Protein level in concentrates for dairy cows in subtropical pasture systems supplemented with maize silage

Abstract

Proc. Aust. Soc. Anim. Prod. 1994 Vol. 20 PROTEIN LEVEL IN CONCENTRATES FOR DAIRY COWS IN SUB-TROPICAL PASTURE SYSTEMS SUPPLEMENTED WITH MAIZE SILAGE R.J. MOSSA, P.R. MARTINB and N.D. CHAPMAN A *Dept of Primary Industries, Mutdapilly Research Station, M.S. 825, Ipswich, Qld. 4306 BDept of Primary Industries, Animal Research Institute, Yeerongpilly, Qld. 4105 SUMMARY The effects on milk production of substituting cottonseed meal (CSM) (43% crude protein (CP)) for grain (13% CP) in iso-energetic supplements fed to autumn calved Holstein-Friesian cows were investigated. Animals grazed a sub-tropical pasture system of rain grown tropical grass and irrigated clover/ryegrass. Maize silage was offered to appetite. Concentrates were fed once daily at 6 kg/cow.day throughout lactation, supplying approximately 13, 15, 16.5, 18 or 22% CP. Inclusion of CSM in the supplement increased milk yields from 5800 L for cows fed grain alone to 6480 L for cows offered supplements with 16.5% CP or higher, but had no effect on milk composition. Keywords: milk, supplementation, protein, tropical pasture, maize silage. INTRODUCTION Productivity of cows in sub-tropical pasture systems can be improved by increased use of fertiliser, temperate forages and concentrate feeding (Moss and Lowe 1993). The primary limitation to production is intake of digestible energy, though cows can also respond to additional protein as energy supplementation is increased. Requirement for additional protein will be influenced by the protein content of the forage and the level of concentrate fed. To investigate the adequacy of forage protein for dairy cows in subtropical feeding systems, a series of studies were conducted with autumn and spring calving cows over 3 consecutive lactations. Responses to inclusion of cotton seed meal (CSM) in isoenergetic concentrates fed to cows grazing fertilised rain grown tropical pastures in summer/autumn and irrigated temperate forages in winter/spring were examined. In the initial studies, increasing the CSM content of the concentrate increased milk yield and composition of cows grazing tropical grass pastures for the major part of their lactation, but had no effect on production of autumn calving cows whose predominant forage source was irrigated temperate pastures (Moss et al. 1992). This paper describes the 3rd lactation for the autumn calved cows when maize silage was also fed to increase the energy content of the animals' diet. MATERIAL AND METHODS The experiment was conducted at Mutdapilly Research Station 80 km south west of Brisbane (27'46' 152'40' altitude 40 m). Average rainfall is 800 mm/year. This study was conducted from S, E, spring 1989 to late summer 1993 and covered 3 complete lactations. Pastures were established on heavy alluvial black (Ug 5.16) and grey (Ug 5.24) clay soils. Rain grown permanent pastures of Rhodes grass (Chloris gayana cv. Pioneer (70%) and Callide (30%)) provided feed in summer-autumn and a separate area of irrigated ryegrass/clover was used in winter-spring. This pasture consisted of annual ryegrass (Lolium mcrltiflorum cv. Concord) and Shaftal clover (Trifolium resupinatum) oversown into stand-over white clover (Trifolium repens cv. Haifa) from the preceding year. Each pasture received annual applications of 22.5 kg phosphorus/ha and 60 kg potassium/ha at the start of its growing season. Ryegrass/clover was fertilised with 50 kg nitrogen (N)/ha as ammonium sulphate at planting and as urea after first and second grazings in winter. Rhodes grass received a total of 300 kg N/ha as urea applied between September and March each year. Rhodes grass pastures were stocked at 2 cows/ha, and irrigated temperate pastures at 5 cows/ha and grazed between morning and afternoon milkings from June to early December. Thirty multiparous Holstein-Friesian heifers, half calving in autumn (April-June) and the remainder in spring (September-November) each year, were stratified on calving date, production and age and randomly allocated to 5 treatments based on level of protein in concentrate supplements. Cows remained in a treatment throughout the project unless removed because of infertility or misadventure when they were replaced by animals of equivalent status. COWS grazed pastures in rotation as a single herd and were 0 individually fed 6 kg concentrate once daily after mornin, milking throughout lactation. Cottonseed meal (43% crude protein (CP)) was substituted for grain (G) (rolled sorghum or barley as available, 13% CP), in the ratios of 100/O, 20/l, S/l, 5/l and 2.4 G/l CSM to provide iso-energetic rations (13 MJ ME/kg 124 Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 DM) varying in crude protein content. Cottonseed meal was selected to increase protein content of the concentrate while maintaining similar energy levels and because it can provide both rumen degradable and undegraded protein (UDP) for intestinal digestion providing an estimated 50% UDP (Moss and Murray 1992). In year 3 maize silage was offered to appetite at night with the Rhodes grass pasture from May 1992, the level varying with pasture availability to ensure cows were offered a minimum of 20 kg dry matter (DM)/day as green pasture plus silage. Milk yields were recorded at 2 consecutive milkings each week and a composite sample analysed for butterfat, protein and lactose (Milkotester Mk III, Foss Electric). Yield and composition of pasture on offer at each grazing during the growing season were determined by hand cutting and sorting. Oven dried samples of pasture on offer were bulked for each rotation cycle and analysed for protein content and in vitro dry matter digestibility (IVDMD). Rumen liquor samples were collected from cows by stomach tube immediately after morning milking (0700 hours, before supplementation) and before afternoon milking (1430 hours, 6 hours post supplementation) in winter-spring (2 September) and summer (28 January) and analysed for ammonia (NH$. These times were selected to cause minimal disturbance to the cows' normal routine and because they would represent approximately equilibrium levels for the differing diets (Moss and Murray 1992). Milk production data were analysed by analysis of variance, separately for cows in each calving season. RESULTS Rhodes grass pastures in autumn and spring provided 1.5 t green leaf/ha or about 21 kg leaf/cow.day (Table l), but with drought conditions from January 1993, green leaf and protein contents of the pasture declined (Table 1). Feed on offer from the temperate pastures was lower at lo-15 kg/cow.day with higher protein and digestible energy contents. Maize silage contained 7.5% CP and an IVDMD of 670 g/kg (9.5 MJ ME/kg DM). Voluntary intake of maize silage was high when cows grazed tropical pasture in early lactation and decreased as availability of temperate pasture increased. Intake of silage was negligible in November-December when quality and quantity of both temperate and tropical pastures were high, and increased as the aualitv of tropical pastures declined with lack of rain from Januarv to April (Table 1). Table 1. Yield and composition of pasture on offer and levels of feed available In the first 2 lactations, increasing the CSM content of the concentrate did not affect milk yield or composition of cows calved in autumn and grazing temperate forages for the major part of their lactation (M OSS et al. 1992). Milk yields for autumn cows in year 2 were 5765, 5616, 5748, 5803 and 5830 125 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 L/lactation (SED 2 485; P = 0.99) respectively for cows receiving increasing levels of CSM in their concentrate. In year 3 when cows also received maize silage, milk and 4% fat corrected milk yield were lowest for cows receiving grain alone and increased with increasing level of CSM in the concentrate (Table 2). There were no consistent effects on milk composition but yields of butterfat, milk protein and lactose were all increased as CSM was increased (Table 2). Rumen ammonia levels in all cows were low with values of 12.6 and 7.1 mg NH,-N/100 mL at 1430 and 0700 hours respectively in unsupplemented, non-lactating cows grazing clover/ryegrass pastures during the day and offered 5 kg maize silage with frosted Rhodes grass pastures overnight. In lactating cows concentrate supplementation reduced these levels. Rumen ammonia concentration increased with increasing level of CSM in the concentrate, averaging 4.7 and 10.2 mg NH,-N/100 mL with concentrate of 13 and 22% CP respectively. Table 2 Effect of cottonseed meal (CSM) in concentrates on yield and composition of milk produced by Holstein-Friesian cows supplemented with maize silage DISCUSSION Maize silage increased milk production of autumn calved cows compared with the preceeding lactation only when protein level in concentrates was above 16.5% and cows produced above 6000 L from a sub-tropical pasture based feeding system. The 10% increase over previous lactation is below the 18% predicted by Kerr et al. (1991) but productivity of our pasture based system was already high. By feeding maize silage to appetite according to pasture yield and quality it is estimated that these COWS would have consumed about 2 t DM in 1 year for a response of approximately 600 L milk/cow, a similar response to that achieved by Moran and Stockdale (1992). Our earlier studies showed that increasing the protein content of concentrates had no effect on milk production of COWS grazing temperate forages (Moss et al. 1992). The 11% increase in milk yield in the current experiment with supplements containing 16.5% CP and higher is similar to responses obtained with tropical grass pastures (Davison et al. 1990; Moss et al. 1992) though with tropical grass pastures both milk yield and concentrations of milk solids were increased (Moss et al. 1992). In the present study increasing protein level in the concentrate had no consistent effect on milk composition. Energy density of the diet would have been higher than that of cows grazing tropical pastures and cows had access to sufficient energy to meet production requirements provided protein in the diet was adequate (Schwab et al. 19%). Low rumen ammonia levels in cows grazing clover/ryegrass suggest part of the response to added protein may have been a direct effect on rumen microbial growth. With high maize silage intakes in 126 Proc. Aust. Sot. Anim. Prod. 1994 Vol. 20 autumn and winter (5-10 kg DM) rumen ammonia levels were below those recommended for optimum microbial growth (Satter and Slyter 1974). Cottonseed meal has the potential to supply protein postruminally (Kempton et al. 1977) but can be substantially broken down in the rumen (Moss and Murray 1992). Increasing the supply of CSM in the concentrate would have increased the amount of rumen degradable protein in the diet. Protein supplementation may have improved efficiency of digestion and increased feed intake. Added dietary protein as CSM will increase milk production of cows grazing temperate pastures supplemented with substantial amounts of maize silage. ACKNOWLEDGMENTS We gratefully acknowledge the support of the staff at Mutdapilly Research Station for management of the animals and assistance with experimental procedures, and at the Animal Research Institute, Yeerongpilly for chemical analyses. This research was supported by the Dairy Research and Development Corporation. REFERENCES DAVISON, T-M., JARRETT, W.D. and CLARK, R. (1990). Aust. J. Exp. Agric. 30: 451-5. KEMPTON, T-J., NOLAN, J.V. and LENG, R.A. (1977). WZdAnim. Rev. 22: 2-10. KERR, D-V., COWAN, R.T. and CHASELING, J. (1991). Agric. Syst. 35: 313-20. MORAN, J.B. and STOCKDALE, C.R. (1992). Aust. J. Exp. Agric. 32: 279-85. MOSS, R.J., EHRLICH W.K., MARTIN P.R. and McLACHLAN B.P. (1992). Proc. Aust. Sot. Anim. Prod. 19: 100-2. 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