PHYSIOLOGY AND MANAGEMENT

Effects of Bovine Somatotropin and Evaporative Cooling Plus Shade on Lactation Performance of Cows During Summer Heat Stress

M. TARAZO? N-HERRERA,1 J. T. HUBER,2 J. SANTOS,3
H. MENA,4 L. NUSSO,5 and C. NUSSIO5
Department of Animal Sciences,
University of Arizona, Tucson 85721-0038

ABSTRACT

Thirty-two Holstein cows (8 per treatment) averaging 195 d in milk were assigned to 70 d of treatment on the basis of production during a 14-d pretreatment period, which was used for covariate analysis. The experiment was a randomized block design with a 2 ? 2 factorial arrangement of treatments. Factors were normal shade or shade plus evaporative cooling with pressurized spray, plus with or without the administration of bovine somatotropin (bST). Cows receiving bST were injected with 500 mg of bST every 14 d. All cows were fed the same total mixed rations twice daily at approximately 10% in excess of appetite, and water was offered free choice. There were no interactions between bST and the cooling system for any of the variables measured. Milk yield was increased by bST and tended to be greater for cooled cows. Fat percentages were increased by bST, and yields of fat, protein, and 3.5% fat-corrected milk, and the efficiency of conversion of dry matter to milk, whereas evaporative cooling increased body weights and protein yields, but decreased SNF and milk protein percentages. Rectal temperatures and respiration rates also were lower for cooled cows. And, bST increased nonesterified fatty acids in blood serum, suggesting that a part of the energy for increased milk production came from mobilization of body fat. Administration of bST effectively improved performance of cows under hot summer conditions whether evaporatively cooled or not.

(Key words: bovine somatotropin, evaporative cooling, summer heat stress)

Abbreviation key: KK = Korral Kool., THI = temperature- humidity index.

INTRODUCTION

Meteorological conditions, environmental modifications, and animal factors influence milk production of cows (3, 12). Important meteorological factors affecting milk yield are temperature, humidity, wind, radiation, and photoperiod (1). Ambient temperature and humidity are combined to derive the temperature-humidity index (THI; 9). When the THI exceeds 72, high producing cows become heat stressed, with severe heat stress occurring at a THI above 80 (31). Cows that are heat stressed for extended periods (at THI above 80 for 5 to 6 wk) produce 25 to 35% less milk if they are not cooled to reduce thermal discomfort, which often manifests itself by a decrease in feed intake (16).

Heat stress results when dairy cows are exposed to hot or hot and humid environments, which causes increased maintenance costs and decreasedDMI and milk yield (16).

High producing cows have more metabolic activity and produce more body heat than low producers; thus, a higher milk yield may increase heat stress if the cause of stress is not mitigated (30). In addition, heat stress of lactating cattle dramatically reduces roughage intake and rumination (16). Such decreases in roughage intake contribute to decreasedVFA production and may contribute to an alteration in the ratio of acetate to propionate (16). Several studies have shown that evaporatively cooled cows had lower rectal temperatures and respiration rates than those that were not cooled (2, 3, 4, 11). A greater DMI for cooled cows was associated with higher milk production (2.5 kg/d of increase) compared with those receiving only shade (11).

Environmental modifications that can alleviate severe heat stress in dairy cattle include water spray and fans (2, 3), evaporative cooling, or the Korral Kool. (KK) system (Mesa, AZ) (2, 3, 4, 27). Cows cooled with KK, which involves the release of a pressurized spray above the cows in a shaded area, produced 2.3 kg of milk/d more than when cooled with spray and fans (3). However, a later study (2) showed that cows cooled with spray and fans had greater milk yields (37.6 kg/d) than those cooled with KK (36.2 kg/d).

The administration of bST does not change maintenance requirements or partial efficiencies of milk yields (24). Cows treated with bST exhibited slightly higher rectal temperatures during the hot summer months than did control cows, but no difference in response to bST was noted during periods of moderate versus high ambient temperatures (29). The greater heat stress reported in some studies when bST was used (30) was probably due to increased metabolic activity and heat production associated with higher milk yield. However, it was suggested (18, 30) that even though bST increases heat production, it also increases heat dissipation. Management strategies are needed that will minimize the effects of heat stress and that will maintain sufficient DMI to sustain the potentially increased milk yields because of bST technology (30). The objective of this study was to determine responses to bST in heat stressed (shade only) or shaded and evaporatively cooled cows.

MATERIALS AND METHODS

Thirty-two lactating Holstein cows averaging 195 DIM and 33.6 kg of milk/d were assigned to each of four treatments in a randomized block design with a 2 ? 2 factorial arrangement of treatments. Blocks were based on milk production during a 14-d pretreatment period during which all cows were fed the normal herd ration. Factors were normal shade versus evaporative cooling plus shade (KK), and injection versus no injection of bST. Thus, the four treatment combinations (8 cows per treatment) were shade without bST injection (SH), shade plus injection (SH + bST), shade plus evaporative cooling without injection (KK), and shade plus evaporative cooling with injection (KK + bST). Cows were injected in the tailhead with 500 mg of bST (Monsanto Co., St. Louis, MO) every 14 d. Treatments were balanced for DIM and numbers of primiparous versus multiparous cows. Pretreatment milk production was used for covariate adjustments in statistical analyses. Treatment occurred from June 1 to August 9, 1996, at the University of Arizona Dairy Research Center in Tucson.

The diet (DM basis) contained 1.7 Mcal of NE/kg (23), 16.6% CP, and 21.2% ADF. Ingredients were 45% chopped alfalfa hay, 41% steam-flaked sorghum grain, 10% whole cottonseed, 3% mineral-vitamin mix, and 0.8% prilled fatty acids (Energy Booster., Milk Specialities, Inc., Dundee, IL).

Cows were housed in open dirt lots (10 ? 38 m) equipped with shades and Calan gates (American Calan, Inc., Northwood, NH). Feed was offered at 10% in excess of appetite, and individual feed intakes were recorded daily. The diet was fed twice daily as a TMR. Before treatment and at weekly intervals during treatment, cows were weighed and body condition score (BCS; 32) was assessed. Ambient temperatures and humidities were measured twice weekly during the pe riod of hottest daily temperatures (1400 to 1530 h), and

THI was calculated as follows:

THI = 0.45 T + 0.55 TH – 31.9 H + 31.9

where

T = dry bulb temperature expressed in °F and H = relative humidity/100, according to Chambers (9).

Rectal temperatures and the respiration rates of individual cows also were determined at the same time as ambient temperatures.

Samples of TMR and refusals (orts) were collected weekly and analyzed forDMand CP according to AOAC (5), ADF and NDF according to Robertson and Van Soest (26), and starch by enzymatic hydrolysis using the starch and sugar autoanalyzer (YSI 2700 Select, Yellow Spring Instrument Co., Yellow Spring, OH) according to procedures of Poore et al. (25).

Heparinized blood samples (20 ml) were collected weekly from the coccygeal vein and centrifuged within 30 min, and plasma was stored at –20°C until analyzed for NEFA according to Johnson and Peters (19).

Cows were milked twice daily at 0430 and 1630 h, and milk yields were recorded at each milking. Individual milk samples were collected weekly from two consecutive milkings (a.m. and p.m.), and daily composites were analyzed for fat, protein, lactose, and SCC by infrared procedures (5; Foss 360, Foss Technology, Eden Prairie, MN) at the Arizona DHIA Laboratory in Phoenix. The content of SNF was determined by difference.

Individual cow means for DMI, feed efficiency (FCM/ DMI); milk, fat, and protein yields; SNF, fat, protein, and lactose percentages; SCC; 3.5% FCM; BW; BCS; NEFA; rectal temperature; and respiration rate were adjusted for covariate effects with the data from the 14-d pretreatment period. They were analyzed by the general linear models procedures (GLM) of SAS (28), using the following statistical model:

Yijkl = M + Bi + Sj + Ck + SCjk + COV1 + Eijkl

where: Yijkl = observation, M = overall mean, Bi = blocking effect, Sj = bST effect, Ck = evaporative cooling effect, SCjk = interaction between Sj and Ck, COV1 = covariate effect, and Eijkl = random residual error.

RESULTS AND DISCUSSION

Ingredient and nutrient composition of the diet are in Table 1. All nutrients in the TMR were balanced to meet requirements (23) of the higher producers in the experiment, which had milk yields of about 41 kg/d.

( TABLE 1. Ingredient and nutrient composition of the diet. table1

Items	Amount
Ingredient, %of DM
Alfalfa hay	45.0
Whole cottonseed	10.0
Stream-flaked sorghum	41.2
Prilled fatty acids1	0.8
Mineral and vitamin premix2	3.0
Nutrients
DM,%	88.0
CP,%	16.6
NEl.Mcal/kg3	1.70
Starch	36.7
ADF,%	21.2
NDF,%	25.9

1 Energy Booster. (Milk Specialties Co., Dundee, IL).

2Ingredients = 29% molasses, 25% NaHCO3, 22% dicalcium phosphate, 13% MgO, 7% NaCl, 1% niacin, 0.5% Zinpro., 2000 mg of Zn/ kg, 1300 mg of Mn/kg, 333 mg of Cu/kg, 29 mg of I/kg, 10 mg of Se/ kg, 3.3 mg of Co/kg, 67,000 IU of vitamin A/kg, 6,700 IU of vitamin D/kg, and 700 IU of vitamin E/kg. 3Based on NRC (23).)

Milk yield was increased (P < 0.01) by bST and tended (P < 0.07) to be greater for cooled cows (Table 2). When cows were in shade only, the addition of bST to the diet increased milk yield 10%, but when cows were cooled, the response to bST was 12.4%. Cows injected with bST were 7.2% higher in milk yields when they were evaporatively cooled than when not cooled; but when bST was not administered, the cooling of cows tended to increase milk yield only 4.7%. However, the increase was 17.7% when cows were cooled with bST versus cows in shade only without bST. Although the response to bST in milk production was in the range of reported results (8, 14, 15, 17), the effects of bST and evaporative cooling in the present experiment were smaller than expected because of the high THI during the summer of 1996 compared with previous years (10).

Percentage of fat (P < 0.05) and yields of fat, protein, and 3.5% FCM, and the efficiency of feed conversion to milk were increased (P < 0.01) by the addition of bST, but protein and SNF percentages were not affected by treatment (Table 2). No change in milk composition in response to bST (6, 15, 18, 20, 22) was observed, but the yield of components was increased. The percentage of lactose was not affected by bST as reported earlier (6, 15), probably because lactose is the osmotic factor of milk synthesis and is required in proportion to the amount of milk produced.

Evaporative cooling increased (P < 0.05) fat, protein, and FCM yields, and decreased protein and SNF percentages, but did not affect the percentage of fat or the efficiency of feed conversion to milk. Other research (2, 3, 4, 11) has shown little or no effect of evaporative cooling on percentages of milk components, but component yields were generally increased because of higher milk yields. Neither cooling system nor bST affected SCC, and large variations in SCC values were observed. Moreover, all treatment groups had some cows that developed either mastitis or foot lesions, which would increase SCC of milk.

The intake of DM was not significantly affected by bST or evaporative cooling, but some variations in selectivity of individual feeds were observed among treatments. The administration of bST usually increases DMI several weeks after commencement of injections to sustain the increased milk production (9, 14, 15, 24). In this study, bST resulted in 5% higher DMI in cooled cows but less in noncooled cows. Alleviation of heat stress often increases feed consumption and overcomes

TABLE 2. Effect of bST and evaporative cooling on performance of heat-stressed cows.

Treatment1effect
Item	SH	SH+bST	KK	KK+bST	SEM	bST	KK	bST * KK
(N=8 cows/treatment)
Milk yield,kg/d	27.7	30.4	29.0	32.6	0.45	0.01	0.07	NS3
DMI,kg/d	23.4	23.8	23.4	24.6	0.36	0.33	0.67	NS
3.5%FCM,kg/d	26.4	29.0	27.4	31.3	0.39	0.01	0.05	NS
FCM/DMI	1.10	1.24	1.14	1.31	0.02	0.01	0.21	NS
Fat,
%	3.14	3.26	3.13	3.31	0.03	0.05	0.77	NS
kg/d	0.88	0.98	0.91	1.07	0.01	0.01	0.04	NS
Protein,
%	3.12	3.12	3.05	3.05	0.02	0.99	0.05	NS
kg/d	0.87	0.95	0.88	0.99	0.01	0.01	0.04	NS
Lactose,%	4.97	4.94	4.89	4.95	0.02	0.77	0.45	NS
SNF,%	8.68	8.67	8.51	8.59	0.03	0.49	0.03	NS
SCC,cells/ml(x 10 3)	316	347	213	389	39.2	0.51	0.82	NS

1 Treatments: SH = shade only, without bST, SH + bST = shade only plus bST, KK = shade and Korral Kool (Mesa, AZ) system without bST, and KK + bST = shade and KK plus bST; 8 cows/treatment for 70 d.

TABLE 3. Daily temperatures, humidities, and THI from the experimental site, taken at 1330 h and 1530 h.1

SH KK
		T	H	THI	T	H	THI
Avg. pre-treatment		35.2	14.0	75.2	…	…	…
Treatment
June	4	37.2	18.0	80.5	33.3	25.0	78.1
	6	37.2	18.0	80.5	33.9	26.5	79.0
	11	37.5	18.0	80.8	32.5	27.5	77.7
	13	36.7	40.0	84.8	34.4	40.0	82.3
	18	41.4	28.0	87.3	31.9	33.5	78.1
	20	37.8	32.5	84.4	29.4	50.0	7.8
	25	38.1	24.5	82.9	28.1	38.5	74.4
	27	35.6	31.5	81.7	26.4	48.5	73.6
July	2	41.1	22.5	85.5	34.4	41.5	82.6
	4	39.7	28.5	85.6	31.7	34.5	78.0
	9	33.3	31.0	79.1	31.1	44.0	78.8
	11	37.8	20.0	81.5	35.0	45.0	84.0
	23	37.8	12.5	79.8	35.38	37.0	83.3
	25	38.3	30.0	84.4	32.5	35.5	79.1
	30	41.7	29.0	87.9	36.7	30.0	82.7
Aug.	1	35.0	27.0	80.1	29.2	27.0	74.0
	6	41.7	30.0	88.1	32.2	38.0	79.2
	8	40.0	28.0	85.8	33.3	36.0	80.2
Avg. treatment		38.2	26.1	83.4	32.3	36.6	79.0

1 T = Temperature, H = humidity, THI = temperature-humidity index, SH = shade only, and KK = shade and Korral Kool (Mesa, AZ) system without bST.

The negative nutrient balance caused by excessive heat (13, 16). The reason that DMI was not increased by cooling may be because heat stress was only partially alleviated (Table 3).

The effects of bST and evaporative cooling on changes in BW and BCS, NEFA in plasma, rectal temperatures, and respiration rates are shown in Table 4. Changes in BCS were not affected by bST or evaporative cooling. Most research has shown little or no effect of bST on BCS (14, 15, 17), and little information is available on the effects of evaporative cooling on BCS. Neither were changes in BW affected by bST, which agrees with (14, 16); however, BW was affected negatively by bST in some studies (21, 22). Less (P < 0.05) loss ofBWoccurred in evaporatively cooled cows. Changes in BW and BCS are related to energy balance, so the tendency toward increased milk in cooled cows with no effect on intake should have caused a greater energy deficiency in these cows. The reduced loss ofBWbecause of cooling, coupled with increased milk suggests that alleviation of some heat stress increased efficiency of utilization of dietary energy. The average difference in BW change between cooled and noncooled cows was 0.2 kg/d, which should furnish sufficient NEL for production of 1.6 kg/d of milk (23). Evaporative cooling increased milk yield an average of 1.7 kg/d.

Rectal temperatures and respiration rates were not affected by bST, but they were decreased (P < 0.01) by evaporative cooling, which agrees with many trials conducted with heat-stressed cows (1, 2, 3, 4, 11, 16).

TABLE 4. Effect of bST and evaporative cooling on measurements of heat-stressed cows.

	Treatment1					Effect
Item	SH	SH+bST	KK	KK+bST	SEM	bST	KK	bST+KK
(n=8 cows/treatment)
BW change,kg/d	-0.16	-0.12	0.00	0.12	0.05	0.37	0.04	NS2
BCS 3 change	0.08	0.06	0.06	0.14	0.03	0.61	0.69	NS
NEFA,meq/L	0.14	0.21	0.15	0.21	0.01	0.01	0.82	NS
Rectal temperature,°c	39.5	39.4	38.7	38.8	0.04	0.88	0.01	NS
Resp. rate, breaths/min	84.4	86.2	67.3	65.7	0.83	0.95	0.01	NS

1 Treatments: SH = shade only, without bST, SH + bST = shade only plus bST, KK = shade and Korral Kool (Mesa, AZ) system without bST, and KK + bST = shade and KK plus bST; 8 cows/treatment for 70 d. 2NS = Not significant (P > 0.10). 3BCS = Body condition score.

The concentrations of NEFA in plasma were not affected by the cooling system but were increased (P < 0.01) by bST, suggesting greater mobilization of adipose tissue to supply the extra nutrients required for the increased milk production. Cows in early lactation generally have greater concentrations of NEFA because of a negative energy balance that stimulates mobilization of adipose tissue. A lag time for feed intakes to adjust to accommodate greater milk yields resulting from bST administration was shown previously (7, 24). Cows in mid or late lactation fed balanced diets generally do not mobilize tissue fat because the milk production is declining and the cow is gaining body fat in preparation for the coming lactation. In the present study, increased fat mobilization of bST-treated cows was probably because of the heat stress, which was only partially alleviated by evaporative cooling.

In Table 3, ambient temperatures, humidities, and THI taken at the experimental site are given. The data suggest that evaporative cooling was not sufficient to completely eliminate heat stress in cows because maxima for THI measured under the cooling system remained high enough to decrease milk yield (78.9), according to (1, 31). However, THI might not accurately reflect heat stress in evaporative cooling systems that deliver a pressurized spray with considerable air movement above the cow’s back, resulting in higher humidities but also causing a strong cooling effect. The signifi- cant decreases (P < 0.01) in rectal temperatures and respiratory rates of cows showed that a partial alleviation of heat stress resulted from evaporative cooling, the effect of which was confirmed by the increased milk production and reduced BW losses.

CONCLUSIONS

Data from this study show that the administration of bST and evaporative cooling during hot summer months increases yields of milk and FCM of lactating Holstein cows. Fat percentage and yields of fat, protein; 3.5% FCM; and efficiency of feed conversion to milk also were greater in cows treated with bST. Evaporative cooling decreased rectal temperatures and respiration rates but increased yields of milk, fat, protein, and FCM. The concentrations of NEFA in blood serum tended to be higher in cows treated with bST, which suggests that a part of the response in milk production to bST in heat-stressed cows is at the expense of body fat mobilization.

ACKNOWLEDGMENTS

The authors wish to thank the Monsanto Company (St. Louis, MO) for the provision of the recombinant bST used in this experiment. The help of M. Townsend for grain processing, G. Ghenniwa, and M. Pessarakli for assistance in laboratory analyses, and L. George- Smith for aid in developing the manuscript also are appreciated. Finally, deep appreciation is expressed to Consejo Nacional de Ciencia y Tecnologı?a (CONACYT, Mexico City, Mexico) and the Universidad de Sonora for the scholarship awarded to M.A. Tarazo?n-Herrera, the senior author of this manuscript.

REFERENCES

1 Armstrong, D. V. 1994. Heat stress interaction with shade and cooling. J. Dairy Sci. 77:2044–2050.

2 Armstrong, D. V., S. K. DeNise, F. J. Delfino, E. J. Hayes, P. J. Grundy, S. Montgomery, and A. Correa. 1993. Comparing three lactational performances of Holstein cows in hot weather. J. Dairy Sci. 64:844–849.

3 Armstrong, D. V., F. Wiersma, T. J. Fuhrmann, J. M. Tappan, and S. M. Cramer. 1985. Effect of evaporative cooling under a corral shade on reproduction and milk production in a hot arid climate. J. Dairy Sci. 68(Suppl. 1):167.(Abstr.)

4 Armstrong, D. V., M. E. Wise, M. T. Torabi, F. Wiersma, R. Hunter, and E. Kopel. 1988. Effect of different cooling systems on milk production of late lactation Holstein cows during high ambient temperature. J. Dairy Sci. 71(Suppl. 1):212.(Abstr.)

5 Association of Official Analytical Chemists. 1990. Official Methods of Analysis. 15th ed. AOAC, Arlington, VA.

6 Barbano, D. M., J. M. Lynch, D. E. Bauman, G. F. Hartnell, R. L. Hintz, and M. A. Nemeth. 1992. Effect of prolonged-release formulation of N-methionyl bovine somatotropin (sometribove) on milk composition. J. Dairy Sci. 75:1775–1793.

7 Bines, J. A., and I. C. Hart. 1982. Metabolic limits to milk production, specialty roles of growth hormone and insulin. J. Dairy Sci. 65:1375–1389.

8 Chalupa, W., B. Vecchiarelli, D. T. Galligan, J. D. Ferguson, L. S. Baird, R. W. Hemken, R. J. Harmon, C. G. Soderholm, D. E. Otterby, R. J. Annexstad, J. G. Linn, W. P. Hansen, F. R. Ehle, D. L. Palmquist, and R. G. Eggert. 1996. Responses of dairy cows supplemented with somatotropin during weeks 5 through 43 of lactation. J. Dairy Sci. 79:800–812.

9 Chambers, A. B. 1970. A psychometric chart for physiological research. J. Appl. Physiol. 29:406–408.

10 Chan, S. C., J. T. Huber, K. H. Chen, J. M. Simas, and Z. Wu. 1997. Effects of ruminally inert fat and evaporative cooling on dairy cows in hot environmental temperatures. J. Dairy Sci. 80:1172–1178.

11 Chen, K. H., J. T. Huber, C. B. Theurer, D. V. Armstrong, R. C. Wanderley, J. M. Simas, S. C. Chan, and J. L. Sullivan. 1993. Effect of protein quality and evaporative cooling on lactational performance of Holstein cows in hot weather. J. Dairy Sci. 76:819–825.

12 Collier, R. J., D. K. Beede, W. W. Thatcher, L. A. Israel, and C. J. Wilcox. 1982. Influences of environment and its modifications on dairy animal health and production. J. Dairy Sci. 65:2213–2227.

13 Collier, R. J., R. M. Ely, A. K. Sharma, R. M. Pereira, and D. E. Buffington. 1981. Shade management in subtropical environment for milk yield and composition in Holstein and Jersey cows. J. Dairy Sci. 64:844–849.

14 Gibson, J. P., B.W. McBride, J. H. Burton, I. Politis, and X. Zhao. 1992. Effect on production trials of bovine somatotropin for up to three consecutive lactations. J. Dairy Sci. 75:837–846.

15 Hartnell, G. F., S. E. Franson, D. E. Bauman, H. H. Head, J. T. Huber, R. C. Lamb, K. S. Madsen, W. J. Cole, and R. L. Hintz. 1991. Evaluation of sometribove in a prolonged-release system in lactating dairy cows production response. J. Dairy Sci. 74:2645–2663.

16 Huber, J. T.,G.Higginbotham, R. A. Gomez-Alarcon, R. B. Taylor, K. H. Chen, S. C. Chan, and Z. Wu. 1994. Heat stress interactions with protein, supplemental fat and fungal cultures. J. Dairy Sci. 77:2080–2090.

17 Huber, J. T., Z. Wu, C. Fontes, Jr., J. L. Sullivan, R. B. Hoffman, and G. F. Hartnell. 1997. Administration of recombinant bovine somatotropin to dairy cows for four consecutive lactations. J. Dairy Sci. 80:2355–2360.

18 Johnson, H. D., R. Li, W. Manalu, K. J. Spencer-Johnson, B. A. Becker, R. J. Collier, and C. A. Baile. 1991. Effects of somatotropin on milk yield and physiological responses during summer farm and hot laboratory conditions. J. Dairy Sci. 74:1250–1262.

19 Johnson, M. M., and J. P. Peters. 1993. Technical note: An improved method to quantified nonesterified fatty acids in bovine plasma. J. Anim. Sci. 71:753–756.

20 Laurent, F., B. Vignon, D. Coomans, J. Wilkison, and A. Bonnel. 1992. Influence of bovine somatotropin on the composition and manufacturing properties of milk. J. Dairy Sci. 75:2226–2234.

21 Lean, I. J., H. F. Troutt, M. L. Bruss, T. B. Farver, R. L. Baldwin, J. C. Gallard, D. Kratzer, C. A. Holmberg, and L. D. Weaver. 1991. Postparturient metabolic and production responses in cows previously exposed to long-term treatment with somatotropin. J. Dairy Sci. 74:3429–3445.

22 McGuffey, R. K., R. P. Basson, D. L. Snyder, E. Block, J. H. Harrison, A. H. Rakes, R. S. Emery, and L. D. Muller. 1991. Effect of somidobove sustained release administration on the lactation performance of dairy cows. J. Dairy Sci. 74:1263–1276.

23 National Research Council. 1989. NutrientRequirements of Dairy Cattle. 6th rev. ed. Natl. Acad. Sci., Washington, DC.

24 Peel, C. J., and D. E. Bauman. 1987. Somatotropin and lactation. J. Dairy Sci. 70:474–486.

25 Poore, M. H., J. A. Moore, R. S. Swingle, T. P. Eck, and W. H. Brown. 1991. Wheat straw or alfalfa hay in diets with 30% neutral detergent fiber for lactating Holstein cows. J. Dairy Sci. 74:3152–3159.

26 Robertson, J. B., and P. J. Van Soest. 1981. The detergent system of analysis and its application to human foods. Pages 123–158 in Analysis of Dietary Fiber in Food. Vol. 3. W.P.T. James and O. Theander, ed. Marcel Dekker, Inc., New York, NY.

27 Ryan, P. D., M. P. Boland, E. Kopel, D. Armstrong, L. Minyakasi, R. A. Godke, and R. H. Ingraham. 1992. Evaluating two different evaporative cooling management systems for dairy cows in hot, dry climates. J. Dairy Sci. 75:1052–1059.

28 SAS. User’s Guide: Statistics, Version 5 Edition. 1985. SAS Inst., Inc., Cary, NC.

29 Sullivan, J. L., J. T. Huber, S. K. DeNise, R. G. Hoffman, L. Kung, Jr., S. E. Franson, and K. S. Madsen. 1992. Factors affecting response of cows to biweekly injections of sometribove. J. Dairy Sci. 75:756–763.

30 West, J. W. 1994. Interactions of energy and bovine somatotropin with heat stress. J. Dairy Sci. 77:2091–2102.

31 Wiersma, F. 1990. Temperature-humidity index table for dairy producers to estimate heat stress for dairy cows. Dept. Agric. Eng., Univ. Arizona, Tucson.

32 Wildman, E. E., G. M. Jones, P. E. Wagner, R. L. Boman, H. F. Troutt, Jr., and T. N. Lesch. 1982. A dairy cow body scoring system and its relationship to selected production characteristics. J. Dairy Sci. 65:495–501.

Source: University of Arizona
Author: Tarazon-Herrera