Effect of pregnancy and feeding level on voluntary intake, digestion and microbial N production in Nellore cows*

Abstract Text: The objective of this experiment was to evaluate the effects of pregnancy and feeding level on intake, digestibility and efficiency of microbial N production in Nellore cows. Forty-four multiparous Nellore cows (32 pregnant and 12 non-pregnant) with average initial body weight of 451 ± 10 kg were fed either HIGH (ad libitum) or LOW (restricted feeding 1.2 times maintenance according to the NRC) feeding level. The diet consisted of corn silage (85%), ground corn, soybean meal, urea and mineral mixture. The intake was controlled daily and the DMI was evaluated weekly. In vivo apparent total digestibility was estimated using indigestible NDF as an internal marker and microbial N synthesis was estimated using the technique of purine derivatives in urine. Fecal and urine samples were collected every 28 days. The voluntary feed intake reduced as the pregnancy advances in Nellore cows and can be calculated as DMI (kg/d) = (16 – 0.0093 × days of pregnancy) / 1000 × SBW, where SBW is shrunk body weight. The average DMI of LOW-fed cows corresponded to 102, 98 and 67% of the amount of energy necessary to attend the maintenance and pregnancy energy requirements suggested by NRC for a cow at 0, 135 and 270 d of pregnancy, respectively. However, LOW-fed cows had 0.26 kg/d of average shrunk body gain indicating that the nutrient energy requirements of Zebu cows are likely lower than those suggested by NRC. The interaction between the feeding level and days of pregnancy was significant (P < 0.05) for the digestibility of DM, OM, CP, ether extract (EE), NDF corrected for ash and protein (NDF_ap) and GE, and the values of TDN. In all these cases there was a reduction in digestibility with increasing gestation age in HIGH-fed cows, while digestibility of OM, CP, EE, NDF_ap and GE increased as function of days of pregnancy in LOW-fed cows. The reduction in the digestibility of neutral detergent fiber occurs faster than in dry matter digestibility. These data suggests that the reduction of the digestibility as pregnancy increases in ad libitum fed cows is caused by an increase in the rate of passage as compensation factor for the ruminal volume reduction. There were no direct effects of pregnancy on microbial N production in Nellore cows. 

*Funded by INCT-CA, CAPES, CNPq, and FAPEMIG.

Keywords: beef cattle, Bos indicus, dry matter intake, gestation, total digestible nutrients, Zebu