3.0 McaVd of NEL. The fat source used was a commercial product consisting of hydrolyzed animal fat (Alifet@; Alifet USA, Inc., Cincinnati, OH). Orts were collected and weighed once daily in the p.m. Composites of feed and orts were collected and used to calculate nutrient consumption. Cows were housed in a tie-stall barn and released into an exercise lot prior to the a.m. and p.m. milkings.

The bST (480 mg) was the Ala-1-Va1126 variant and was administered S.C. at 2-wk intervals as compressed pellets prior to the p.m. milking. This method of administration results in a 14-d release profile (10).

Composite milk samples were collected weekly at p.m. and a.m. milkings and preserved with bromopol (D & F Control Systems, Inc., San Ramon, CA). Samples were analyzed for fat and protein contents (Dairy Lab Services, Dubuque, IA). Composition of feed and orts was analyzed from composites of samples collected weekly for each diet (Livestock Nutrition Laboratory Services, Columbia, MO). Plasma samples were collected weekly via coccygeal venipuncture beginning l wk prior to treatment initiation in the a.m., prior to the time that feed was offered. Samples were analyzed for IGF-I by RIA (26). Intraassay and interassay coefficients of variation were 8.2 and 9.8%, respectively. The BW were determined weekly after the a.m. milking. Body condition (27) was scored weekly (1 = thin to 5 = obese) in .25-unit increments by two independent scorers.

Data were analyzed by ANOVA for a completely randomized block design. Pretreatment values were used as covariates when appropriate. Single degree of freedom comparisons were used to determine effects of fat supplementation, bST, and their interaction. Differences were declared significant at P < -05. There were no effects (P > .05) of the blocking factor (DIM), and, therefore, means are not presented for this effect.

TABLE 2. Effect of supplemental dietary fat with or without bST administration on milk production and feed intake.

===========table2 page=2—-==============

1 Significant single degree of freedom contrast (P < .05) for effects of supplemental fat, bST (B), or their interaction.

2 Difference between BW or body condition score at beginning and end of treatment.

3 Body condition scored (1 = thin to 5 = obese) in increments of .25.

Protein Infusion Study

Data were analvzed bv ANOVA. Means for Four ruminally fistulated multiparous Holstein cows between 70 and 100 DIM were used. The study was conducted as a randomized block design in which each of four complete blocks encompassed 11 d with 7 d between blocks. Treatments were 1) ad libitum feed plus water infusion, 2) ad libitum feed plus casein infusion, 3) restricted feed plus water infusion, and 4) restricted feed plus casein infusion. Infusions were administered for all 11 d of each block, and bST was administered during the last 5 d.

All cows were fed a TMR (diet 2 in Table 1) formulated to meet or to exceed NRC (19) nutrient requirements. Between periods, cows were fed the same diet for ad libitum intake. For restricted treatments, cows were offered amounts that provided 80% of their requirement for NEL based on BW and milk production prior to imposition of treatments within each block. Infusions were administered postruminally at 6 L/d, and casein was administered at 450 g/d, as described by Cohick et al. (7). The Ala-1-Va1126 bST was solubilized at 10 mg/ml in sterile water, and 40 mg were injected S.C. daily.

Composite a.m. and p.m. milk samples were taken during each period prior to bST initiation and at the end of the period. Samples were analyzed as described. Blood samples were collected daily from the coccygeal vein at 12 h after the bST treatment. Plasma was analyzed for IGF-I as described, and intraassay and interassay coefficients of variation were 6.5 and 5.0%. respectively.

Data were analvzed bv ANOVA. Means for the 3 d prior to 6ST trkatment were used to estimate main effects and their interactions prior to bST treatment. The bST response was estimated by the difference between means from the last 3 d of each period and the 3 d prior to bST administration. Single degree of freedom comparisons were used to determine effects of feed intake, casein infusion, and their interactions. Differences were declared to be significant at P < .05.

RESULTS

Fat Supplementetlon Study

Administration of bST increased milk production and 3.5% FCM production by 7.1 and 9.7 kg/d, respectively, compared with that of untreated cows (Table 2). The top-dressing of ruminally protected fat did not affect milk production of cows fed the basal diet and did not improve the milk production response to bST administration. Milk fat percentage was significantly higher when cows were given bST but was unaffected by fat supplementation.

Feed consumption was not significantly affected by bST administration (Table 2) during the 42-d treatment period, and cows given supplemental fat did not consume additional NEL, in spite of the additional 3.0 Mcal offered by the top-dressing of fat. Based on the composition of feeds and orts, cows ate the top-dressed fat in proportion to the total quantity of feed consumed. During the 42-d treatment period, cows in all treatment groups gained BW, but those given bST gained 19.2 kg less BW than untreated cows. These data illustrate that cows do not always go into negative energy balance during the period immediately following initiation of bST treatment. Supplemental fat did not alter the change in BW during the treatment period. Changes in body condition during the study reflected the change in BW.

Concentrations of IGF-I in plasma were higher for bST-supplemented cows than for controls for samples collected 1 wk after bST administration, when concentrations should have been near maximal, and at 2 wk postinjection, when concentrations were at their nadir (Figure 1). Fat supplementation did not affect concentrations of IGF-I.

Casein infusion Study

As expected, milk production was higher when cows were fed for ad libitum intake than when cows were fed DM to supply 80% of their NEL requirement (Table 3). Similarly, the milk production response to bST was higher for cows fed for ad libitum intake than for those offered restricted amounts of feed. Postruminal infusion of casein failed to alter milk production significantly and did not affect the milk production response to bST administration. Fat percentage of milk tended (P < .lo) to increase with bST treatment for cows fed restricted amounts offered.

As dictated by treatments, DMI and intakes of CP and energy were significantly lower when cows were offered restricted quantities of feed (Table 4). Similarly. postruminal infusion of casein increased intake of total CP by cows.

Plasma IGF-I concentrations in the period prior to bST administration were unaffected by diet or postruminal infusion of casein (Figure 2). Concentrations of IGF-I increased in response to bST administration for all treatment groups. This change in IGF-I was greater (P c .05) for cows fed for ad libitum intake than for cows fed restricted amounts of feed. Although the mean IGF-I response was higher for cows infused with casein for both amounts of feed intake, this difference was not significant.

DISCUSSION

Achieving an overall increase in energy consumption, regardless of bST treatment, is a major reason for addition of dietary fat. This objective can be accomplished not only by increasing the energy density of the diet, such

TABLE 3. Effect on milk production of postruminal infusion of casein to cows fed for ad libitum intake or fed restricted amounts of DM before bST administration and the change because of bST.

===========table3 page=4==============

1 restricted diets were offered at 80% of feed required to meet NRC (19) requirement.

2 Water (6 Ud) or casein (454 g/d) infused postruminally.

3 Significant (P < .OS) single degree of freedom contrast for effects of feed intake 0, casein infusion, or their

4 Mean for 3 d prior to bST administration.

5 Means for last 3 d of bST treatment minus 3 d prior to bST treatment.

NUTRIENT SUPPLEMENTS AND SOMATOTROPIN RESPONSE

Figure 1. Concentrations of IGF-I from plasma of cows fed diets with or without supplemental fat and with or without bST administration. Treatments are basal diet @), basal diet with supplemental fat e), basal diet and bST (0). and basal diet with supplemental fat and bST (0). Each point is the mean from 10 cows, and the overall mean (* SEM) is 201 f 7.5 nglml.

TABLE 4. Effect on DMI, CP, and energy of postruminal infusion of casein for cows fed for ad libitum intake or fed restricted amounts before bST administration and change due to bST.

====table4 page=5========

1 Restricted diets were offered at 80% of feed required to meet NRC (19) requirement.

2 Water (6 Ud) or casein (454 gld) infused postruminally.

3 Significant (P e .05) single degree of freedom contrast for effects of feed intake 0, casein infusion (0. or their

4 Means for 3 d prior to bST administration.

5 Means for last 3 d of bST treatment minus 3 d prior to bST treatment.

6 Consumed plus infused. interaction.

as addition of dietary fat, but also by increasing total feed consumption. Addition of fats that are not ruminally inert can inhibit ruminal fermentation, resulting in a depression in DMI and nullifying the increased energy density of the diet (8). The fat source used in the present study was a hydrolyzed animal fat, and saturated fats do not inhibit ruminal fermentation (8). However, increasing the energy density of the diet by top-dressing the diet with ruminally inert fat did not change intakes of NEL. Responses were similar for other studies (12, 20) in which ruminally inert fats were added to the diet and cows reduced their intake of other dietary components, resulting in no change in total energy intake.

The short-term milk production response to bST administration is at the expense of BW gain because the increased nutrient intake does not meet the requirements of the incremental milk production for several weeks. The hypothesis tested herein was that changing the diet to increase energy consumption might increase short-term milk production response to bST or retard BW changes. The amount of supplemental fat offered to the cows would have supplied an additional 3.0 Mcal/d of NEL if cows had consumed the same amount of the other dietary ingredients. This additional energy could have accounted for more than one-half (4.4 of the 7.1 kg) of the additional milk produced in response to bST. Without the additional nutrients, approximately .7 kg/d of mobilized body fat, or a reduction in a similar amount of BW gain, would be required to account for the energy needed to produce the 7.1 kg/d of additional milk based on the published energy value (28) of BW gain for dairy cows. This change in daily BW gain is in close agreement with the difference of .5 kgld between actual BW gains of control and bSTtreated cows. However, in our study, supplementation with fat did not improve energy intake, and the milk production responses to bST and BW change were similarly unaffected.

The milk production response to bST was not improved in other studies (12, 13) when cows were given dietary fats. Schneider et al. (24) reported that the combination of ruminally

(Figure 2. Concentrations of IGF-I from plasma of cows fed restricted or ad libitum quantities of feed and given postruminal infusions of water or casein. Cows fed restricted diets were offered an amount of feed that supplied 80% of NRC (19) requirements for energy. Treatments are restricted feed plus water infusion (0). restricted feed plus casein infusion (0). ad libitum feed plus water infusion @), and ad libitum feed plus casein infusion (1). Each point is the mean from 4 cows, and the overall mean f SEM is 181 f 5.9 ng/ml.)

inert fat and bST resulted in a synergistic enhancement in milk production; however, their tables indicated that the F test for this interaction was nonsignificant (P > .IO). Also, in the design by Schneider et al. (24), cows were given bST beginning 2 wk postpartum, and fats were not fed until wk 4 to 8 or 10 to 14. Therefore, the relevance of their data (24) to practical use of fats and bST is not apparent. Currently, the only approved recommendation for bST administration is to begin treatment when cows are in wk 9 of lactation. Data from several studies indicate that bST administration in very early lactation results in little to no increase in milk production. Cows given bST prior to peak lactation are usually in negative nutrient balances, and cows deprived of feed or in early lactation may completely or partially uncouple bST for increasing milk production (see IGF discussion). A lack of milk production response to bST in early lactation was observed in two additional studies (15, 25), regardless of whether cows were given a basal diet alone or a diet supplemented with fat.

Postruminal infusion of casein did not increase the milk production response to bST. McGuffey et al. (16) obtained a higher milk production response to bST administration when ruminally undegradable protein was increased from 33 to 40%. In that study (16), the increase in milk production cannot be attributed entirely to the change in ruminally degradable protein because the major carbohydrate sources also were changed. Peel et al. (22) and Lynch et al. (14) did not obtain an improved milk production response to bST administration by postruminal infusion of casein or Met and Lys, respectively. Also, milk responses to bST administration were not improved when the major protein source was changed to a more ruminally undegradable protein (5).

In the study of Lormore et al. (12), additional energy and undegradable protein did not improve the milk production of cows treated with bST from 25 to 150 DIM. Net energy intake and body condition scores were not improved by the additional nutrients when cows were supplemented with bST (12). In one additional study (I), addition of energy and protein by use of extruded soybeans and longchain fatty acids actually reduced the incremental milk production response to bST treatment from 5 to 20 wk postpartum compared with that of cows fed a control diet.

Supplemental energy or protein greater than that of a well-balanced diet did not improve the milk production response to bST; however, the bST response of cows that were underfed in the protein infusion trial was only about one-half of that for cows fed for ad libitum intake (3.2 vs. 7.2 kg/d). This finding is consistent with those of other studies (11, 18) in which little to no increase in milk production followed bST administration when cows were not adequately fed. The diminished response of cows beginning bST treatment when nutrients are limited is consistent with IGF-I changes in the blood. The bST response for FCM was less affected by feed restriction because of the nonsignificantly greater fat percentage obtained when cows fed restricted diets were given bST. Treatment with bST may increase fat percentage when cows are treated at or near negative energy balance (2).

In the first trial. concentrations of IGF-I in plasma were increased by bST administration but were unaffected by supplementation with fat (Figure 1). This result is not surprising, considering that fat supplementation did not alter intake of NEL. In the second trial, IGF-I concentrations in plasma were unaffected by treatments during the pre-bST period but were significantly elevated during the period of bST administration (Figure 2). Although not significant, concentrations were greatest when cows were fed for ad libitum intake and when casein was infused. This result is consistent with data of McGuire et al. (17), which showed that the IGF-I response to bST administration was highest when cows were fed diets high in protein and energy. Similarly, Vicini et al. (26) demonstrated that the IGF-I response to bST was greater for late lactation cows in positive nutrient balance than for those in negative nutrient balance during early lactation. The differential regulation of the bST response with differences in nutrient supply may be regulated by the number of hepatic bST receptors. Hard et al. (9) measured more 1251-labeled bST binding to hepatic membrane fragments from cows fed diets high in energy and protein compared with fragments from cows fed medium or low energy and protein diets. Uncoupling of the IGF-I response to bST administration during periods of deficient nutrient intake may act as a safety factor to prevent cows from increasing milk production in response to supplemental bST when the nutrient supply is limited (3).

The experiments reported herein were designed to examine only the short-term bST responses. These types of studies should be distinguished from long-term studies because the responses in the longer studies are influenced by the magnitude of increase in DMI. In several studies, effects of bST administration on milk production response were examined under a wide variety of nutritional and management conditions [reviewed in (6)]. Even though the production of control cows in those studies was variable, the bST response has been fairly consistent when cows at similar stages of lactation and similar dosing regimens were compared. Similarly, the bST response has been unchanged in studies in which protein or fiber source was changed or when supplemental vitamins and minerals, buffers, or other additives were included in the diet. The major factor dictating the milk production response to bST when cows are fed wellbalanced diets apparently is maximization of DMI. This factor was evident in studies (11, 18, 21) in which the ability of bST-treated cows to increase feed intake was limited; in those studies, the bST response was lower than expected or essentially nonexistent. As with any high producing dairy cow, conditions that maximize DMI, such as access to fresh feed, cow comfort, and forage quality, appear to be the critical factors in optimization of the longterm bST response. Changes in the diet have only been indicated when composition is inadequate to meet NRC (19) guidelines.

CONCLUSIONS

Data from the present study demonstrate that, in short-term studies, when cows were fed well-balanced diets for ad libitum intake, the inclusion of additional amounts of protein or energy did not improve the increase in milk production because of bST administration. In contrast, cows that were fed inadequate amounts of nutrients, especially energy, had lowered bST responses.

ACKNOWLEDGMENTS

The technical support of T. L. Curran, P. K. Olsson, and the staff of Dardenne Dairy Center is greatly appreciated.

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Source: Monsanto Agricultural
Author: Vicini, Hartwell