Body condition change and foraging strategy of gestating beef cows in response to herbage allowance and cow genotype

The objective was to study the effect of herbage allowance (HA) and cow genotype (CG) on BCS and grazing behavior of beef cows during gestation (180 to 0 d prepartum [DPP]). Multiparous beef cows (n = 24) were submitted to HA (3 [LO] and 5 kg DM/kg BW [HI]) and CG, purebred [PU] Angus–Hereford (437 ± 53 kg BW and 4 ± 0.5 BCS) and F₁ crosses (CR; 462 ± 29 kg BW and 4 ± 0.7 BCS). Herbage mass and height were 1,300 vs. 1,100 kg DM/ha and 3 vs. 2 cm in HI and LO, respectively, without differences in chemical composition. Body condition was determined monthly. From 88 to 77 ± 11 DPP, probability of grazing (PGT) and rumia (PRT) were visually recorded every 5 min. During the major grazing session, we estimated the time to take 100 prehension bites (bite rate [BR]) and the number of patches (PH) and feeding stations (FS) per hour. Forage intake was estimate based on an internal indicator (n-alkanes). Effects of HA, CG, DPP, on BCS and PGT, PRT, BR, PH, and FS were analyzed with repeated measures. The BCS were higher in HI cows (5.3 ± 0.1 vs. 4.2 ± 0.2 and 4.6 ± 0.07 vs. 4.3 ± 0.1 at 90 and 60 DPP, respectively), whereas CR cows had higher BCS than PU cows at calving (4.8 vs. 4.3 ± 0.09 for CR and PU cows, respectively). The HI increased PRT (0.15 vs. 0.05) but did not affect PGT (0.85 for HI vs. 0.87 for LO) PH (14 ± 2 PH/h), and FS (285 ± 25 FS/h). The PGT was reduced for CR cows (0.80 for CR cows vs. 0.85 for PU cows; P < 0.05). Patches per hour was lower for CR cows than for PU cows (10 vs. 18 ± 2; P < 0.05) but FS per hour was higher for CR cows than for PU cows (320 vs. 263 ± 26; P = 0.09). The BR increased in LO–PU with respect to HI–CR, HI–PU, and LO–CR (58 vs. 50 bite/min ; P < 0.05). Forage intake was affected by HA (2.6 vs. 2.2% live weight [LW] in HI vs. LO, respectively) but not by CG (2.2% LW). Foraging strategy was oriented to increased energy intake in HI HA cows or improved energy efficiency in CR cows, which could explain improved BCS during gestation.