Responses to protein supplementation by dairy cows grazing nitrogen fertilised forages.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 19 RESPONSES TO PROTEIN SUPPLEMENTATION BY DAIRY COWS GRAZING NITROGEN FERTILISED FORAGES R. J. MOSSA, W. K. EHRLICHA, P. R. MARTINB and B. P. McLACHLAN Mutdapilly Research Station, M.S. 825, Ipswich, Qld 4306. Animal Research Institute, Yeerongpilly, Qld 4105. A SUMMARY A study with 30 Holstein-Friesian cows and heifers at pasture was conducted to assess the effects on milk production of substituting cottonseed meal for grain in iso-energetic supplements containing 10, 12, 14, 16 and 20% crude protein. Cows calved either in spring or in autumn and grazed dryland Rhodes grass pastures in summer and intensive irrigated ryegrass in winter. The pastures were fertilised with nitrogen. Cottonseed meal increased fat-corrected milk yield, and yields of butterfat, protein and lactose in milk from cows grazing tropical grass pasture, but had no effect on yield or composition of milk from cows calved during autumn and grazing ryegrass pastures. Keywords: milk, supplementation, protein, tropical pasture, ryegrass. INTRODUCTION Dairy production systems in northern Australia often use tropical pastures during summer and temperate forages such as irrigated ryegrass during winter. Concentrates are given to increase the energy intake of cows, and at higher levels of concentrate feeding, dietary protein may be insufficient for optimum utilisation of high energy supplements. Responses to added bypass protein have been demonstrated with tropical pastures (Stobbs et al. 1977). In this study, a feeding system based on rain grown tropical pastures, irrigated annual ryegrass and grain based concentrates was established. Effects on milk yield of increasing the protein content of this supplement were examined. Cottonseed ` meal (CSM) was used to maintain energy content of the supplement whilst increasing its crude protein (CP) content. CSM can provide both rumen degradable and some slowly degraded protein to the cow (Moss et al. 1986). MATERIALS AND METHODS The experiment was conducted at Mutdapilly Research Station south-west of Brisbane (lat. 27O 46' S; long. 152O 40'E; ah. 40 m). Rain grown perennial Rhodes grass pastures [Chloris gayana cvv. Pioneer (70%) and Callide (30%)] provided summer feed, and a separate area of high density annual ryegrass (Lolium multijlorum cv. Aristocrat) was planted for winter feed. Each pasture received annual applications of 22.5 kg P/ha and 60 kg K/ha at the start of the growing season. Urea was applied to the Rhodes grass pastures at 300 kg nitrogen/ha.annum in 3 equal applications (September, December, February), while ryegrass pastures were fertilised with 50 kg nitrogen as urea and irrigated after each grazing (400 kg nitrogen/ha.annum). Rhodes grass pastures were grazed throughout the year at 2 cows/ha while irrigated ryegrass was stocked at 5 cows/ha and grazed between morning and afternoon milkings from June to November inclusive. Standover Rhodes grass pastures were grazed at night. Ten Holstein-Friesian cows plus 5 heifers calved in spring 1989 (September-November) and the same number in Autumn 1990 (April-June). Animals were stratified on age, production and calving date and randomly allocated to five treatments. Cows were fed 5.5 kg DM of concentrates once daily after the morning milking throughout lactation. Cottonseed meal (CSM) (42%CP) was substituted for grain (rolled sorghum (11% CP) or barley (10% CP)) to provide iso-energetic (NRC 1989) supplements containing about 10, 12, 14, 16 or 20% CP. Cows grazed pastures in rotation as a single herd. 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 were determined monthly for each pasture type by hand cutting and sorting. Botanical composition of the tropical pasture diet was determined using the microscope point-hit technique on samples collected by oesophageal fistulated cows in late surnmer. Rumen liquor samples were collected by stomach tube from cows immediately after the morning milking (before supplementation) and before the afternoon milking (5 h post feeding) in summer (December) and winter (July) and analysed for ammonia content. Data were analysed by analysis of variance. RESULTS Rhodes grass pastures provided over 3 t DM/ha and 1 t green leaf/ha on offer throughout the summer with an average CP content of 10% in the pasture on offer and 12% in the green leaf fraction (Table 1). Proc. Aust. Sot. Anim. Prod. Vol. 19 Table 1. Yield (kg/ha) and composition (%) of pasture on offer Oesophageally fistulated cows were able to select a diet containing 14% crude protein from these pastures in March (mid season). Feed on offer for annual ryegrass pastures was in excess of 1.7 t DM/ha throughout the growing season (June-November), with a CP content of 25-30% and in vitro digestible dry matter of 71-75%. Milk yields of spring calved cows grazing the tropical grass pastures were lowest for cows receiving grain only and tended to increase with increased level of CSM in the supplement (Table 2). Cows fed grain only produced milk with lower butterfat (P < O.lO), protein (n.s.) and lactose (P < 0.10) contents, and yields of these components were significantly greater for cows receiving supplements containing CSM meal at 14% CP and above (P < 0.05). Increased level of protein supplementation had no effect on milk yield or composition of cows grazed on irrigated annual ryegrass pastures (P > 0.05) (Table 2). Rumen ammonia levels of cows given no CSM and grazing Rhodes grass pastures in summer were low (< 8 mg NH,-N/100 mL) and increased with level of CSM in the supplement to 15 mg Table 2. Effect of supplement protein content and forage type on milk yield (L) and composition of Holstein-Friesian cows 101 Proc. Aust. Sot. Anim. Prod. Vol. 19 NH,-N/100 rnL (P < 0.01). Rumen ammonia levels in cows in winter were high (> 24 mg NH,-N/100 mL) across all treatments at the afternoon sampling but declined overnight (< 10 mg NH3-N/100 mL) when cows grazed frosted Rhodes grass pastures. DISCUSSION Quality of forage on offer from tropical grass pastures was lower than for irrigated ryegrass, and productivity of spring calved cows was considerably less than for autumn calved animals utilizing ryegrass for the major part of their lactation. These production differences support the conclusions of Stobbs (1971) that animal production is limited by intake of digestible nutrients from tropical pastures. Although energy is the primary limitation, protein content of the Rhodes grass pastures was always low and dietary intake of protein from these pastures was below the 15% recommended for lactating cows (Davison et aZ. 1991). Any protein deficit therefore would be increased by supplementation with cereal concentrates. Substitution of CSM for grain in iso-energetic supplements to increase dietary protein intake increased FCM yield, improved milk composition and increased the yield of milk components for cows grazing tropical grass pastures. The 9.4% milk yield response we obtained throughout lactation was similar in magnitude to that obtained in early lactation by Davison et al. (1990) with cows grazing tropical grass pastures and offered a supplement of molasses and meat and bone meal, and greater than measured with cows fed at a similar level with tropical grass legume pastures (Davison et al. 1991). However, our response was below the 20% obtained by Stobbs et al. (1977) with formaldehyde treated casein fed to cows grazing fertilised Rhodes grass pastures. Degradability of CSM is higher than formaldehyde treated casein or meat and bone meal, with a rumen degradability of 50% at 6 h and a residual crude protein at 24 h of 6% (Moss et al. 1986) but provides a similar energy density to grain. CSM in this experiment would have provided both rumen degradable protein and some post ruminal feed protein. Rumen ammonia concentrations in cows grazing tropical pastures were generally low and levels in cows receiving grain only may not have been sufficient for optimum microbial fermentation (Satter and Slyter 1974). Protein supplementation therefore may have improved efficiency of digestion and increased intake of tropical pastures. In contrast, increasing the level of CSM in supplements fed with intensive annual ryegrass pastures had no effect on milk production or composition. Rumen degradable protein was in excess in cows grazing irrigated ryegrass and any post ruminal digestion of the protein supplement had no effect on milk yield or composition of these cows. We conclude that additional dietary protein can increase milk production and yield of milk components when concentrates are fed at a moderate level with a tropical grass pasture. When protein content of the forage is high, as with annual ryegrass pastures, added protein in the concentrate is not required. ACKNOWLEDGMENTS We thank MS N. Chapman, Mr I. Buchanan and Mr W. O K for technical support and Mutdapilly staff for the care of the animals. Mr H. Kramer and Mr B. Burran QDPI, ARI, Yeerongpilly assisted with the chemical analysis. Financial support for this project was provided by the Dairy Research and Development Corporation. REFERENCES DAVISON, T. M., JARRE' IT, W. D. and CLARK, R. (1990). Aust. J. Exp. Agric. 30: 451-5. DAVISON, T. M., WILLIAMS, D., ORR, W. N. and LISLE, A. T. (1991). Aust. J. 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