Dietary nitrogen and phosphorus depletion in cattle and their effects on liveweight gain, blood metabolite concentrations and phosphorus kinetics

Abstract

In a 15-week animal-house experiment, 24 steers were offered one of six diets based on molasses and ad libitum barley straw. Three levels of dietary nitrogen (N) and three levels of dietary phosphorus (P), in factorial combination, were formulated by the addition of urea, formaldehyde-treated wheat gluten and monosodium orthophosphate. Food intake, liveweight gain, plasma metabolites and P kinetics were measured under dietary regimens similar to those experienced by cattle grazing Australia's northern semi-arid rangelands.The adverse effect of the low dietary N on both liveweight change and feed intake was greater and more immediate than that of the dietary P deficiency. The reduction in feed intake due to the P deficiency approached that caused by the N deficiency after 10 weeks. Under conditions of adequate dietary N, there was a trend for the effects of P deficiency on liveweight gain to be exacerbated. Dietary N and P deficiency reduced the concentrations of plasma urea-N and inorganic P respectively. Dietary N deficiency had no effect on cortical rib bone thickness but P deficiency markedly decreased bone thickness. Faecal endogenous loss of P and P absorption efficiency ranged from 9 to 21 mg/kg LW and 063 to 0-82 respectively for P intakes from 6 to 41 mg/kg LW. Faecal endogenous losses were closely related to dry matter intake and plasma inorganic P together. Dietary N deficiency affected the efficiency of absorption of P.The results of this experiment indicate that cattle consuming diets containing low levels of N and P require supplementary N and P in combination to avoid severe depletion since an increase in N intake alone exacerbated the P deficiency. The results are also discussed in relation to the published findings regarding P metabolism and the implications for the calculation of P requirements.

Effect of offering dairy cows diets differing in phosphorus concentration over four successive lactations: 1. Food intake, milk production, tissue changes and blood metabolites

“…For example, failing to meet the P requirements of rumen microflora may have a negative effect on ration digestibility and microbial protein synthesis, as reviewed by Breves and Schroder (1991), and this in turn may have a negative effect on food intake. However, reduced food intakes have been observed with cows offered 'low' P diets, despite ration digestibility being unaffected (Milton and Ternouth, 1985;Bortolussi et al, 1996). The latter authors have suggested that this may be mediated via an effect at a cell metabolism level.…”

Desempenho de novilhas alimentadas com rações contendo milho ou casca de mandioca como fonte energética e farelo de algodão ou levedura como fonte protéica

“…Segundo WITT e OWENS (1983), a disponibilidade de fósforo (P) para os microrganismos do rúmen é importante para atingir a máxima eficiência de digestão dos nutrientes. BORTOLUSSI et al (1996), investigando os efeitos dos níveis de nitrogênio (N) e P na ingestão e no ganho em peso de novilhos confinados, observaram que a adição de N e P na dieta composta de glúten de trigo, melaço, palha de cevada e uréia melhorou o ganho em peso e a ingestão de alimentos de bovinos confinados durante quinze semanas.…”

Perda endógena e exigência de fósforo para mantença de cabritos Saanen

“…Considerando-se a quantidade de P ingerido, a variação encontrada está próxima às obtidas por Ternouth (1989), Hendricksen et al (1994) e Bortolussi et al (1996), ficando pouco abaixo dos valores observados por Bueno (1997). Isso evidencia que a perda endógena não é um valor constante, como concluíram também outros autores (Braithwaite, 1984;Coates & Ternouth, 1992;Louvandini & Vitti, 1994;Ternouth & Coates, 1997).…”

“…Young et al (1966) observaram que, diferentemente do cálcio, a excreção de fósforo endógeno fecal variou com a quantidade ingerida de P. As perdas de P aumentaram de 16,8 para 24,9 mg/kg PV/dia, quando o conteúdo de P da dieta de bezerros de seis meses aumentou de 12,7 para 31,6 g/dia (Annenkov, 1982). Bortolussi et al (1996) determinaram, em bovinos consumindo entre 6 e 41 mg P/kg PV/dia, perdas endógenas de P variando de 9 a 21 mg/kg PV/dia, as quais foram também relacionadas com a ingestão de matéria seca e P no plasma. No Brasil, Louvandini & Vitti (1994) determinaram 10,65 mg/kg PV/dia para a perda endógena de P, em ovinos de 40 kg de peso vivo, mas o nível mais baixo de P ingerido correspondeu a 51,11 ± 14,45 mg/kg PV/dia, constando também que, a partir de ingestão de 100 mg P/kg PV/dia, há tendência de diminuição nos valores de P endógeno.…”