Genotype x environment interactions in milk yield and quality in Angus, Brahman, and reciprocal-cross cows on different forage systems

Milk yield and quality were observed on 93 Angus, Brahman, and reciprocal-c ross cows over 3 yr to evaluate the interactions of direct and maternal bre ed effects and heterosis with forage environment. Forage environments were common bermudagrass (BG), endophyte-infected tall fescue (E+), and a rotati onal system (ROT) of both forages, in which each forage (BCT or E+) was gra zed during its appropriate season, usually June through October for BG and November through May for E+. Milk yield was estimated each of 6 mo (April t hrough September) via milking machine and converted to a 24-h basis. Milk f at, milk protein, and somatic cell count were analyzed by a commercial labo ratory. Heterosis for milk yield was similar among forages, averaging 2.4 k g (P < 0.01). Expressed as percentages of purebred means, heterosis for mil k yield was largest on E+ (52.8%), intermediate on ROT (39.3%), and smalles t on BG (23.7%). Direct breed effects for milk yield favored Brahman, and t hey were similar among forages but tended to be larger for E+ (2.5 kg) and ROT (2.8 kg) than for BG (1.3 kg). Direct breed effects for milk fat favore d Brahman and were similar among forages but tended to be larger for E+ (1. 0%) and ROT (1.0%) than for BG (0.6%). Purebred cows exceeded crossbreds in milk protein by 0.1% on ROT (P < 0.10). Crossbred cows had lower somatic c ell counts than purebreds on BG (P < 0.05), E+ (P < 0.01), or ROT (P > 0.30 ). Heterosis for somatic cell counts as percentages of purebred means was s imilar for BG (-68.3%) and E+ (-68.9%) and less favorable for ROT (-31.6%). Maternal breed effects for somatic cell count favored Angus on ROT (P < 0. 10) with a similar nonsignificant trend on BG and E+. Direct breed effects for somatic cell count favored Brahman on ROT (P < 0.10) with similar nonsi gnificant trends on BG and E+. These results suggested that a rotation of c ows from E+ to BG in the summer can partially alleviate negative effects of E+ on milk yield. Conclusions also indicated an advantage to crossbred cow s in somatic cell count and provided evidence of both direct and maternal b reed effects for this trait. The results also suggested that direct breed e ffects for milk yield, milk fat, and somatic cell count and heterosis for m ilk yield and somatic cell count las percentages of purebred means) tended to vary with forage environment, indicating a potential for genotype x envi ronment interaction for these traits.