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* Centro de Estudos de Ciência Animal do Institutode Ciências e Tecnologias Agrárias e Agro-Alimentares,
Faculdade de Ciências and
Instituto de Ciências Biomédicas de Abel Salazar, Universidade do Porto, Campus Agrário de Vairão, 4485-661 Vairão VC, Portugal,
Institute of Grassland and Environmental Research, Plas Gogerddan, Aberystwyth, Ceredigion SY23 3EB, U.K., and
|| Divisão de Leite e Lacticínios, Estação Experimental, Direcção Regional de Agricultura do Entre-Douro e Minho,4594-909 Paços de Ferreira, Portugal
Corresponding author: A. R. J. Cabrita; e-mail address: rita.cabrita{at}mail.icav.up.pt.
Feed intake and milk production responses to N supplementation of corn silage-based diets were measured in three 3 x 3 Latin square experiments. In each experiment, 9 Holstein cows received total mixed rations (TMR), based on corn silage. In Exp. 1, midlactation cows were used to study effects of diets with different ratios of effective rumen-degradable protein (ERDP; g) to fermentable metabolizable energy (FME; MJ), providing a large deficiency (RL), a slight deficiency (RM), and a slight excess (RH) in relation to the target level of 11 g of ERDP/MJ of FME, respectively, for lactating cows. Diets were formulated to be isoenergetic, and to satisfy the metabolizable protein requirements. In Exp. 2, early-lactation cows were used to evaluate effects of different proportions of quickly and slowly rumen-degradable protein (RDP), achieved by replacing soybean meal with urea in the concentrates (0, 0.5, and 1% urea). Experiment 3 investigated effects of synchronizing the availability of FME and ERDP in the rumen. Midlactation cows received a diet containing, on a dry matter (DM) basis, 45% corn silage, 5% ryegrass hay, 35% energy-rich concentrate, and 15% protein-rich concentrate (crude protein: 38% of DM; urea: 2% of DM). The protein-rich concentrate was fed either once (D1) or twice (D2) per day before the meal, or included in the TMR (DU). Treatment RL led to lower DM intake and milk yield, but higher milk production efficiency; there were no significant differences between treatments RM and RH. There were no significant treatment effects on DM intake, milk yield, or milk composition in Exp. 2. Manipulating rumen synchrony by altering the timing of feeding affected milk yields, with D1 cows producing significantly less than D2 and DU cows, which were similar. The amount of ERDP in the diet should be matched to the amount of fermentable energy available to maximize intake, milk yields, and the conversion of feed N into milk protein. However, this study showed only small benefits to altering the diurnal pattern of supply of RDP and FME, and only with extreme feeding strategies that would not be used in practice. Urine volume increased in response to increased or unbalanced protein supply. Analysis of the allantoin:creatinine ratio in spot samples of urine was not useful in identifying predicted differences in microbial protein yield from the rumen.
Key Words: corn silage • dairy cow • nitrogen supplementation • rumen synchrony
Abbreviation key: A/c = allantoin/creatinine, D1 = protein-rich concentrate fed once a d before the a.m. meal, D2 = protein-rich concentrate fed twice a d before both meals, DU = protein-rich concentrate given as a TMR with the basal diet, ERDP = effective rumen-degradable protein, FME = fermentable metabolizable energy, QDP = quickly rumen-degradable protein, RH = diet with a slight excess of ERDP in relation to FME, RL = diet with a large deficiency of ERDP in relation to FME, RM = diet with a slight deficiency of ERDP in relation to FME, SDP = slowly rumen-degradable protein, U0 = concentrate with 0% urea, U5 = concentrate with 0.5% urea, U10 = concentrate with 1% urea
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