Effects of stocking rate and physiological state of meat goats grazing grass/forb pastures on forage intake, selection, and digestion, grazing behavior, and performance

Abstract

Effects of forage conditions with different stocking rates on performance and grazing behavior of goats could vary with animal physiological state, as influencing nutrient demand and usage. Therefore, Boer goat does nursing two kids (D; 1 month after kidding), growing wethers (G; 4 month initial age), and yearling wethers (Y; 14 month initial age) grazed 0.4-ha grass/forb pastures, with one animal per type in each pasture (four per stocking rate; SR) for a low SR and two for the high SR. The experiment started in late spring and was 114 days in length, with four periods of 33, 28, 30, and 23 days (P1, P2, P3, and P4, respectively). Data were analyzed by mixed models with a repeated measure of period. Forage mass was 2517, 2433, 2506, and 2452kg/ha for the low SR and 2680, 1932, 1595, and 1393kg/ha for the high SR in P1, P2, P3, and P4, respectively (SE=335.1). Botanical composition of the diet determined from n-alkane concentration in simulated grazed forage samples and feces was similar among animal types (P>0.10). Likewise, chemical composition of forage samples did not differ between animal types (P>0.10), with average dietary levels of 11% CP and 53% NDF. Digestibility of OM, determined from the concentration of the n-alkane hentriacontane (C31) in forage samples and feces, was the greatest for growing wethers (P<0.05; 63.5%, 67.2%, and 62.0% for D, G, and Y, respectively) and greater (P<0.05) for the low than high SR (66.1% vs. 62.3%). Intake of ME estimated from digestibility and fecal output was 1015, 855, and 692kJ/kg BW0.75 for D, G, and Y, respectively (SE=57.4) and greater for the low than high SR in P1 (1204, 789, 682, and 445 for high SR and 1732, 767, 683, and 531kJ/kg BW0.75 for low SR in P1, P2, P3, and P4, respectively; SE=93.5). There was an interaction (P<0.05) between animal type and period in ADG (13, −12, −44, −8, 83, 25, −28, 73, 127, 51, −43, and −7g; SE=21.5) and time spent grazing (7.5, 5.3, 7.4, 8.6, 78.6, 5.6, 10.0, 9.1, 4.8, 5.9, 8.4, and 9.5h for D-P1, D-P2, D-P3, D-P4, G-P1, G-P2, G-P3, G-P4, Y-P1, Y-P2, Y-P3, and Y-P4, respectively; SE=0.88). Rate of ME intake was greater (P<0.05) for D vs. G and Y (49.5, 21.9, and 33.9kJ/min for D, G, and Y, respectively; SE=5.68) and differed (P<0.05) among periods (57.5, 45.3, 24.8, and 12.9kJ/min in P1, P2, P3, and P4, respectively; SE=5.17). In conclusion, with this forage of moderate nutritive value, levels of forage mass above 1400kg/ha would not be of benefit to performance of meat goats regardless of physiological state with different nutrient requirements.